The one load you may size below its nameplate

When to use this: wiring a welder outlet in a shop or garage, or sizing a feeder for several machines.

Almost every load in the code is sized at 100% of its rating or more. A welder is the exception: NEC 630.11 lets you size the conductors below the nameplate current, because a machine at 30% duty cycle welds three minutes in ten and the conductors never reach the temperature a continuous draw would produce.

A 50 A welder at 30% duty cycle needs conductors rated 27.5 A. That is a saving of 22.5 A, and it is entirely legitimate.

Table 630.11(A) — the multiplier is a square root

Conductor heating goes as I²t, so the multiplier is √(duty cycle) rather than the duty cycle itself. Half the duty cycle is not half the heat, and it is not half the conductor.

The tabulated values are the square root rounded to two places. The 60% row is the one that does not round cleanly — √0.6 is 0.7746, which rounds to 0.77, and the code publishes 0.78. The table is the legal value, so that is what this calculator uses; the discrepancy is worth knowing about rather than assuming somebody mistyped it.

Because the table is a square root, unlisted duty cycles are not a problem: a machine rated 35% simply takes √0.35.

Input amps, not output amps

The most common error on this page's topic, and it is a factor of five. A welder nameplate carries two current ratings:

• Output — welding amps at the electrode. 180 A, 250 A. The number the machine is sold on.
• Input — primary amps drawn from the panel. Typically 25 to 50 A at 240 V. The number the circuit cares about.

Everything in Article 630 uses the input figure. Sizing a circuit from a 180 A output rating gives conductors three times larger than needed and a breaker that will never trip.

Inverter machines complicate the intuition further: an inverter rated 200 A output may draw 25 A input where an old transformer machine would draw 50. Read the plate.

The breaker is much larger than the conductors

630.12 permits overcurrent protection at up to 200% of the rated primary current — not of the derated conductor ampacity. For our 50 A welder that is 100 A, so a 100 A device ahead of conductors rated 27.5 A.

Like a motor circuit, this looks like a serious violation under 240.4 and is exactly what the code requires. The device has to let the machine strike an arc without opening; the conductors only have to survive the average. Our motor breaker calculator covers the same idea from the Article 430 side.

Several welders on one feeder

630.11(B) applies diversity more generously than almost anywhere else in the code, because welders in a shop genuinely do not all strike an arc at the same instant.

Everything from the fifth machine onward counts at 60%, so a large shop's feeder grows much more slowly than the machine count suggests.

Duty cycle falls as you turn the machine up

Manufacturers quote the best number, and duty cycle is stated at a specific output current. A machine rated 60% at 200 A may be 20% at its 250 A maximum — a multiplier of 0.78 against 0.45, which is most of a conductor size.

Size on the setting you actually weld at. If you routinely run the machine flat out, use the duty cycle at that output rather than the headline figure.

Other things about welder circuits

• The receptacle is usually the limit. A 50 A welder on a 50 A receptacle is fine; the 200% breaker allowance does not license a larger receptacle.
• Voltage drop matters more than usual. Arc characteristics change with supply voltage, so a long run to a shop can affect weld quality as well as compliance. Our voltage drop calculator covers the run.
• Resistance welders are different. 630 Part III applies its own rules, and the duty cycle approach here is for arc welders and plasma cutting equipment.
• Generators need the surge, not the average. If the machine runs off a generator, size on starting and striking behaviour rather than the derated figure — our generator sizing calculator handles that.

How this calculator is checked

Technical reviewer

Salamot Hok

Electrician · 10+ years of installation work in Bangladesh and the wider South Asian region

He reads the result the way an installer would: are the defaults values people actually meet, does the warning fire where you would stop and think, and is the answer something you could buy and fit? The code figures themselves come from the published standards cited below, not from him — that boundary is set out on his profile.

• The maths lives in a pure function with its own test suite, asserted against worked examples from published references and standards. A calculator does not ship until those tests pass.
• 2 sources cited by name and linked, so any figure on the page can be traced back to the document it came from.
• Last reviewed 17 August 2026. Review dates are advanced only when the page is actually re-read, never to look fresh.
• Unusable input returns no answer. Where the inputs do not describe a real design, the calculator says so and withholds the number rather than printing a plausible-looking wrong one.

The full process is written up in the methodology and editorial policy. Results are engineering guidance, not a code sign-off — see the disclaimer. If a result looks wrong, tell us; corrections are answered before anything else.