A welding shop electrical panel needs enough amperage to handle all connected equipment simultaneously, with breaker slots for every circuit and room for future expansion. For a single-welder home shop, a 100-amp sub-panel with 20-24 breaker spaces covers current needs and growth. For a multi-welder shop with a plasma cutter and compressor, step up to 200 amps.
Understanding Electrical Load Calculation
An electrical load calculation determines how much power your shop needs. It accounts for every piece of equipment, applies demand factors (because not everything runs simultaneously at full power), and produces a total demand in amps. This number sizes your panel, feeder cable, and possibly your utility service.
The NEC (National Electrical Code) provides the calculation method. For welding shops, the key articles are:
- NEC Article 220 - Branch circuit and feeder calculations
- NEC Article 630 - Electric welders (demand factors)
- NEC Article 430 - Motors (for compressors, ventilation fans)
Step 1: List All Equipment
Document every piece of electrical equipment in the shop, including its voltage, amperage, and whether it runs continuously or intermittently.
| Equipment | Voltage | Input Amps | Watts | Circuit Size |
|---|---|---|---|---|
| MIG welder (primary) | 240V | 40A | 9,600W | 50A / 6 AWG |
| TIG welder / second welder | 240V | 30A | 7,200W | 40A / 8 AWG |
| Plasma cutter | 240V | 25A | 6,000W | 30A / 10 AWG |
| Air compressor (5 HP) | 240V | 24A | 5,760W | 30A / 10 AWG |
| Lighting (LED) | 120V | 3A | 360W | 15A / 14 AWG |
| General outlets (4 circuits) | 120V | 15A each | 1,800W each | 20A / 12 AWG each |
| Exhaust fan | 120V | 5A | 600W | 15A / 14 AWG |
| Bench grinder | 120V | 8A | 960W | 20A / 12 AWG |
Step 2: Apply NEC Demand Factors for Welders
NEC Article 630.11 provides demand factors for groups of welders. These factors reduce the calculated load because multiple welders rarely operate at maximum output simultaneously.
For a single welder, calculate the effective current using the duty cycle multiplier (per NEC Table 630.11(A)):
Effective current = Nameplate amps x Duty cycle multiplier
For multiple welders per NEC 630.11(B):
- Largest welder: 100% of effective current
- Second largest: 85% of effective current
- Third largest: 70% of effective current
- All remaining: 60% of effective current
Worked Example: Full Shop Load Calculation
Using the equipment list above:
MIG welder: 40A nameplate, 60% duty cycle. Effective = 40 x 0.78 = 31.2A. At 100% (largest welder) = 31.2A
TIG welder: 30A nameplate, 60% duty cycle. Effective = 30 x 0.78 = 23.4A. At 85% (second welder) = 19.9A
Plasma cutter: Treated as a welder per NEC 630. 25A nameplate, 50% duty cycle. Effective = 25 x 0.71 = 17.75A. At 70% (third) = 12.4A
Total welding load at 240V: 31.2 + 19.9 + 12.4 = 63.5A
Compressor motor: 24A at 240V. NEC 430 requires 125% of the largest motor for the feeder: 24 x 1.25 = 30A
120V circuits converted to 240V equivalent:
- Lighting: 360W / 240V = 1.5A
- General outlets (4 x 1,800W, at 40% demand per NEC 220): 4 x 720W = 2,880W / 240V = 12A
- Exhaust fan: 600W / 240V = 2.5A
- Bench grinder: 960W / 240V = 4A
Total calculated demand at 240V: 63.5 + 30 + 1.5 + 12 + 2.5 + 4 = 113.5A
This shop needs a 125-amp or 150-amp sub-panel (125-amp is common). With room for growth, a 200-amp panel makes more sense.
Panel Size Options
| Panel Size | Typical Breaker Spaces | Suitable For |
|---|---|---|
| 60A sub-panel | 8-12 | Single small welder, lighting, few outlets |
| 100A sub-panel | 16-20 | One full-size welder, compressor, lighting, outlets |
| 125A sub-panel | 20-24 | Two welders, compressor, full shop circuits |
| 200A panel (main or sub) | 30-42 | Multi-welder production shop, full equipment complement |
Always buy a panel with more breaker spaces than you currently need. Adding circuits later is cheap if you have empty slots. It’s expensive if the panel is full and you need to swap it out.
A general rule: buy 30-40% more breaker spaces than your initial plan calls for. If you’ve planned 12 circuits, get a 20-space panel. If you’ve planned 18 circuits, get a 30-space panel.
Feeder Cable Sizing
The feeder cable connects the main house panel (or meter) to the shop sub-panel. It must carry the full calculated demand plus be sized for voltage drop over the run distance.
| Sub-Panel Size | Feeder Wire (Copper) | Feeder Wire (Aluminum) | Conduit Size (EMT) |
|---|---|---|---|
| 60A | 6 AWG | 4 AWG | 1 inch |
| 100A | 3 AWG | 1 AWG | 1.25 inch |
| 125A | 1 AWG | 2/0 AWG | 1.5 inch |
| 200A | 2/0 AWG | 4/0 AWG | 2 inch |
For runs over 100 feet (common for detached shops), voltage drop becomes significant. Upsize the feeder one gauge to compensate. A 100-amp sub-panel fed by a 150-foot run needs 1 AWG copper instead of 3 AWG to keep voltage drop under 3%.
Aluminum feeder cable costs substantially less than copper for larger sizes and is commonly used for feeder runs. The trade-off is larger wire gauge (aluminum requires one to two sizes larger than copper for the same ampacity) and the need for anti-oxidant compound on all connections.
Sub-Panel Requirements for Detached Buildings
Running power to a detached garage or shop building requires a sub-panel at the building with:
Main breaker or disconnect. NEC 225.31 requires a means of disconnect at or near the building entrance. A main breaker panel satisfies this. A separate disconnect switch ahead of a main lug only (MLO) panel also works.
Separate grounding electrode. NEC 250.32 requires a grounding electrode (ground rod) at each building served by a feeder. The ground rod connects to the sub-panel’s grounding bus. In the sub-panel, the neutral bus and ground bus must be separate (not bonded together, as they are in the main panel).
Four-wire feeder. The feeder from the house to the shop must include two hots, a neutral, and a ground. Even if you don’t plan any 120V circuits, the neutral is required by code for the four-wire feeder to a separate building. Requirements vary by jurisdiction, so verify with your electrician.
Underground vs. Overhead Feeder
| Method | Advantages | Disadvantages |
|---|---|---|
| Underground (UF-B cable or conduit) | No visual clutter, protected from weather, permanent | Trenching required (18-24 inch depth), more labor |
| Overhead (triplex or conduit) | Less labor, easier to install, no trenching | Minimum height clearances (10-18 ft depending on use), visible, vulnerable to storm damage |
Underground is the standard for permanent installations. Direct-burial UF-B cable requires 24-inch depth. Schedule 80 PVC conduit with THWN conductors can be as shallow as 18 inches. Metal conduit (rigid or IMC) can go as shallow as 6 inches. Requirements vary by jurisdiction.
Circuit Planning
Plan every circuit before the electrician starts. Changing your mind after the wiring is in the wall costs time and money. Here’s a template for a well-equipped home welding shop:
| Circuit # | Description | Voltage | Breaker | Wire |
|---|---|---|---|---|
| 1-2 | MIG welder (NEMA 6-50) | 240V | 50A 2-pole | 6 AWG |
| 3-4 | TIG welder / spare 240V (NEMA 6-50) | 240V | 50A 2-pole | 6 AWG |
| 5-6 | Air compressor | 240V | 30A 2-pole | 10 AWG |
| 7-8 | Plasma cutter (NEMA 6-30) | 240V | 30A 2-pole | 10 AWG |
| 9 | Lighting | 120V | 15A | 14 AWG |
| 10 | Exhaust fan | 120V | 15A | 14 AWG |
| 11 | General outlets - welding area | 120V | 20A | 12 AWG |
| 12 | General outlets - bench area | 120V | 20A | 12 AWG |
| 13 | General outlets - layout area | 120V | 20A | 12 AWG |
| 14 | Bench grinder / dedicated tool outlet | 120V | 20A | 12 AWG |
That’s 14 circuits using 18 breaker spaces (each 240V circuit takes two spaces). A 20-space panel handles this with two spaces to spare. A 24-space panel gives you room for future circuits like a heater, dust collector, or additional 240V outlet.
Service Upgrade Considerations
If your home’s main electrical service is 100 amps (common in older homes), adding a shop may push you past the service capacity. A 100-amp service can barely support a modern home’s HVAC, water heater, dryer, and range, with little room for a 50-amp welder circuit.
Signs you need a service upgrade:
- Main breaker trips when the welder runs and the dryer or AC kicks on
- The load calculation shows demand exceeding 80% of service capacity
- Your electrician recommends it after performing a formal load calculation
A service upgrade from 100A to 200A involves the utility company, a new meter base, new service entrance cable, and a new main panel. Cost ranges from $2,000 to $5,000 depending on the scope. It’s a significant investment, but it’s often necessary for a proper shop installation and it increases your home’s value.
Common Mistakes
Undersizing the feeder for voltage drop. A 100-amp sub-panel fed by 100 feet of 3 AWG copper loses about 4% voltage at full load. Welders on the sub-panel see 230V instead of 240V. Upsize the feeder or accept reduced welder performance.
Bonding neutral and ground in the sub-panel. In a sub-panel, the neutral bar and ground bar must be separate. Only the main panel bonds neutral to ground. Bonding them in the sub-panel creates parallel paths for neutral return current, which can cause stray voltage on metal surfaces and is a code violation.
Insufficient breaker spaces. Buying a 12-space panel for 10 circuits leaves you stuck when you add one more tool. Buy 30-40% more spaces than you think you need.
Running all circuits from the house panel. If you have more than two or three shop circuits, a dedicated sub-panel is cleaner, safer, and often required by code. Individual home runs for each shop circuit crowd the house panel and create excessively long wire runs.
Forgetting about future loads. If you might add a second welder, a heater, a dust collector, or an EV charger in the next 5 years, size the panel and feeder for those loads now. Upgrading the feeder later costs far more than doing it right the first time.
Panel sizing is the foundation of your shop’s electrical system. Get it right and every future circuit, tool, and upgrade plugs in without drama. Get it wrong and you’re back in the panel with the electrician every time you add a piece of equipment. Spend the time on a thorough load calculation, add margin for growth, and install once.