Welder Duty Cycle Calculator
Work out a welding machine’s duty cycle at any current, how many minutes of arc that allows in ten, and the current it will run at continuously.
How to use this calculator
- 1Take both halves of the rating off the plate. "60%" on its own is not a specification.
- 2Enter the current you actually intend to weld at, not the machine’s maximum.
- 3Compare machines on their continuous current rather than on the duty cycle percentage - the percentage can be quoted at any flattering current.
- 4Tick the hot-environment box if the machine will be in the sun, in a corner, or anywhere it cannot breathe.
How the calculation works
DC(actual) = DC(rated) x (rated amps / actual amps)^2
Arc minutes = DC / 100 x 10
Continuous current = rated amps x sqrt(DC rated / 100)- DC(rated)
- The duty cycle on the plate, which always comes paired with a current
- Rated amps
- The current that duty cycle applies at. Either half of the pair alone means nothing
- The square
- Resistive heating is I squared R, so the heat rises with the square of current and the sustainable time falls with it
The rating period is ten minutes in both the NEMA and IEC conventions, so a 60% duty cycle is six minutes of arc and four of cooling - not six hours in ten.
IEC 60974-1 ratings are taken at 40 C ambient, which is a hot shop. A machine rated to it will usually beat its plate figure in a comfortable one. NEMA ratings are generally taken cooler and are less forgiving of heat.
The continuous current - where duty cycle reaches 100% - is the single most useful number for comparing machines, and the one least often advertised.
Worked example
A 60% at 200 A machine pushed to 250
- 1.Heating goes as the square of current, so at 250 A the machine makes (250/200)^2 = 1.56 times the heat.
- 2.Duty cycle scales inversely: 60 x (200/250)^2 = 60 x 0.64 = 38.4%.
- 3.That is 3.8 minutes of arc in every ten, then 6.2 minutes with the fan running.
- 4.The same machine runs continuously at 200 x sqrt(0.60) = 155 A.
Result: 38% - under four minutes of arc in ten
The same machine turned down
- 1.At 150 A the raw calculation gives 60 x (200/150)^2 = 107%.
- 2.Anything over 100% just means continuous - the machine is making less heat than it can shed.
- 3.Its true continuous rating is 155 A, so 150 is comfortably inside it.
- 4.This is why the continuous current is the number worth comparing: it is the one point on the curve that is not a trade-off.
Result: 100% - it will run all day at 150 A
A long pass that does not fit
- 1.A 30% at 175 A hobby machine run at 200 A: 30 x (175/200)^2 = 23%, so 2.3 minutes of arc in ten.
- 2.In a hot corner, knock that back again - call it 18%, or 1.8 minutes.
- 3.The weld is 120 in at 10 in/min, which is 12 minutes of arc.
- 4.So around seven stints with cooling between them, and well over an hour elapsed. Stopping and restarting mid-pass seven times is also seven restart defects waiting to happen.
Result: Seven stints and over an hour - the machine is too small for this weld
What duty cycle is measuring
A welding machine turns mains power into a low-voltage, high-current arc, and every component in that path has resistance. Resistance times current squared is heat, and that heat has to leave the machine through its fins and its fan faster than it arrives, or the temperature climbs until something is at risk.
Duty cycle is the manufacturer's statement of how long it can arrive faster than it leaves. It is expressed over a ten minute period: 60% duty cycle means six minutes of welding and four minutes of rest, not six hours out of ten. Both the American NEMA convention and the international IEC 60974-1 use the same ten minute window.
Because the heat goes as the square of current, the relationship is steep. Twenty-five percent more current is fifty-six percent more heat, and the sustainable time falls to about two thirds. It is why the difference between a machine at its rated current and the same machine turned down a little is so much larger than it looks.
Why the percentage alone is meaningless
A duty cycle figure is always half of a pair. "Sixty percent" describes nothing until it is attached to a current, because the same machine is at 100% somewhere lower and at 20% somewhere higher. Advertising exploits this routinely, quoting a high duty cycle at a current nobody would actually use for the work the machine is sold for.
The number that cannot be gamed is the continuous current - the point where duty cycle reaches 100%. It falls out of the rated pair as the rated current times the square root of the rated duty cycle, and it is the honest measure of what a machine will do without stopping.
For a machine rated 60% at 200 A that comes out at 155 A. For one rated 20% at 200 A it is only 89 A. Those two machines have the same headline current and are not remotely the same machine, and the duty cycle percentage is the only thing on the plate that reveals it.
- 100% at rated current — Industrial. Built for mechanised work and long passes.
- 60% — A serious shop machine. Comfortable for almost all manual work.
- 30% to 40% — Light industrial and good hobby machines. Fine for manual welding, limiting for long passes.
- 20% and below — Entry level. Adequate for short welds and repairs, frustrating for anything sustained.
Why it rarely matters, and when it suddenly does
Arc-on time in a real fabrication shop is far lower than people assume. Studies of manual welding operations put it between 20% and 30% of the working day once tacking, positioning, chipping slag, grinding, changing electrodes, inspecting and reading the drawing are all counted. Even a hard-working welder on a good day spends most of it not welding.
That means a 30% machine is rarely the constraint on a manual job, and a 60% one essentially never is. The welder is the bottleneck long before the machine is.
The picture inverts completely for anything mechanised. A carriage running a fillet the length of a beam does not stop to reposition, and a robot does not stop at all. There the duty cycle is the whole specification, which is why production machines are rated 100% at currents that look absurdly conservative next to their maximum. The other case is long uninterrupted passes on heavy plate, where stopping mid-run is itself a defect risk - and there the answer is usually a bigger machine rather than a planned pause.
What this assumes, and where it stops
Assumptions
- The ten minute rating period used by both NEMA and IEC 60974-1.
- Duty cycle scales with the inverse square of current, which is the standard relationship and holds well across the usable range.
- The plate rating is accurate and the machine is in good order with a clear air path.
- The hot-environment adjustment is a broad allowance, not a tested derating.
Limitations
- It cannot predict the actual trip point. Real thermal behaviour depends on ambient temperature, airflow, dust, altitude and the machine’s own thermal mass.
- Some manufacturers rate optimistically, and cheap machines are the usual offenders. A plate figure is a claim, not a measurement.
- Inverter and transformer machines heat and cool at different rates, so two machines with the same rating can behave differently on stop-start work.
- It says nothing about the input side - a machine at high current may exceed what the supply circuit or the generator feeding it can deliver.
- Wire feeders, torches and cables have their own duty cycle ratings, and a gun rated below the machine is the limit in practice.
Common questions
What does 60% duty cycle mean?
Six minutes of welding in every ten, at the current that rating was measured at, with four minutes of rest for the machine to cool. It is not six hours in ten - the period is always ten minutes. And the current is essential: the same machine is at 100% somewhere lower and 20% somewhere higher.
How do I work out duty cycle at a different amperage?
Multiply the rated duty cycle by the square of the ratio of rated current to actual current. A machine rated 60% at 200 A, run at 250, gives 60 x (200/250)² = 38%. Run at 150 it gives 107%, which just means continuous.
What happens if I exceed the duty cycle?
The thermal overload trips, the output stops and the fan keeps running until the machine has cooled, then it restores itself. That is protection working, not damage. What does harm a machine is preventing it - blocking the airflow, running in a hot corner, or sitting at the limit for hours so the windings never get a chance to cool.
Is a 20% duty cycle machine any good?
For manual welding, often yes. Real arc-on time in a shop is 20% to 30% of the day, so a 20% machine keeps up with a person doing ordinary fabrication with tacking, positioning and grinding between welds. It becomes limiting on long continuous passes and it is useless for anything mechanised.
Which number should I compare between machines?
The current at which duty cycle reaches 100% - rated current times the square root of the rated duty cycle. It is the one figure that cannot be quoted flatteringly, and it tells you what the machine will genuinely do without stopping. A machine at 60% and 200 A runs continuously at 155; one at 20% and 200 A runs continuously at 89.
Sources
- IEC 60974-1 - arc welding equipment, power sources — International Electrotechnical Commission
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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