Detached Garage Subpanel: Complete Wiring Guide
Bill of materials
Feeding a detached garage involves more code requirements than a simple circuit extension. NEC 250.32 requires a separate grounding electrode, the feeder must be 4-wire, and the subpanel neutral must not be bonded to ground.
Detached garages need a separate grounding electrode per NEC 250.32. All garage receptacles require GFCI. For runs over 75 feet, check voltage drop and consider upsizing wire. Underground feeder requires UF cable or THWN in conduit.
60A subpanel (basic workshop)
| Component | Specification | NEC basis |
|---|---|---|
| Wire (copper) | 6 AWG THHN | Table 310.16, 110.14(C) |
| Wire (aluminum alt.) | 4 AWG THHN | Table 310.16, 110.14(C) |
| Breaker | 60A double-pole | 240.4 |
| Voltage | 240V single-phase | |
| Conduit (EMT) | 3/4" EMT | Chapter 9 Table 4 |
| GFCI | Required (GFCI breaker or device) | 210.8(A)(2) |
| Max distance (3% VD) | 153 ft at 240V copper | 210.19(A) Note 4 |
100A subpanel (full workshop + EV)
| Component | Specification | NEC basis |
|---|---|---|
| Wire (copper) | 3 AWG THHN | Table 310.16, 110.14(C) |
| Wire (aluminum alt.) | 1 AWG THHN | Table 310.16, 110.14(C) |
| Breaker | 100A double-pole | 240.4 |
| Voltage | 240V single-phase | |
| Conduit (EMT) | 1" EMT | Chapter 9 Table 4 |
| GFCI | Required (GFCI breaker or device) | 210.8(A)(2) |
| Max distance (3% VD) | 184 ft at 240V copper | 210.19(A) Note 4 |
Installation notes
This project requires GFCI protection per NEC 210.8(A)(2). The simplest approach is a GFCI breaker at the panel, which protects the entire circuit. For 240V circuits, use a 2-pole GFCI breaker sized to match the circuit. GFCI breakers cost $40-80 depending on amperage.
For the wire run, you have two options: NM-B cable (Romex) for interior runs through framing, or individual THHN conductors in conduit for exposed runs, outdoor installations, or runs through unfinished spaces. NM-B is faster to install but cannot be used outdoors or in conduit. For conduit, use EMT (metallic) or PVC Schedule 40.
Run length matters: every foot of wire adds voltage drop, reducing power to your equipment. The table below shows the exact voltage drop at common distances. If your run exceeds the 3% threshold, upsize one gauge.
Voltage drop by distance
Percentage voltage drop at common run lengths. NEC recommends 3% maximum for branch circuits and 5% for combined feeder plus branch.
| Distance | 6 AWG Co | 4 AWG Al | 4 AWG Co |
|---|---|---|---|
| 25 ft | 0.5% | 0.5% | 0.3% |
| 50 ft | 1.0% | 1.0% | 0.6% |
| 75 ft | 1.5% | 1.5% | 0.9% |
| 100 ft | 2.0% | 2.0% | 1.2% |
| 150 ft | 3.0% | 3.0% | 1.9% |
| 200 ft | 3.9% | 4.1% | 2.5% |
Common products for 6 AWG Copper circuits
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Related wire data
Detailed ampacity: 6 AWG copper, 4 AWG aluminum. Wire size lookup: 60A circuit. Calculators: voltage drop · panel load calculator.
Inspection preparation
Before your rough-in inspection, verify these frequently-checked items:
| Check | NEC Ref | Common Fail |
|---|---|---|
| Cable secured within 12 inches of box | NEC 334.30(A) | NM cable not stapled close enough to box |
| Proper wire gauge for circuit amperage | NEC 240.4(D) | 14 AWG on 20A circuit or 12 AWG on 30A circuit |
| Ground wire continuous and properly bonded | NEC 250.148 | EGC not connected in box or spliced incorrectly |
| No NM cable in wet/damp locations | NEC 334.12(B) | Romex used in exterior, underground, or damp location |
| Box fill calculations | NEC 314.16 | Too many conductors in box |
See the full inspection checklist tool for all 19 items with progress tracking.
Frequently asked questions
What wire size do I need for detached garage subpanel?
Use 6 AWG copper or 4 AWG aluminum on a 60A breaker at 240V. This is based on NEC 2023 Table 310.16 ampacity with 110.14(C) termination limits and 125% continuous load sizing per 210.19(A)(1).
Does detached garage subpanel require GFCI?
Yes. GFCI protection is required per NEC 210.8(A)(2).
NEC 2023 references verified April 2026