What Is Secondary Power in Welding? A Complete Guide

You are running a 300-amp machine on a 100-foot lead, and the puddle suddenly goes cold and stubborn halfway through the joint. The dial has not moved, the machine sounds fine, and the amps readout still says 300. The problem is not your arc technique, it is what is happening on the secondary side of the welder: the low-voltage, high-current power that actually reaches the electrode, the cables, and the work. Understanding that side of the circuit explains a surprising number of weld problems and safety close calls.

What Secondary Power Actually Means in Welding

You are running a 300-amp machine on a 100-foot lead, and the puddle suddenly goes cold and stubborn halfway through the joint. The dial has not moved, the machine sounds fine, and the amps readout still says 300. Nothing about your technique changed. What changed is what is reaching the arc on the secondary side of the welder, the low-voltage, high-current output that actually feeds the electrode, the cables, and the work.

That side of the circuit explains a surprising number of weld problems and safety close calls. So let’s define it cleanly before anything else.

The Plain Definition

Secondary power is the electrical output of the welding power source, delivered from the secondary winding of the transformer or the output stage of an inverter, and measured as voltage times current on the welding side of the machine.

That is the short, quotable version. Everything else in this guide is really an explanation of what that sentence means in practice, and why the numbers behave the way they do.

Primary Side vs. Secondary Side

The vocabulary comes straight out of transformer theory. A welding power source takes high-voltage, low-current electricity from the shop supply, the primary, and converts it into low-voltage, high-current electricity for welding, the secondary.

The scale of that conversion is what makes it worth understanding. According to the CWB Group, a welding transformer takes primary power at roughly 208 to 600 volts and 15 to 55 amps and turns it into a secondary output of up to about 80 volts, with welding currents that can reach 1000 amps AC or more. Resistance welding pushes the same idea much further: secondary voltages of only 2 to 25 volts while producing 5,000 to 90,000 amps of welding current.

Power in approximately equals power out. A step-down in voltage always comes with a step-up in current. The machine is not creating free energy, it is trading one for the other, and any losses show up as heat in the windings and core.

Why the Word “Secondary” Confuses People

In everyday English, “secondary” means backup, lesser, or auxiliary. In electrical work it means something completely different: the second winding of a transformer, the output side. A welder’s secondary is not a spare system waiting to take over. It is the main event, the power that strikes the arc.

That misunderstanding gets worse because people also use “secondary power” loosely to describe the auxiliary generator output on engine-driven units. On a engine-driven welder, the 120/240 V receptacles for grinders and lights are auxiliary power. The welding output is still the welding secondary. They are separate circuits with separate limits, and confusing them leads to overloaded generators and tripped breakers.

What the secondary circuit includes depends on the process:

• Arc welding: the secondary is the whole welding circuit, meaning the electrode lead, the work lead, the electrode or torch, and the arc itself.
• Resistance welding: the secondary is the low-voltage, high-current output feeding the electrodes through the throat of the machine and the workpiece.

Why This Matters More Than It Sounds

Secondary power is not an academic distinction. It decides four practical things:

• Weld quality: the current that actually reaches the joint, not the number on the display, is what fuses metal.
• Cable sizing: undersized leads waste voltage as heat before the power ever reaches the arc.
• Machine selection: ratings only mean something when you know the duty cycle they are quoted at.
• Shock hazard: low voltage is not the same as low risk, as the safety section will show.

Even the code treats this side of the machine as its own category. NEC Article 630 defines welding cable as cable designed for use in the secondary circuits of electric welders, which is why welding lead is flexible, finely stranded, and rated differently from building wire.

In the sections that follow, we will follow the power from the transformer windings through the three secondary voltages, into resistance welding kVA ratings, down the cables, across the different machine types, and through the safety and diagnostic side. If your welder ever feels weaker than its dial suggests, the answer is almost always hiding somewhere on this list. Processes like aluminum MIG welding are especially sensitive to it, because aluminum conducts heat away fast and punishes any loss of secondary current.

Start with the cable and the connections. They are the cheapest thing to check, and the most common culprit.

From Primary to Secondary: How a Welding Transformer Makes Secondary Power

A welding transformer is best understood as a gearbox for electricity. It does not create energy. It trades voltage for current, taking a relatively small current at high voltage from the supply and turning it into a large current at low voltage on the welding side. That trade is what makes a 300-amp arc possible from an ordinary industrial service.

Two Windings, One Magnetic Link

Inside a conventional welder sit two coils of copper wire wound around a laminated iron core. The primary winding connects to the incoming supply; the secondary winding connects to the electrode lead and work lead. The two are electrically isolated but magnetically coupled. Alternating current in the primary creates a changing magnetic field in the core, and that field links the secondary turns. This is mutual inductance: a changing flux induces a voltage in any coil it passes through.

The laminated core matters. Thin insulated steel plates reduce circulating eddy currents that would otherwise turn the core into a heater. The core also gives the magnetic flux a low-reluctance path, so most of the primary field reaches the secondary instead of leaking away. The Canadian Welding Bureau’s explanation of welding transformers describes this same basic arrangement.

The Turns Ratio Does the Math

Voltage ratio equals turns ratio. If the primary has 48 times as many turns as the secondary, the secondary voltage is one forty-eighth of the primary voltage. Current goes the other way: in an ideal transformer, primary current times primary voltage equals secondary current times secondary voltage, so a 48:1 step-down multiplies current by roughly 48.

A real example makes this concrete. Feed 480 V into a transformer with a 9 to 10 V secondary and you get a turns ratio near 48:1, as the resistance welding equations published by Spot Weld, Inc. illustrate. If the primary is drawing 100 A, the ideal secondary current is about 4,800 A at 10 V. A 50:1 unit does the same trick more aggressively: 100 A of primary current becomes roughly 5,000 A at the secondary, a relationship explained in this resistance welding transformer primer. In practice, losses, magnetizing current, and power factor mean the primary draw is slightly higher than the ideal calculation.

Taps, Cooling, and the Inverter Twist

Tap changers let the operator choose a different effective turns ratio. Moving the tap adds or removes primary turns, which changes secondary open-circuit voltage and therefore the current available at a given arc length. That is why the tap setting feels like a coarse power control on an old stick welder: it is literally changing the transformer’s ratio. Fine control comes from a movable magnetic shunt, a series reactor, or electronic phase control, depending on the design.

Welding transformers are unusual because they are expected to operate near a short circuit. The arc is close to a short, and resistance welding is a controlled short. To survive that, the transformer needs enough impedance to limit current and enough thermal mass or cooling to handle intermittent overloads. High-current resistance welding transformers are often water-cooled because heat inside the copper and core cannot escape fast enough otherwise. Forced-air cooling serves the same purpose on lighter-duty machines.

Inverters change the recipe. Incoming AC is rectified to a DC bus, then switched at high frequency into a much smaller transformer. In one documented 1,000 Hz inverter example, that transformer may see about 650 VAC even though the supply is 480 VAC, according to the same transformer primer. Higher frequency means a smaller core and fewer turns for the same power. The higher voltage on the transformer side also lowers current for a given kVA, reducing copper loss, and the final rectifier delivers DC that suffers less inductive drop in long leads. That steady DC arc is one reason inverters are popular for aluminum MIG welding.

Single-Phase, Three-Phase, and Where the Losses Go

Small shop welders usually run on single-phase 240 V. Industrial machines often use three-phase 480 V because the same power arrives in smaller conductors and the load is balanced across phases. A 0.81 constant appears in some three-phase secondary-to-primary current conversions as an empirical factor bundling phase relationships and typical losses. Treat it as a rough estimating aid, not a substitute for the nameplate or a meter reading.

None of this escapes conservation of energy. The secondary delivers less power than the primary receives; the difference becomes heat in the windings, core, and connections. That is why cooling, duty cycle, and cable size all matter. The simple, robust nature of a transformer-based stick welder is part of the appeal of stick welding, but even the toughest machine will fail if its thermal limits are ignored.

The Three Secondary Voltages You Need to Know: Open Circuit, Arc, and Short Circuit

Voltage is the push behind secondary current, and it is also the part of the welding circuit most likely to bite you. A welding machine does not have one fixed output voltage. It has three distinct voltage conditions, and confusing them leads to bad settings, false diagnoses, and unnecessary risk.

Open Circuit Voltage: The Push Before the Arc

Open circuit voltage, or OCV, is the voltage across the output terminals when the machine is on but no arc is burning. It is the highest voltage the electrode will see. Lincoln Electric notes that arc welding open circuit voltages typically run from about 20 to 100 volts, while voltages inside the machine case can reach 120 to 575 volts or more (Lincoln Electric). That difference is why the secondary side is safer, not harmless.

OCV matters for two reasons. First, it provides the electrical pressure needed to start and restrike an arc, especially with stick electrodes on AC. Second, it is the shock exposure a welder faces when changing electrodes or adjusting a stinger. OSHA 1910.254 caps OCV at 80 volts for manual AC arc welding and cutting, 100 volts for automatic AC, and 100 volts for DC machines (OSHA).

OCV-reducing devices exist because that exposure is real. These controls hold output at a low safe level until the electrode touches the work, then switch to full OCV for arc initiation. They are especially valuable for AC welding in wet, hot, or confined spaces. Higher OCV does make restriking easier, which is one reason stick welding remains popular on dirty field work, but convenience should never override the shock hazard.

Arc Voltage: The Load Voltage While Welding

Once the arc establishes, the voltage sags to the arc voltage, also called load voltage. This is the voltage actually driving current across the gap. Common ranges are roughly 20 to 30 volts for stick welding, 18 to 28 volts for MIG, and 10 to 18 volts for TIG. The exact value depends on arc length, gas, electrode type, and current.

Arc voltage is not the same as a machine’s rated output voltage. A 300-amp machine rated at 32 volts describes a point on its output curve, not its OCV and not necessarily the voltage you will see at every setting. A long arc raises arc voltage; a tight arc lowers it. That is why voltage controls on MIG welders change bead shape so noticeably.

Short-Circuit Voltage and Current

Short-circuit voltage is the near-zero voltage across the output when the electrode touches the work and the circuit is effectively closed. In arc welding, that moment is brief and the current is limited by the machine’s characteristic. In resistance welding, short-circuit current is a design rating: it is the current with the electrodes clamped together and no workpiece resistance in the loop.

That short-circuit current is always higher than the current available through a real joint. IS 4804-1 defines maximum welding current as approximately 0.8 times the maximum short-circuit secondary current (IS 4804-1). It is a useful sanity check when comparing transformer ratings with real weld schedules.

Voltage condition	What it means	How it is measured	What can go wrong
Open circuit (OCV)	No-load output; highest voltage at the electrode	Voltmeter across output terminals, machine idle	Shock exposure; failed OCV-reduction circuit
Arc / load voltage	Voltage across the burning arc	Meter between electrode and workpiece while welding	Long arc, poor technique, incorrect voltage setting
Short-circuit voltage	Near-zero voltage with output shorted	Meter across output during a controlled short	Sticking electrode; excessive current in resistance welding

Treat these three readings as different tools. OCV tells you about starting capability and shock risk. Arc voltage tells you what the puddle is actually experiencing. Short-circuit data tells you how hard the machine can push when the circuit is closed. Mixing them up is one of the fastest ways to misjudge a welder’s real behavior.

Secondary Power in Resistance Welding: kVA, Duty Cycle, and Turns Ratio

In resistance welding, secondary power is the driving force behind the process. A spot welder can take in 480 volts at a few dozen amps on the primary side and deliver as little as 10 volts at up to 20,000 amps on the secondary side. This massive current at low voltage is only possible with a transformer, but the nameplate rating is often the most misunderstood spec on the machine.

kVA Is a Thermal Rating, Not a Continuous Output

The kVA listed on a resistance welding transformer is apparent power, calculated by multiplying secondary voltage and secondary current with no regard for power factor. More importantly, it is a thermal rating. It defines how much apparent power the transformer can handle for a set on/off pattern before excess heat becomes a concern, not how much it can output around the clock.

Since welds are short and parts are moved between them, heat builds during each weld and dissipates during idle time. TWI’s equipment-sizing guidance notes that this rating is based on a 50 percent duty cycle, the industry standard used by ISO 5826 and its equivalent DIN approaches. With a one-minute integration period, that means the transformer is only intended to be energized for 30 seconds out of every 60 at its rated kVA.

Duty Cycle Determines Usable kVA

Duty cycle is simply on-time divided by the integrating period. While 50 percent is the most common baseline, some imported machines are rated at 20 percent or even 4 percent. These are valid ratings, but comparing two “100 kVA” welders without checking their duty cycle basis is a recipe for undersizing.

The relationship between duty cycle and permissible power follows the square-root rule. For the same allowable temperature rise, power scales with the square root of the duty cycle. Using Harms & Wende’s explanation of welding transformer performance, a 100 kVA transformer rated at 50 percent duty can deliver roughly 224 kVA at 10 percent duty, while its continuous 100 percent duty capability drops to about 70.7 kVA. The table below puts this into perspective:

Duty cycle	Approximate short-term capability of a 100 kVA, 50% unit	What it means in practice
50%	100 kVA	Rated thermal design for that pattern
20%	About 158 kVA	Shorter bursts with longer cooling gaps
10%	About 224 kVA	Fast production with substantial idle time
4%	About 354 kVA	Very short, infrequent welds only

Keep in mind that duty cycle is a system limit. For example, twenty welds per minute at 0.2 seconds each equates to about 6.7 percent duty for the transformer, but tips, electrodes, and cables can reach their own thermal limits far sooner.

Turns Ratio: Turning Voltage Into Current

While duty cycle limits heat, the turns ratio dictates how power is delivered. Primary and secondary voltages are proportional to winding turns (Vp/Vs = Np/Ns), and current is inversely proportional. As explained by the step-down action of a resistance welding transformer, a 480-to-10-volt ratio (48:1) can take 100 primary amps and turn it into roughly 4,800 secondary amps, barring losses.

However, secondary voltage must be sufficient to overcome the total impedance of the secondary loop. This includes the transformer secondary, jumpers, arms, electrodes, workpiece, and return path. A long-throat machine or a worn jumper adds impedance, meaning more kVA is required to achieve the same weld current. Even surface conditions play a role. For materials like aluminum, oxide and contamination can increase contact resistance, which is why proper prep, such as using an aluminum cleaner for welding, helps keep current flow stable.

Specifying With Margin in Mind

Theoretically, the nameplate is only half the equation. TWI advises adding at least 20 percent to the estimated required current to allow for adjustment and automatic heat control. This margin accounts for worn tips, voltage fluctuations, and normal production wear. Both ISO 5826 and DIN recognize this type of overload capacity, rather than treating the rating as an absolute ceiling.

With that in mind, the best practice is to specify the secondary power needed at the duty cycle you actually run, factor in loop losses, and leave room for adjustment. More often than not, when a resistance welder feels weak, the culprit isn’t the transformer rating. It’s a long, lossy secondary loop.

The Secondary Circuit: Cables, Work Lead, and Voltage Drop

Secondary power is only as good as the path it travels. A machine can deliver a perfect 300 amps at its output terminals and still leave you with a cold, stubborn puddle if the cable, connections, and return path eat the voltage before the arc ever sees it. That is why the secondary circuit deserves as much attention as the power source itself.

The Complete Secondary Loop

The welding circuit is a closed loop made up of the power source output, the electrode cable, the electrode holder or torch, the arc, the workpiece, the work lead, and the return to the machine. Every part carries the same current, so every part contributes resistance. The total loop resistance, not just the arc resistance, determines how much voltage is left for welding.

• Electrode cable: carries current from the machine to the holder or torch.
• Holder, torch, and consumables: introduce contact and conductor resistance.
• Arc: the useful part of the circuit, where heat is generated.
• Work lead and clamp: return current to the machine.
• Workpiece path: current must travel through the part, sometimes a long way from the clamp.

Why Four Volts Is the Design Limit

A 4-volt drop has been the accepted ceiling for welding cable since the 1938 AWS Welding Handbook, and it is still the practical benchmark today. The limit exists to protect arc characteristics, not simply to keep cables from overheating. Excess voltage drop robs the arc of the voltage needed to start reliably, maintain length, and transfer metal smoothly. On a constant-voltage process, the machine may try to compensate, but it cannot recover what is lost in a long, undersized loop.

Sizing Cable With the AWS Formula

AWS uses a simple copper-cable formula:

Circular mils = (10.37 × amps × total cable length in feet) ÷ 4 volts

The length is the entire loop, out and back. For 250 amps with a 150-foot run each way, the total is 300 feet:

(10.37 × 250 × 300) ÷ 4 = 194,438 circular mils

The next standard size above that is 4/0 AWG, at about 211,600 circular mils. That matches the common field rule: 250 amps over a 300-foot total circuit needs 4/0 copper. Lincoln Electric’s guidance on voltage drop through weld cables follows the same logic and is worth bookmarking for quick checks.

Cable size	150 A	200 A	250 A	300 A
#2 AWG	171 ft	128 ft	102 ft	85 ft
1/0 AWG	272 ft	204 ft	163 ft	136 ft
2/0 AWG	343 ft	257 ft	205 ft	171 ft
3/0 AWG	432 ft	324 ft	259 ft	216 ft
4/0 AWG	545 ft	408 ft	327 ft	272 ft

Approximate maximum total loop length for a 4-volt drop, calculated from the AWS formula. Ambient temperature and conduit derating are not included.

What Undersized Cable Actually Does

An undersized cable turns secondary power into heat. The jacket softens, the copper oxidizes, and resistance climbs as the conductor warms. That creates a frustrating loop: hotter cable means more drop, which means the arc runs colder, so the operator turns the machine up, which heats the cable further. The result feels like a weak welder when the real problem is a straw instead of a hose. Cable catalogs commonly list welding cable from 8 AWG to 500 kcmil, rated 600 volts and 90 to 105°C, and those temperature ratings assume the jacket and terminations are intact.

Connections, Clamps, and Return-Path Abuse

Loose lugs, corroded terminals, and spatter-covered work clamps add resistance at the worst possible places. A clamp biting through paint or mill scale may look connected while carrying current through a few small contact points. Coiled cable also creates a neat inductor on AC, and the reactance grows with the loop area rather than the copper cross-section.

Using structural steel, pipe, cranes, or wire rope as a return path is another common shortcut. OSHA 1910.254 specifically prohibits chains, wire ropes, cranes, hoists, elevators, and conduits from being used as part of the work-lead circuit. Even a clean steel table is a poor substitute for a proper work lead, and thin sections such as an aluminum boat hull can be damaged by return current taking an unintended path.

AC Inductance Versus DC Output

In AC secondary loops, inductive losses increase with throat area, the open space enclosed by the cables and work. Wide, looping runs and long-throat resistance welders suffer most. DC inverter output largely removes that reactance, which is one reason a compact inverter can feel stronger on long leads than an older AC transformer of similar rating.

Secondary Power Sources Compared: Transformer, Inverter, Engine-Driven, and Multi-Operator

Two welders can dial in the same 200 amps and still feel like completely different machines on the same job. One runs a smooth arc down a hundred feet of lead; the other stumbles and leaves you fighting the puddle. The number on the dial isn’t the difference. It’s how each machine makes its secondary power, and what that method does to voltage, ripple, and control as the load shifts.

Every welder does the same basic job: take high-voltage, low-current primary power and convert it into low-voltage, high-current secondary power. There’s more than one way to make that conversion, and each route leaves fingerprints on the arc.

Conventional Transformer Machines: The Heavyweight Classic

The traditional stick and TIG workhorse uses a laminated iron core with a primary and secondary winding. Line voltage goes in, magnetic flux steps it down, and low-voltage AC comes out. There’s very little to break — no circuit boards, no switching devices, just copper, iron, and a tap changer or moving shunt. That simplicity is why they survive decades in shops that treat equipment badly. They’re also heavy: a 300-amp transformer machine can weigh several hundred pounds and wants a serious primary circuit behind it.

Two secondary-side behaviors matter. First, output follows input — when plant voltage sags under load, secondary voltage sags with it and the arc feels softer or colder. Second, some designs produce a strong low-end arc because of how the magnetic shunt shapes the volt-ampere curve. Plenty of stick welders still prefer that feel for root passes and thin material.

Inverter Machines: Small Box, Better Control

Inverters take a different route. Incoming AC is rectified to DC, switched at high frequency, passed through a much smaller transformer, then rectified again to produce DC welding output. Because the transformer sees high-frequency AC, it can be tiny compared to a 60 Hz unit of the same rating.

The payoff is real: lighter weight, lower primary draw for the same output, and a stable arc with far less ripple. Feedback circuits adjust output thousands of times per second, so the machine compensates as arc length changes. That’s why a 40-pound inverter can out-weld a 200-pound transformer on a portable job. The trade-off is more electronics and more failure points, and dirty power, heat, and moisture are hard on them. Also worth knowing: because the output is DC, inductive losses in the secondary loop are virtually eliminated, according to TWI’s guidance on resistance welding equipment sizing. Long leads stop punishing you the way they do on AC.

Engine-Driven Welders: Secondary Power From a Generator

On a pipeline right-of-way or a remote structural job, the welder is its own power plant. An engine spins an alternator or generator, and that generator’s output becomes the welding secondary. The welding terminals are still the secondary side of the system.

Confusion creeps in around the panel receptacles. The 120/240 V outlets are auxiliary power for grinders, lights, and tools — not welding secondary power, and they behave nothing like it. Treat them as a separate system with separate limits, because that’s exactly what they are. Engine drives also change with altitude: generator output drops as air density falls, so a machine rated 300 amps at sea level may not deliver it at 8,000 feet. Under OSHA 1910.254(c)(2)(i), engine-driven welding machines are exempt from the frame grounding requirement that applies to other arc welding machines, though grounding practices still deserve attention.

Multi-Operator Installations: One Source, Many Stations

Large fabricators sometimes run one big transformer feeding several operator stations, each with its own secondary control. It saves space and capital, but it creates a shared electrical environment. Voltage drop and isolation become the design drivers: a heavy draw at one station can pull down the others, and no-load voltages between separate electrode holders are not simply the OCV of one machine. OSHA’s 1910.254(c)(3)(iv) warns that with several welders on one structure, no-load voltages between electrode holders can reach 2 times normal on DC, or 1, 1.41, 1.73, or 2 times normal on AC machines. That’s a serious shock consideration, not a footnote.

Side-by-Side Comparison

Type	Typical OCV	Output	Portability	Efficiency	Best Use
Transformer	~50–80 V	AC (some DC)	Poor	Moderate	Shop stick/TIG, rough service
Inverter	~60–90 V	DC	Excellent	High	Field work, TIG, long leads
Engine-driven	~60–90 V	DC or AC	Mobile (vehicle)	Lower	Remote sites, pipeline, repairs
Multi-operator	Varies by station	AC or DC	Fixed	High at scale	High-volume fabrication bays

Duty cycle comparisons deserve a caution. A 60 percent machine at 200 amps is not automatically better than a 40 percent machine rated at a higher amperage. The percentage only means something paired with the current it was measured at, so read both numbers together and check the amperage basis before comparing price tags.

Matching a machine to the process matters as much as matching it to the building. If you’re branching into aluminum, the arc stability differences between these power source types show up fast — our guides to aluminum MIG welding and aluminum flux core welding both explain how output characteristics change the game.

Secondary Power and Safety: Shock, OCV Limits, and Grounding

When dealing with secondary power, safety needs to be looked at as an engineering problem, not a warning label. The welding circuit is intentionally designed to be low voltage and high current, however that inversion does not make it harmless. To understand the risk, you first have to know the difference between the two types of electrical shock possible around a welder.

The most dangerous of the two is primary shock. This comes from the incoming power supply, anywhere from 115 to 600 volts, before it reaches the welding transformer or inverter. With that much voltage and the direct path to your facility’s electrical system, a primary shock has a much higher potential to be fatal. Secondary shock, on the other hand, comes solely from the welding output. It is far lower in voltage, but it can still deliver a tremendous amount of current if the right conditions are met, and as a result, it still deserves your full respect.

The Exact Condition For A Secondary Shock

A secondary shock requires one specific thing: contact with both sides of the welding circuit at the same time while the output is energized. The circuit consists of the electrode lead, the arc (or load), the workpiece, and the work lead back to the machine. If you touch a bare spot on your electrode cable with one part of your body, and simultaneously touch the workpiece with another, you have just bridged both sides of that circuit. The current will take the path of least resistance, and unfortunately, that path can be through your body.

There are many everyday multipliers that drastically increase this risk. Working on wet concrete, using sweat-soaked gloves, sitting or kneeling directly on the work, or wearing metallic clothing such as jewelry or watchbands all increase conductivity and lower the body’s natural resistance. These conditions are especially prevalent in humid shops or outdoor jobs, and even in precision work such as aerospace TIG welding, complacency around moisture can be dangerous.

OCV Limits and When They Matter

The voltage side of secondary power is what creates the greatest shock potential while the machine is idling. This is known as open circuit voltage (OCV), the no-load voltage present at the output terminals before an arc is struck. Once welding begins, this voltage sags down to a much lower arc voltage.

Due to this idle voltage, OSHA 1910.254(b)(3)(iv) states that for AC welding in wet conditions or warm surroundings where perspiration is a factor, reliable automatic controls that reduce no-load voltage are recommended. Many modern machines utilize OCV-reducing devices for this exact reason. The goal isn’t to reduce the welding power, it’s to remove the unnecessary voltage the moment the arc isn’t established.

Grounding, Insulation, and Proper Circuit Practices

Good grounding is one of the most overlooked engineering controls. OSHA requires that the frames or cases of welding machines, except engine-driven machines, be grounded under the conditions of Subpart S. Just as importantly, the work lead is not a grounding conductor. It is a current-carrying conductor for the welding circuit, not a substitute for a proper equipment ground.

This is taken a step further for resistance welding. Where technically practical, the secondary of welding transformers used in multispot, projection, and seam welding machines must be grounded, or a center-tapped grounding reactor or safety disconnect used. This requirement exists due to the massive secondary currents and the sheer size of these circuits.

Even with proper grounding, the physical condition of your circuit is just as vital. Pay extra attention to insulation and connector condition, especially damaged cable jackets, cracked electrode holders, and loose lugs. A common, and avoidable, hazard is the electrode holder accidentally contacting a grounded surface while the machine is energized. Be diligent with your equipment, as these small points of failure are the ones most likely to create the two-sided contact needed for a secondary shock.

At the end of the day, the safest mindset is simple: treat every energized welding output as capable of delivering a shock if bridged. With proper grounding, well-insulated cables, OCV-reducing controls where applicable, and awareness of your environment, you can mitigate the risks without sacrificing the power that makes secondary power so useful in the first place.

How to Measure Secondary Power and Diagnose Common Problems

When a welder starts acting up, the instinct is to blame the machine itself. In reality, the majority of “weak welder” complaints are not a failed transformer or inverter, but issues within the secondary circuit. With the right measurements and a methodical approach, most of these problems can be tracked down before any major repairs are needed. The goal is not to guess, but to test the path that the low-voltage, high-current power actually takes to the arc.

The Right Tools To Take Accurate Readings

Before diagnosing anything, you need to know what can and cannot be measured. Every test you take will depend on using the correct instrument for the job at hand.

• Voltmeter for OCV: With the machine turned on and no arc struck, place your voltmeter leads directly across the output terminals. This will give you the open circuit voltage (OCV). Never take this reading mid-weld, as it will be heavily skewed by the arc.
• Voltmeter for arc voltage: To get a true load reading, measure voltage at the work and electrode while welding. These readings will always be lower than OCV, as voltage is dropped across the arc itself.
• Standard clamp meter: These are great for many shop tasks, however, a standard clamp meter often cannot accurately read the thousands of amps used in welding. They also struggle with the distorted waveforms that many inverters produce. Relying on them alone can be extremely misleading.
• Dedicated weld current meter or current-sensing weld control: For resistance welding specifically, these are a necessity. Resistance welding current is commonly measured in kilo-amperes, with about 10,000 amps at about 5 volts typical for low-carbon steel per RWMA data. Only a purpose built current transducer or weld control will be able to read these extreme values with any consistency.

Step-By-Step Diagnostic Workflow

With your tools in hand, work your way from the simplest checks to the more involved ones. Keep in mind that a poor connection can get very hot and increase resistance further as copper heats, so warm spots in cables or lugs indicate a problem according to Lincoln Electric. Additionally, coiled welding cable should be spread out before use to avoid overheating and insulation damage as stated in OSHA 1910.254.

For larger fabrication jobs, such as the ones seen when working with heavier sections covered under our guide for Alloy Steel Welding Electrodes: Selection, Use & Safety, these voltage drops become far more noticeable. The same can be said for many setups similar to those explained in our piece, Advantages of Stick Welding: Why SMAW Still Wins.

Troubleshooting Table: Symptoms, Causes, and First Checks

Symptom	Likely Secondary-Side Cause	First Check To Perform
Machine welds cold at the end of a long lead	Voltage drop from undersized cable or excessive total circuit length	Measure voltage at the work and electrode while welding. Compare against your OCV. Look for any drop nearing or over 4 volts.
Machine trips breakers	Primary supply issue or inrush, not secondary power	Inspect primary voltage, extension cords, and breaker size. Do not immediately resize secondary cables for this symptom.
Inconsistent resistance welds	Poor secondary connections, worn tips, or duty cycle limits	Inspect all secondary lugs, jumpers, and tips for looseness, pitting, or heat discoloration. Verify your weld schedule against the machine’s rated duty cycle.
Machine hums and heats up excessively	Cooling failure or exceeded duty cycle	Check fans, air vents, or water flow. Time your weld cycle against the duty cycle rating and allow for adequate cool down between long runs.

At the end of the day, start with the secondary path first. Check your connections for warmth, spread any coiled cable, verify your voltage drop under load, and confirm your duty cycle is not being exceeded. More often than not, tightening a lug, swapping to the correct cable size, or simply uncoiling your leads will bring that cold, stubborn puddle right back to life. These small checks are the difference between chasing a “bad machine” and fixing the circuit that feeds it.

Common Mistakes and Myths About Secondary Power

Most bad welder purchases and plenty of mysterious weld defects trace back to the same handful of misunderstandings. Secondary power is simple in principle, but the labels, ratings, and marketing numbers around it invite wrong conclusions. Here are the myths worth retiring.

Myth 1: Higher Secondary Voltage Means a More Powerful Welder

Secondary voltage helps start and sustain an arc, but it does not set weld heat by itself. Heat comes from current flowing through the resistance of the joint: I²R. A machine with a high open-circuit voltage and modest current can produce a beautiful arc and still lack penetration on thick plate.

Real capability comes from the transformer’s kVA rating, its duty cycle, and the current it can deliver at that duty. Chasing voltage can leave you with a machine that is harder to control at low amps and carries a higher shock risk. Match the process and thickness first, then look at voltage.

Myth 2: A 50 kVA Machine Is Always Equal to Another 50 kVA Machine

This is the most expensive myth in the resistance welding market. One transformer can be labeled 50 kVA at 50 percent duty cycle while another is 50 kVA at 20 percent. They are very different machines. The first can run longer or hotter before its insulation cooks.

In North America, welding transformers are generally rated at 50 percent duty cycle, while imported equipment may quote 20 percent or as low as 4 percent. A “0 percent duty cycle” rating would, in principle, mean the machine could never run at all, which shows how rating math can be stretched.

TWI explains that a transformer’s kVA rating defines the power it can draw over a long period without overheating at that duty cycle. Two identical-looking 50 kVA labels can therefore hide a two-and-a-half-fold difference in thermal capacity.

Myth 3: Secondary Power Is Harmless Because the Voltage Is Low

Low voltage reduces one risk; it does not remove the hazard. A secondary shock requires contact with both sides of the welding circuit, such as a bare spot on the electrode cable and the workpiece, while the output is energized. Sweat-soaked gloves, wet clothing, and kneeling on the work make that far more likely.

The result can be a fall, a violent reaction, or cardiac disruption. Lincoln Electric’s safety guidance treats secondary shock as a genuine hazard, not a nuisance. Inspect cable jackets and keep gloves dry.

Myth 4: The Work Lead Is a Ground Wire

The work lead completes the welding circuit. It is not a protective grounding conductor, and it must never be used as one. The machine frame needs its own ground connection. Confusing the two can energize the frame, create stray current paths through building steel, or leave a machine ungrounded while appearing properly connected.

OSHA 1910.254 covers grounding and secondary circuit requirements; treat it as the minimum, not a suggestion.

Myth 5: A Bigger Machine Fixes Long-Lead Problems

Long leads usually create voltage drop and resistive heating in the cable, not a shortage of transformer capacity. A larger machine pushes more current into the same undersized cable and often makes the heat problem worse. The cheaper fix is usually larger cable and clean connections. Buy the machine for the process and duty cycle, then size the secondary circuit to the distance.

Myth 6: Duty Cycle Only Matters for Production Shops

A small shop can exceed a 40 percent duty cycle on a single long repair or a run of tack welds faster than expected. When the thermal protection trips, the machine is not broken; it is enforcing its rating. Duty cycle is a purchase decision, not a production-only concern.

Myth 7: The Amps Display Tells You What Is Happening at the Arc

The display may show an internal value, a calculated estimate, or a reading taken somewhere other than the arc. Long leads, poor connections, and a hot cable can all reduce actual arc current while the display barely moves. Use a clamp meter or dedicated weld meter for diagnosis. On aluminum, where a cold arc quickly becomes lack of fusion, this matters even more; our aluminum MIG welding guide shows how arc behavior and setup interact.

Spec-Sheet Sanity Check

• Is the kVA or amp rating tied to a stated duty cycle?
• Is the duty cycle based on a standard 10-minute period?
• Does the rated output match your thickest common joint?
• Is the open-circuit voltage suitable for the process and environment?
• Are cable gauge and length included in the quoted package?
• Does the machine have proper grounding and OCV-reduction features?

If a seller cannot answer the first two questions clearly, treat the headline number as advertising rather than engineering data.

Practical Checklist, Buying Advice, and Final Thoughts

Secondary power stops being abstract the moment you specify, buy, or maintain a machine. The goal is simple: get enough current to the arc or the resistance weld, through a circuit that does not waste it as heat, without creating a shock hazard. These checklists turn the earlier theory into decisions you can defend.

Checklist: Specifying a New Machine

Start with the job, not the brochure. A machine is only correctly sized when its output, duty cycle, and installation match the work.

• Required weld current and process. Confirm the amps you need for the thickest joint and the process you actually run. A 200-amp MIG machine and a 200-amp stick machine are not interchangeable.
• Duty cycle at that current. Compare ratings at the same amperage, not at different points on the curve. A 60 percent machine at 200 amps may be less capable than a 40 percent machine at 250 amps.
• Secondary open-circuit voltage for the circuit impedance. Long leads, a large work loop, and aluminum cable all add impedance. If OCV is marginal, the arc will struggle at the far end of the lead.
• Primary supply capacity and voltage. Match the machine to the available service. OSHA 1910.254 requires supply conductors for an individual welding machine to have a current-carrying capacity not less than the machine’s rated primary current.
• Cable length and gauge. Use the manufacturer ampacity charts and the AWS F42 guide, Safety Guidelines for Proper Selection of Welding Cables. F42 covers minimum copper content, gauge sizing, electrical performance, and resistance criteria, and it is a better reference than a generic extension-cord table. Keep the total loop short and the copper generous.
• Cooling method. Air-cooled guns and torches have lower duty limits than water-cooled versions. For integrated welding-gun transformers, ISO 10656:2016 specifies a cooling water flow of 4 liters per minute for the transformer types it covers, so verify the manufacturer’s requirement rather than guessing.
• Safety features. Look for OCV-reducing controls where required, a grounding terminal on the frame, and connections that accept the correct work-lead lug. Add these items to your accessory order through a reputable supplier’s welding equipment guide so the machine arrives ready to run.

Checklist: Maintaining Secondary Power

Most weak-welder complaints start on the secondary side, and most of them are preventable with a short weekly routine.

• Inspect cable jackets and lugs for cuts, crushed spots, corrosion, and loose strands.
• Clean work clamps and remove spatter from contact faces; a dirty clamp is resistance you did not budget for.
• Spread coiled cable rather than leaving it in a tight reel, which concentrates heat and adds inductance on AC.
• Check cable and connection temperature after heavy use. Warm is normal; hot insulation is a warning.
• Verify cooling water flow on water-cooled equipment and confirm the duty cycle the shop actually runs.
• Measure voltage at the work clamp while welding, not just at the terminals. The old AWS 4-volt maximum drop recommendation, now carried into ANSI Z49.1, remains a useful field check.

Bigger Machine or Bigger Cable?

When a welder feels weak at the end of a long lead, the instinct is to buy more machine. Often the cheaper and more effective fix is larger cable. Voltage drop across undersized copper steals output before the current reaches the arc, and a bigger machine pushing through the same restrictive cable may only add heat. If the existing machine meets the duty cycle and current requirement, upgrade the cable first. If the machine is already running near its duty limit, no cable upgrade will create more thermal capacity.

Final Thoughts

Primary power is high voltage and low current; secondary power is low voltage and high current. The transformer or inverter exists to make that inversion safely and efficiently, and every cable, lug, clamp, and cooling passage after the output terminals is part of the welding circuit.

Back to the 300-amp machine on the 100-foot lead: before blaming the dial, check the work clamp, inspect the lugs, confirm the cable gauge, and measure voltage at the arc. In most cases, the machine is fine. The secondary circuit is asking for help.

Frequently Asked Questions

Is secondary power the same thing as secondary voltage?

No. Secondary voltage is one half of the picture, and secondary current is the other. Secondary power is the product of the two on the output side of the welder, which is why a machine can have a low secondary voltage and still deliver enormous power through very high current.

Why is secondary welding current so high when the voltage is so low?

Because a transformer trades voltage for current. Power in roughly equals power out, so when the secondary voltage drops from hundreds of volts to under a hundred, the current rises by nearly the same ratio. In resistance welding that ratio can push tens of thousands of amps at only a few volts.

What is a safe open circuit voltage on a welder?

Under US OSHA rules, manual AC arc welding machines are limited to 80 volts open circuit, while automatic AC and DC machines are limited to 100 volts. Higher values are only permitted with measures that prevent accidental contact, and OCV-reducing controls are recommended in wet or hot conditions.

Can secondary power give you a shock?

Yes, though it is generally less dangerous than primary voltage. A secondary shock happens when you touch both the electrode circuit and the work at the same time while the output is energized, especially with wet gloves or clothing. The risk is highest at open circuit voltage, before the arc starts.

What does a 50 percent duty cycle rating mean on a welder?

It means the machine can run at its rated output for 5 minutes out of a 10-minute period without exceeding its thermal limit. For resistance welding transformers, the rating is usually given at 50 percent duty cycle with a one-minute integration period, which is why the number is not directly comparable to a 20 percent rating.

How do I calculate secondary current from primary current?

Divide or multiply by the turns ratio. If the ratio is 50:1 and the primary current is 100 amps, the secondary current is about 5,000 amps. Going the other way, a 25,000 amp secondary current with a 72:1 ratio implies roughly 347 amps on the primary.

What size welding cable do I need for a long run?

Size for a voltage drop of no more than about 4 volts over the total circuit length, which includes both the electrode lead and the work lead. At 250 amps and 150 feet each way, that works out to 4/0 copper cable. Longer or hotter runs may need even larger conductors.

Is secondary power AC or DC?

It depends on the machine. Conventional transformer welders usually deliver AC on the secondary, while inverter welders rectify the output to DC. Resistance welding machines may use AC, DC, or medium-frequency DC depending on the control and transformer design.

What is the difference between secondary power and auxiliary power on an engine-driven welder?

Secondary power is the welding output that feeds the electrode and work circuit. Auxiliary power is the separate 120 or 240 volt generator supply used for tools and lights. They come from the same engine-driven unit but serve different circuits with different voltage and current characteristics.

Does secondary power affect weld quality?

Significantly. Too little secondary current, excessive voltage drop in the cables, a dirty work clamp, or a machine pushed past its duty cycle all show up as cold, inconsistent, or spattery welds. Checking the secondary circuit first explains many problems that look like operator error.