How to wiring a detached garage

Plan the Feeder Before You Dig

A detached garage is a separate structure, so feeder sizing, grounding, and disconnect rules all shift from what you do inside the dwelling. Start by totaling the load per NEC Article 220. For most residential garages with lights, receptacles, a door opener, and maybe a compressor or EV charger, a 60A or 100A feeder is the common call. Don't undersize. Future EVSE loads alone can eat a 60A feeder fast.

Run your calculation with both general lighting VA and any specific appliance or EV load. NEC 220.87 lets you use measured demand data for existing service upsizing checks, but for a new detached structure you're working from 220.12 and 220.14. Document it. Inspectors want to see the math.

Pick your wiring method next. Direct burial USE-2, PVC conduit with THWN-2, or a UF cable run, each has trade-offs. PVC with individual conductors is usually cleanest for long runs and gives you pull capacity for future circuits.

• Burial depth: 18 inches for PVC with conductors, 24 inches for direct burial cable, 6 inches under 4 inch concrete slab. See NEC Table 300.5.
• Mark the trench with detectable tape 12 inches above the conduit.
• Size conductors for voltage drop on runs over 100 feet. Aim for 3% or less on the feeder.

Grounding and the Four-Wire Rule

This is where older electricians trip up. Since the 2008 NEC cycle, a detached structure fed from the main building requires four wires: two hots, a neutral, and an equipment grounding conductor. The neutral and ground must be kept separate at the garage subpanel. Bond the ground bar to the enclosure. Float the neutral bar.

You still drive grounding electrodes at the detached structure per NEC 250.32(A). Two ground rods 6 feet apart, or a single rod if you can prove 25 ohms or less resistance, which almost nobody tests for in practice. Just drive two and move on. Bond them to the ground bar in the subpanel with a #6 copper minimum.

Field tip: If you're replacing an old three-wire feeder to an existing garage, you must upgrade to four-wire. The grandfathering allowance in 250.32(B) Exception was removed. Pull a new feeder or rework the existing conduit.

Disconnect and Panel Requirements

Every detached structure needs a disconnecting means per NEC 225.31 and 225.32. This can be a main breaker in the subpanel, a separate disconnect ahead of it, or a panel with a main lug kit plus a listed disconnect. Most installs use a main breaker panel, which is the simplest path to compliance.

Mount the panel at a readily accessible location, nearest the point of feeder entry. Working space per 110.26 still applies: 36 inches of depth, 30 inches of width, 6.5 feet of headroom. Don't put it behind the workbench you're about to build.

Label the disconnect clearly. If the garage has more than one supply, such as a generator or PV backup, each disconnect needs a permanent plaque at each location per 225.37.

Branch Circuits Inside the Garage

Garage receptacles have specific rules that changed in recent cycles. NEC 210.52(G) requires at least one receptacle per vehicle bay, and 210.11(C)(4) requires a dedicated 20A branch circuit for garage receptacles that serves no other areas. GFCI protection is required on all 125V through 250V receptacles up to 50A in a garage per 210.8(A)(2).

Lighting should be on a separate circuit from the receptacles. A dead receptacle circuit shouldn't leave you in the dark. Use LED fixtures rated for the ambient, especially if you're in an unconditioned space with temperature swings.

1. One 20A dedicated receptacle circuit minimum, more if the shop load warrants it.
2. Separate 15A or 20A lighting circuit.
3. EVSE circuit sized per 625.41 and 625.42, typically 40A or 50A for a Level 2.
4. Door opener on its own 15A circuit or shared with lighting, per manufacturer spec.

EV Chargers, Welders, and Shop Equipment

Working electricians know the garage is never just a garage. Plan for the 240V loads now, even if the owner says they'll never add them. Pull a spare 1 inch PVC from the panel to a junction box near the likely EVSE location. Costs twenty bucks and saves a drywall job later.

For welders, NEC 630 allows duty cycle adjustments to the branch circuit sizing, so a 50A welder on a 40A circuit can be compliant if the duty cycle math works. Verify the nameplate and apply Table 630.11(A). For continuous EV charging loads, apply the 125% factor per 625.41.

Field tip: If the customer is considering an EV within five years, install a 60A feeder minimum and a panel with at least 12 spaces. Retrofits to accommodate a charger on an undersized feeder almost always mean a service upgrade or a load management device.

Inspection and Finish Details

Before the rough inspection, verify trench depth, conduit joints glued and supported, and conductor identification. White tape on the neutral, green on the ground if you're using black THWN for everything. Bond all metallic piping and structural steel per 250.104 if present.

At final, confirm GFCI operation on every required receptacle, torque all terminations to the manufacturer spec, and verify the panel directory is filled out legibly. NEC 408.4(A) requires circuit identification on every panel, and inspectors check this more often than people expect.

Take photos of the buried conduit before backfill. When the customer calls in three years asking where the feeder runs, you'll have the answer.