Running power to a detached shop or garage is more involved than wiring a bedroom circuit. The building is far from the main panel, voltage drop becomes a real constraint, and the National Electrical Code has specific requirements for separate structures. Most importantly, you need to size the feeder and subpanel based on realistic load projections, not just what you are plugging in today.
The mistake most people make is underestimating future demand. You wire for a few lights and a couple of outlets, then six months later you want to add a welder, an air compressor, a dust collector, and a space heater. At that point your 60-amp subpanel is maxed out and your voltage drop is unacceptable.
This guide walks through the process: listing loads, applying NEC demand factors, sizing the feeder for both ampacity and voltage drop, choosing conduit or direct burial cable, and meeting grounding requirements for a separate building.
Start With the Load List
Before you size a panel or buy wire, make a complete list of every load you plan to run. This includes lighting (typically 10 to 15 amps total for LED fixtures), receptacle circuits (15 or 20 amps each), motor loads (air compressors, dust collectors, table saws), welders (often 40 to 60 amps at 240V), and heating or cooling equipment. Do not forget the loads you will add in the next two to three years.
Be honest about simultaneous use. You will not run every load at the same time, and the NEC allows you to apply demand factors that reflect this reality. But the starting point is the gross connected load. Add up the nameplate ratings for all equipment and circuits.
Common shop loads: MIG welder (30-50A at 240V), air compressor 3-5 HP (20-30A at 240V), dust collector (15-20A), space heater (15-40A), general receptacles (15-20A at 120V), and LED lighting (1-2A per fixture). A well-equipped home shop can easily have 200-300 amps of connected load, but the actual service size needed is much smaller once demand factors are applied.
Panel Load Study
Do you actually need a panel upgrade? Walk your breaker panel with NEC Article 220 demand factors. See connected load vs. calculated demand and test whether an EV charger, heat pump, or hot tub fits.
Subpanel Sizing with NEC 220 Demand Factors
You do not need a subpanel rated for the sum of all connected loads. NEC Article 220 applies demand factors: first 10,000 VA at 100%, remainder at 40% for general loads. Motors at 125% of the largest plus 100% of the rest.
Worked example: 15A lighting (1,800 VA), 40A receptacles (4,800 VA), 30A air compressor (7,200 VA), 20A dust collector (4,800 VA), 50A welder (12,000 VA). Gross connected: 30,600 VA. After demand factors: lighting/receptacles at 6,600 VA (all at 100% since under 10,000 VA), motors at 125% of largest (9,000 VA) plus rest (4,800 VA) = 13,800 VA, welder at 12,000 VA. Total demand: 32,400 VA. At 240V, that's 135 amps. A 150-amp or 200-amp subpanel would be appropriate.
When in doubt, go one size larger. The cost difference between a 100-amp and a 150-amp panel is minimal. The feeder wire is the expensive part, not the panel itself. If your calculation suggests a 100-amp subpanel and you have any uncertainty about future loads, install 150 amps.
General lighting/receptacles: First 10,000 VA at 100%, remainder at 40%.
Motors: 125% of largest motor + 100% of others.
Continuous loads (welders, heaters): 125% of nameplate for conductor sizing.
Feeder Wire and Voltage Drop
This is where detached buildings get tricky. A shop 100-200 feet from the main panel needs heavier wire than the ampacity tables suggest. The NEC recommends keeping voltage drop below 3% for feeders. For a 240V system, 3% is about 7 volts. A motor running at 233V instead of 240V draws more current and runs hotter.
VD = (2 × K × I × D) / CM. For a 100-amp feeder at 150 feet on 2 AWG copper (66,360 CM): VD = (2 × 12.9 × 100 × 150) / 66,360 = 5.83 volts, or 2.4%. Acceptable. If you used 4 AWG (41,740 CM), the drop would be 9.3 volts or 3.9%. Marginal.
For long runs, aluminum is often more economical. A 100-amp feeder at 150 feet might use 1/0 aluminum (105,600 CM) instead of 2 AWG copper. The voltage drop is (2 × 21.2 × 100 × 150) / 105,600 = 6.0 volts or 2.5%. The aluminum is cheaper per foot even at the larger size.
Run the voltage drop calculation before you buy wire. For a detached shop, the wire size is usually driven by voltage drop, not ampacity.
VD = (2 × K × I × D) / CM
K = 12.9 (copper) or 21.2 (aluminum)
I = amps, D = distance (ft), CM = circular mils
%VD = (VD / Vsystem) × 100
Target: 3% or less for feeders
Wire Sizing Calculator
Find the right AWG wire gauge for any electrical run. Enter amps, distance, and voltage to get NEC-compliant sizing with derating, voltage drop, and copper vs aluminum cost comparison.
Conduit, Burial, and Code Requirements
Three options for running wire: direct burial cable (UF-B), PVC conduit, or rigid metal conduit (RMC). Direct burial is cheapest but offers no future flexibility. PVC is the most common compromise. RMC is the most robust but most expensive.
NEC burial depth: direct burial cable (UF-B) requires 24 inches. PVC Schedule 40 conduit requires 18 inches. RMC can go at 6 inches. These are NEC minimums; local codes may require more. Go deeper under driveways.
When using PVC, size it for future capacity. A 2-inch conduit gives you room for additional circuits later. Use sweep elbows for easier wire pulling. If the run is more than 100 feet with multiple bends, add a pull box midway.
The detached building requires a separate grounding electrode per NEC 250.32. Two ground rods at least 6 feet apart, bonded to the subpanel ground bus. The feeder must include a separate equipment grounding conductor. At the subpanel, the neutral and ground buses must be isolated (not bonded together). The bond point is at the main service panel only. This is a common mistake that creates parallel return paths and can cause nuisance trips or ground faults.
Common Shop Loads and Their Gotchas
Welders are misunderstood. The nameplate rating is maximum input at 100% duty cycle, but most hobby welding runs at lower duty cycles. A 50-amp input welder might only draw 30 amps during typical MIG welding. However, the NEC requires you to size the circuit for nameplate, because the welder can pull full current for minutes at a time.
Air compressors have high starting current, typically 5-7 times running current. A 5 HP motor with 25 amps running current might pull 150 amps for a fraction of a second during startup. Dedicated circuits and motor-rated breakers are essential. Lights dimming when the compressor starts means your feeder is undersized.
Three-phase equipment in a single-phase shop needs a VFD or rotary phase converter. VFDs are more efficient and provide soft-start and speed control. Budget $500-1,500 for the converter plus installation.
EV chargers are continuous loads under NEC 625, meaning 125% sizing. A 40-amp Level 2 EV charger requires a 50-amp circuit. If you're planning to add one, include it in your load calculation now. The feeder and subpanel must have capacity, and you may need to upgrade your main panel.
Can I Run This On That?
Check if your circuit breaker and wiring can handle a specific appliance. Enter breaker size, wire gauge, and load wattage for a pass/fail verdict based on NEC standards.