Welder Circuit Calculator

The 630.11(A) duty-cycle method

An arc welder is an intermittent load. It draws its rated primary current only while the arc is struck, and the nameplate duty cycle is the fraction of a ten-minute period the machine can run at that current without overheating. Because the conductor heats on the same intermittent schedule, NEC 630.11(A) lets it be sized to a reduced current rather than the full rated primary current: multiply the rated primary current by the Table 630.11(A) factor for the welder type and duty cycle. A welder run at a lower duty cycle draws less average heating current, so the code allows a smaller conductor.

The factors run from 1.00 at 100 percent duty down to 0.45 for a nonmotor-generator welder at 20 percent or less. Where a nameplate lists a duty cycle between the table rows, this tool rounds up to the next tabulated value, taking the larger multiplier so the conductor is never undersized.

Worked example: 50 A welder at 60 percent duty

1. The nonmotor-generator welder's rated primary current is 50 A at a 60 percent duty cycle.
2. The Table 630.11(A) factor for a nonmotor welder at 60 percent is 0.78.
3. Required conductor ampacity is 50 x 0.78 = 39 A.
4. In the 75 degrees C column of Table 310.16, 10 AWG copper is 35 A (short), and 8 AWG copper is 50 A, so the minimum conductor is 8 AWG copper.
5. The overcurrent device may not exceed 200 percent of the 50 A rated primary current (100 A) or 200 percent of the 50 A conductor ampacity (100 A), so the maximum standard device is 100 A.

The two 200 percent overcurrent ceilings

NEC 630.12 caps welder overcurrent protection with two rules, both at 200 percent. Under 630.12(A) the device may not exceed 200 percent of the rated primary current; under 630.12(B) it may not exceed 200 percent of the conductor ampacity. On a single-welder branch circuit one device serves both roles, so it must satisfy both, and the smaller of the two ceilings governs. When the duty-cycle reduction lands the conductor a size or two below the welder's full rating, the conductor ceiling (630.12(B)) is usually the one that controls. Where the governing ceiling is not a standard 240.6(A) rating, 630.12 permits the next higher standard fuse or breaker.

A group of welders is sized differently

This calculator sizes the branch circuit to one welder. A feeder that supplies several welders is not the sum of their individual conductors: NEC 630.11(B) applies a diversity, taking 100 percent of the two largest welders' 630.11(A) currents, 85 percent of the third, 70 percent of the fourth, and 60 percent of the rest, on the assumption that not every machine strikes an arc at the same instant. That group feeder calculation is outside this tool's v1 scope. Resistance welders (630.31) use a separate method again, keyed on the actual and rated primary currents.

Frequently asked questions

What size wire does an arc welder need?

Multiply the rated primary current by the Table 630.11(A) duty-cycle factor, then pick the smallest Table 310.16 conductor that meets it. A 50 A nonmotor welder at 60 percent duty needs 39 A, which is 8 AWG copper at 75 degrees C.

Why can a welder conductor be smaller than the welder amps?

Because a welder is an intermittent load. It draws its rated primary current only while welding, so 630.11(A) sizes the conductor to a duty-cycle-adjusted current from Table 630.11(A) rather than the full rated current.

How is a welder overcurrent device sized?

NEC 630.12 caps it at 200 percent of the rated primary current and at 200 percent of the conductor ampacity; the smaller ceiling governs, and the next higher standard rating is permitted where that value is not standard.

What is the duty cycle multiplier for a welder?

For nonmotor-generator arc welders: 1.00 at 100 percent, 0.78 at 60 percent, 0.71 at 50 percent, 0.63 at 40 percent, 0.55 at 30 percent, and 0.45 at 20 percent or less. Motor-generator welders are slightly higher at each step.

Does the 240.4(D) small-conductor rule apply?

No. 630.12 permits welder overcurrent protection up to 200 percent of the conductor ampacity, above the 240.4(D) caps, so this tool sizes the conductor on the 630.11(A) ampacity requirement alone.

Related calculators

• Wire Ampacity Calculator: the Table 310.16 ampacity of a chosen conductor, with ambient and fill derating.
• Breaker Size Calculator: the standard 240.6(A) breaker for a general-use load with the 125 percent continuous rule.
• Motor Breaker Sizing Calculator: branch-circuit protection for a motor per NEC Article 430 and Table 430.52.
• Generator Conductor Size Calculator: generator output conductors under the NEC 445.13(A) 115 percent rule.