Complete guide to wiring a detached garage

Plan the Feeder Before You Dig

Start with the load calc per NEC Article 220. Add up general lighting at 3 VA/sq ft, small appliance if applicable, plus every fixed load: EV charger, welder, compressor, mini-split, heaters. Size the feeder to the calculated load, not to the breaker you wish you had.

Most detached garages land on a 60A or 100A feeder. A 100A subpanel gives room for a Level 2 EVSE and a 240V welder without a headache later. Run copper if the distance is short, aluminum SER or URD if the run is long and the budget is tight. Voltage drop matters: keep it under 3% on the feeder per NEC 215.2(A) Informational Note.

Price out #2 AL URD against #4 CU THWN before you commit. On a 150 ft run to a 100A panel, the aluminum spool often wins by half the cost and still meets the 3% drop target.

Trenching, Burial Depth, and Conduit

NEC Table 300.5 governs minimum cover. Direct-buried UF at 24 inches. PVC conduit at 18 inches. Rigid metal at 6 inches. Under a 4 inch concrete slab with a dwelling above, you can go shallower, but for a yard trench stick with the standard numbers and save the inspector conversation.

Use Schedule 40 PVC for most runs, Schedule 80 where the conduit emerges from grade and is exposed to physical damage per NEC 352.10(F). Sweep 90s at both ends, never standard 90s, or you will fight the pull. Leave a pull string. Always leave a pull string.

• Direct burial UF cable: 24 in cover
• Rigid PVC conduit: 18 in cover
• RMC or IMC: 6 in cover
• Under 4 in concrete slab: 18 in cover for PVC
• Residential branch circuit, GFCI protected, 120V 20A max: 12 in cover

Grounding and Bonding the Subpanel

This is where rookies blow it. The detached garage subpanel is fed with four wires: two hots, a neutral, and an equipment grounding conductor. The neutral bar floats. The ground bar bonds to the enclosure. Remove the main bonding jumper. NEC 250.32(B) killed the three-wire feeder exception for new installs back in 2008.

You still need a grounding electrode at the garage per NEC 250.32(A). Two 8 ft ground rods spaced 6 ft apart, or one rod if you can prove 25 ohms or less, which you cannot, so drive two. Bond them with a #6 copper GEC to the ground bar in the subpanel, not the neutral.

If the garage has a metal water line, metal gas line, or structural steel, those are electrodes too. Bond them per NEC 250.104. Ground rods alone are the minimum, not the finish line.

Branch Circuits Inside the Garage

NEC 210.8(A)(2) requires GFCI protection for all 125V through 250V receptacles up to 50A in a garage. That includes the 240V welder outlet and the EV charger receptacle. GFCI breakers in the subpanel are cleaner than deadfront GFCIs at every location.

NEC 210.52(G)(1) requires at least one receptacle in an attached garage, and while detached garages with power are not strictly required to have receptacles, put one on every wall anyway. NEC 210.70(A)(2)(a) requires a lighting outlet controlled by a wall switch at each entrance. Add occupancy sensors or three-way switches if there are multiple doors.

• 20A general purpose receptacle circuit, GFCI protected
• Dedicated 20A circuit for freezer or fridge, still GFCI per 2020 NEC
• Dedicated circuit for door opener per manufacturer
• Lighting on a separate 15A or 20A circuit so a tripped receptacle does not leave you in the dark

EV Charging and Large Loads

If an EVSE is in the plan, size for it now. A 48A continuous charger needs a 60A circuit and #6 copper. NEC 625.42 covers EVSE load calculations. NEC 625.54 requires GFCI protection for any EV receptacle installation, which means a hardwired EVSE is usually the simpler compliance path.

For welders, NEC 630 allows sizing based on duty cycle, which can let a 50A welder live on a 40A circuit if the duty cycle is low. Read the nameplate. For compressors, apply NEC 430 motor rules, not the general branch circuit rules, or you will nuisance trip every startup.

Inspection Checklist

Before you call for inspection, walk the job with fresh eyes. The inspector is looking for the same things every time, and the failures are predictable.

1. Four-wire feeder, neutral isolated, bonding jumper removed
2. Two ground rods, 6 ft apart, #6 GEC continuous to ground bar
3. GFCI protection on every 15A and 20A 125V receptacle, plus any 240V up to 50A
4. Burial depths match Table 300.5, warning tape 12 in above buried cable if using UF
5. Working space at subpanel: 36 in deep, 30 in wide, 6.5 ft high per NEC 110.26
6. Load calc documented and attached to the permit
7. Wire fill within conduit limits per Chapter 9 Table 1

Document everything with photos before backfill. If a wire gets nicked by a shovel two years from now, the pre-backfill photo is your defense. Label the subpanel circuits legibly, not in chicken scratch, and leave the panel schedule inside the door.