Size the breaker based on your welder’s input amperage, not its welding output. Read the nameplate on the machine, find the input current at 240V, and select the next standard breaker size at or above that number. For most home-shop welders, that means a 40 or 50-amp double-pole breaker paired with 8 AWG or 6 AWG copper wire.
How to Read Your Welder’s Nameplate
The nameplate is a metal plate or sticker on the back or bottom of your welder. It contains the electrical specifications you need for circuit sizing. Here’s what to look for:
Input voltage: 120V, 240V, or dual voltage (120/240V). This determines whether you need a single-pole or double-pole breaker.
Input current (amps): This is the current the welder draws from the wall. It’s always lower than the welding output amps because of the transformer or inverter step-down. A welder producing 200 amps at the torch might draw only 33 amps at the plug.
Duty cycle: Expressed as a percentage at a specific output. “30% at 200A” means the welder can run at 200 amps for 3 minutes out of every 10. At lower output settings, duty cycle increases.
KVA or kW rating: Some nameplates list power consumption in kilovolt-amps or kilowatts instead of amps. Divide kVA by 0.24 to get approximate input amps at 240V.
If the nameplate is missing or illegible, check the owner’s manual or the manufacturer’s website for the spec sheet.
Breaker Sizing Formula
The NEC (National Electrical Code) addresses welder circuits in Article 630. For a single welder, the calculation is straightforward:
Step 1: Find the nameplate input current at the rated output.
Step 2: Apply the NEC duty cycle multiplier from Table 630.11(A):
| Duty Cycle | NEC Multiplier | Example: 50A Nameplate |
|---|---|---|
| 100% | 1.00 | 50A effective |
| 90% | 0.95 | 47.5A effective |
| 80% | 0.89 | 44.5A effective |
| 70% | 0.84 | 42A effective |
| 60% | 0.78 | 39A effective |
| 50% | 0.71 | 35.5A effective |
| 40% | 0.63 | 31.5A effective |
| 30% | 0.55 | 27.5A effective |
| 20% | 0.45 | 22.5A effective |
Step 3: Select the next standard breaker size at or above the effective current. Standard double-pole breaker sizes are 15, 20, 25, 30, 35, 40, 45, 50, and 60 amps.
Worked Example
A Miller Millermatic 211 has a nameplate rating of 28 amps input at 240V with a 40% duty cycle at 150 amps output.
- Nameplate input: 28A
- Duty cycle multiplier (40%): 0.63
- Effective current: 28 x 0.63 = 17.6A
- Next standard breaker size: 20A
A 20-amp breaker with 12 AWG wire would technically satisfy NEC minimums for this machine. But most electricians will install a 30-amp breaker with 10 AWG wire for headroom. The welder can momentarily draw more than the effective current, and a breaker right at the edge will nuisance-trip.
Breaker and Wire Gauge Pairing
The breaker and wire must be matched. Never install a larger breaker without upsizing the wire.
| Breaker Size | Minimum Copper Wire (NM-B) | Minimum Copper Wire (THHN) | Typical Application |
|---|---|---|---|
| 20A | 12 AWG | 12 AWG | Small 120V welders |
| 30A | 10 AWG | 10 AWG | Small 240V welders, plasma cutters |
| 40A | 8 AWG | 8 AWG | Mid-range 240V welders |
| 50A | 6 AWG | 6 AWG | Full-size shop welders |
| 60A | 4 AWG | 4 AWG | Large industrial-class welders |
Wire gauge determines the safe current-carrying capacity (ampacity) of the conductor. A 10 AWG copper wire in NM-B cable is rated for 30 amps. Putting a 40-amp breaker on 10 AWG wire means the wire can overheat at full load before the breaker trips. That’s how electrical fires start.
Common Welders and Their Breaker Requirements
Here’s a quick reference for popular home and small-shop welders:
| Welder Model | Input Voltage | Input Amps | Recommended Breaker | Wire Gauge |
|---|---|---|---|---|
| Lincoln Power MIG 210 MP | 120/240V | 25A (240V) | 30A | 10 AWG |
| Miller Millermatic 211 | 120/240V | 28A (240V) | 30A | 10 AWG |
| Hobart Handler 210 MVP | 120/240V | 28A (240V) | 30A | 10 AWG |
| Lincoln Power MIG 256 | 240V | 42A | 50A | 6 AWG |
| Miller Multimatic 255 | 240V | 48A | 50A | 6 AWG |
| ESAB Rebel EMP 235ic | 120/240V | 31A (240V) | 40A | 8 AWG |
These are general recommendations based on published specs. Always verify with your specific machine’s nameplate data. Manufacturers occasionally revise electrical specifications between production runs.
120V Welders: Breaker Considerations
Small MIG and stick welders running on 120V plug into standard NEMA 5-15 or 5-20 outlets. Most 120V welders draw 15-20 amps, putting them right at the limit of a typical 15 or 20-amp household circuit.
The problem: household circuits are rarely dedicated. If your welder shares a circuit with lights, a refrigerator, or a dust collector, the combined load will trip the breaker. For any 120V welder, run it on a dedicated 20-amp circuit with no other loads. If your garage has standard 15-amp outlets on shared circuits, have an electrician add a dedicated 20-amp outlet.
Multiple Welders on One Panel
Running two or more welders from the same panel requires a demand factor calculation per NEC 630.11(B). The first welder is calculated at 100% of its effective current. The second welder is calculated at 70%. Additional welders are calculated at 60% each. This accounts for the fact that multiple welders rarely operate simultaneously at full output.
For a home shop with one welder and a plasma cutter, add both effective currents at 100% and 70% respectively to determine total panel load. Make sure your main breaker and service entrance can handle the combined demand of all circuits.
When to Upsize Your Breaker
Consider going one breaker size above the calculated minimum if:
- Your wire run exceeds 50 feet (voltage drop reduces available power)
- You plan to upgrade to a larger welder in the next few years
- The welder will run at high duty cycles for extended production work
- Ambient temperature in the panel location exceeds 86 degrees F (30 degrees C), which derates conductor ampacity
Upsizing the breaker only works if you also upsize the wire. A bigger breaker on the same wire is never acceptable.
Troubleshooting Breaker Issues
Breaker trips immediately when welding starts. The welder’s inrush current exceeds the breaker’s instantaneous trip rating. Some inverter welders have high inrush. A slow-blow or HACR-rated breaker may help, but consult your electrician first.
Breaker trips after 5-10 minutes of welding. Thermal overload. The sustained current is above the breaker’s continuous rating. You need a larger breaker and wire, or you need to reduce the welder’s output setting.
Breaker trips intermittently. Loose connections cause arcing inside the panel or at the outlet, generating heat that trips the thermal element. Shut off the breaker and have an electrician check all connections.
Breaker feels hot to the touch. Some warmth is normal under load. Hot to the point of discomfort is not. This indicates a loose connection, a faulty breaker, or an overloaded circuit. Kill the power and call an electrician.
Get the nameplate data, do the math, and have a qualified electrician install the correctly sized breaker and wire. This is not a place to guess or cut costs. The materials for a properly sized circuit cost a few dollars more than an undersized one. The consequences of getting it wrong cost orders of magnitude more.