What size breaker and wire for a welder?
Enter the numbers off the welder's nameplate and get the copper wire size, the breaker that carries full output, and the largest breaker NEC 2023 Article 630 allows, with the reason.
Updated November 2026
Quick answer
A 240 V welder with 30 A I1max at 30% goes on a 30 A breaker, which carries full output, with 10 AWG NM-B (Romex) or UF cable to cover the full breaker. NEC 630 sizes welder wire to I1eff, or to I1max times the Table 630.11(A) factor for the duty cycle: here 16.5 A. So on a circuit that feeds only the welder, 12 AWG is the code minimum, with up to 40 A on it. That is per the 2023 NEC; check your local code, and whether a home garage receptacle needs a GFCI breaker.
How NEC 630 sizes a welder circuit
When twice the number isn't a standard breaker size, 630.12 allows the next size up. The code sets the ceiling only. The calculator suggests the smallest standard breaker that carries I1max, so it doesn't trip at full output.
Worked example: the calculator's starting plate
- 240 V welder with 30 A I1max, rated 200 A at 30%, no I1eff: 30 A × 0.55 = 16.5 A.
- Smallest copper for 16.5 A in NM-B (Romex) or UF cable (60 °C column): 12 AWG (20 A).
- Breaker that carries 30 A: 30 A.
- Ceilings: 60 A for the welder, 40 A on 12 AWG. Most allowed: 40 A.
- Wire rated for the full breaker: 10 AWG. 12 AWG only works on a circuit that feeds the welder and nothing else.
Worked example: Lincoln AC/DC 225/125 (K1297)
- From Lincoln's spec sheet: 230 V, 50 A input at rated output, 20% duty cycle, no I1eff.
- Wire current: 50 A × 0.45 (20% or less row) = 22.5 A, so 10 AWG in NM-B (Romex) or UF cable (60 °C column).
- Breaker 50 A. Most allowed on 10 AWG: 60 A. Wire rated for the full breaker: 6 AWG.
NEC Table 630.11(A): duty cycle factors
For arc welders other than motor-generator sets. When the plate has no I1eff, multiply I1max by the factor for the rated duty cycle. A duty cycle between rows takes the next higher duty cycle row, which gives the bigger wire.
Copper wire ampacity (NEC Table 310.16)
Copper, no more than three current-carrying wires together, 30 °C air. NM-B and UF cable use the 60 °C column (334.80, 340.80). THHN in conduit uses 75 °C only when the breaker and receptacle are listed for it.
Standard breaker sizes (NEC Table 240.6(A))
The sizes 630.12 rounds up to when twice the number falls between them, in amps:
10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000
Other welder power questions
Questions welders keep asking about breakers and wire
What size breaker for a 240V welder?
It depends on the plate, not the voltage. A 240 V welder with 30 A I1max at 30% goes on a 30 A breaker, the smallest standard size that carries full output, with 10 AWG NM-B (Romex) or UF cable to cover the full breaker. On a welder-only circuit Article 630 allows down to 12 AWG, with up to 40 A on it. That is per the 2023 NEC; check your local code.
Can I run a welder on a 30 amp breaker?
Yes, if its I1max is 30 A or less and the wire is sized for it. A 240 V welder with 30 A I1max fits a 30 A breaker exactly: 10 AWG copper covers the full 30 A, and on a welder-only circuit the code allows down to 12 AWG. A breaker under I1max isn't against the code, which only sets a ceiling, but it can trip when you run the machine wide open.
What size wire for a 50 amp welder breaker?
Copper rated for the full 50 A is 6 AWG in NM-B or UF cable (60 °C column), or 8 AWG THHN in conduit when the breaker and receptacle are listed for 75 °C. Article 630 lets a welder circuit run smaller: the wire only has to carry I1eff (or I1max times the duty-cycle factor), and the breaker can be up to twice its ampacity, so 10 AWG NM-B (or 12 AWG THHN at 75 °C) is the smallest that can sit on 50 A. Lincoln's AC/DC 225/125 (K1297) works out to 10 AWG NM-B minimum on a 50 A breaker.
Why can the breaker be bigger than the wire?
Because the welder doesn't pull full current all the time, and the wire only heats up on the average. NEC 240.4(G) sends welder circuits to Article 630, which sizes the wire to that average (I1eff) and allows a breaker up to twice the wire's ampacity. 12 AWG carries 20 A, so it can take up to a 40 A breaker on a welder circuit. On anything else it couldn't. If the circuit might feed something else later, run wire rated for the breaker.
Do I use the 60 °C or the 75 °C column?
It depends on the wire. NM-B (Romex) and UF cable always size from the 60 °C column (NEC 334.80 and 340.80), whatever the breaker is rated. Single THHN/THWN wires in conduit can use the 75 °C column, but only when every termination (breaker, receptacle, lugs) is listed for 75 °C (110.14(C)); otherwise it's 60 °C. For a 240 V welder with 30 A I1max at 30%, it changes the minimum wire: 12 AWG in NM-B, 14 AWG THHN at 75 °C.
Does a welder outlet need a GFCI?
It can. The 2020 NEC widened the home GFCI rule (210.8(A)) to 125 through 250 V receptacles, and the 2023 NEC kept it, so a 240 V welder receptacle in a home garage can need a GFCI breaker. Garages that aren't part of a home fall under 210.8(B). States adopt and amend the code on their own schedule, so check what yours requires, and ask the welder's maker whether it runs on a GFCI breaker.
What size breaker for a 120V welder?
Same rules. Say the plate reads 20 A I1max and 12 A I1eff at 120 V. The smallest standard breaker that carries 20 A is 20 A. The wire has to carry 12 A, so 14 AWG copper is the minimum, and the most breaker allowed on it is 30 A. Plug-in 120 V machines also have to match the receptacle they plug into.
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