Welder power calculator
Type in the numbers off your welder's nameplate and get three answers: the breaker and wire it goes on, the generator it takes to run it, and how long it can weld at the amps you run. Circuit sizing follows NEC 2023 Article 630.
Updated November 2026
Quick answer
A 240 V welder with 30 A I1max and no I1eff on the plate (200 A at 30%) goes on a 30 A breaker with 10 AWG NM-B (Romex) or UF cable, which covers the full breaker. On a welder-only circuit Article 630 allows down to 12 AWG (per the 2023 NEC; check your local code, and whether a home garage needs a GFCI breaker). It pulls 7,200 W (7.2 kVA) at full output, the least running watts an inverter's generator needs for full output. At 150 A it can weld about 5.3 minutes in every 10.
How the welder power math works
Breaker and wire (NEC 630)
When twice the number doesn't land on a standard breaker size, the code allows the next size up. Worked through for the calculator's starting plate, a 240 V welder with 30 A I1max rated 200 A at 30%:
- No I1eff, so the wire has to carry 30 A × 0.55 = 16.5 A (30% row of Table 630.11(A)).
- In NM-B (Romex) or UF cable, which sizes from the 60 °C column, the smallest copper that carries it is 12 AWG, good for 20 A.
- The smallest standard breaker that carries the full 30 A: 30 A.
- Ceilings: 60 A for the welder and 40 A on 12 AWG. The lower one, 40 A, is the most allowed.
- Wire rated for the full 30 A: 10 AWG. 12 AWG is only for a circuit that feeds the welder and nothing else.
Generator size
240 V × 30 A = 7,200 W, or 7.2 kVA. For a 200 A inverter, Weldclass's published rule of thumb (180-200A row) is 8 kVA minimum and 10 kVA ideal. Transformer machines can need well over the plate number.
Duty cycle at other amps
30 × (200 ÷ 150)² = 53.3% at 150 A, about 5.3 minutes welding and 4.7 cooling. At 100% it holds about 110 A. This square law is an estimate. The chart in the manual wins.
Need a starting point for the amps? The welding settings chart has amps by process and metal thickness.
One question per page
Generator size for a welder
Running watts and kVA from the plate, plus the inverter rule of thumb.
Breaker and wire for a welder
NEC 630 wire size, the breaker that carries full output, and the max allowed.
Duty cycle at your amps
Minutes welding and cooling in every 10, and the most amps for a duty cycle.
Questions welders keep asking about welder power
What size breaker do I need for my welder?
Go by the nameplate. NEC 630 sizes the wire to I1eff, or to I1max times the Table 630.11(A) factor for the rated duty cycle when the plate has no I1eff, and lets the breaker go as high as twice I1max. A 240 V welder with 30 A I1max at 30% goes on a 30 A breaker, the smallest standard size that carries full output. Run 10 AWG copper and the wire covers the whole breaker. On a welder-only circuit the code allows down to 12 AWG. That is per the 2023 NEC; check your local code.
What size generator do I need to run a welder?
Multiply the input volts by I1max. A 240 V welder with 30 A I1max pulls 7,200 W (7.2 kVA) at full output, so a generator with fewer running watts than that can't give you full output. For a 200 A inverter, Weldclass's rule of thumb is 8 kVA minimum and 10 kVA ideal. Transformer machines need more: Lincoln says its AC/DC 225/125 (K1297) needs at least a 15,000 W generator, against 11.5 kVA on its plate.
What does duty cycle mean on a welder?
It's how many minutes out of 10 the welder can run at a given output before it has to cool. 200 A at 30% means 3 minutes welding and 7 minutes cooling at 200 A. Turn it down and the duty cycle goes up: about 53.3% at 150 A.
What are I1max and I1eff on a welder nameplate?
I1max is the most current the welder pulls from the supply. I1eff is the effective current, the steady current that would heat the wire the same as the welder does over its duty cycle, so it is always lower. NEC 630.11(A) sizes the wire to I1eff. Older plates list rated primary current instead (input amps at rated output); use it as I1max. With no I1eff, the code multiplies I1max by a factor for the duty cycle: 0.55 at 30%, so 30 A becomes 16.5 A.
Why can the breaker be bigger than the wire on a welder?
A welder doesn't pull full current all the time. NEC 240.4(G) sends welder circuits to Article 630, which sizes the wire to the duty-cycle current and lets the breaker go up to twice the wire's ampacity. That is how 12 AWG can sit on a 30 A breaker for a 240 V welder with 30 A I1max: the most allowed on that wire is 40 A. If the circuit might ever feed something else, run 10 AWG, which is rated for the full breaker.
Will a transformer welder run on a generator?
Yes, but it takes a big one. Lincoln's AC/DC 225/125 (K1297) pulls 50 A at 230 V at rated output, which is 11.5 kVA by the plate, and Lincoln says it needs at least a 15,000 W generator. Where the maker publishes a generator size, go by it.
Other tools in the Tack Box
Welding Shop Rate Calculator
What should you charge per hour? Break-even and recommended rate.
Welding Cost Calculator
Cost per inch of weld: labor, filler, gas, and markup by process.
Metal Weight Calculator
Plate, pipe, bar, tube. Steel, aluminum, stainless, brass.
Gas Usage Calculator
Cylinder count and cost for a project. MIG, TIG, flux-core.
Filler Metal Calculator
Pounds of wire or rod for a joint. Spool count and cost.
Job Pricing Calculator
Build a bid from scratch: tasks, materials, overhead, margin.
Welding Quote Calculator
Build a customer-ready quote: work lines, materials with markup, PDF.
Welding Settings Chart
Amps, voltage, wire speed by process, metal, and thickness.
Wire & Gas Selection Guide
Which wire and shielding gas for your process and base metal.
Welding Symbol Chart
Every AWS A2.4 weld symbol. Searchable with visual diagrams.
Fillet Weld Strength Calculator
What a fillet holds, or the size and length for a load. AISC 360.
Stair Stringer Calculator
Risers, treads, stringer length and angle, checked against IRC or IBC.
Railing Picket Spacing Calculator
Pickets per bay, the real gap, and the steel to order by weight.
Pipe Coping Template
Full-size printable fish-mouth template for tube onto tube, 15 to 90°.
Why shops choose Bead Board
How to Quote Welding Jobs
Turn these numbers into a price: rate, markup, margin, and a worked example.
Bead Board vs. Spreadsheets
Why shops outgrow Excel for job tracking.
Bead Board vs. Paper & Notebooks
Paper loses jobs. Software does not.
Mobile Welder App
Native on iPhone and Android. Works offline in the field.
Circuit rules from NFPA 70 (NEC) 2023 Article 630. Generator and duty cycle figures from the makers listed above. Other Tack Box tools.