Wire Fill for Detached Garage Sub-Feed
Detached garage feeder wire fill: 60A uses 6 AWG copper in 1-inch PVC, 100A uses 3 AWG in 1-1/4 inch. Separate GES required per NEC 250.32.
Wire fill for detached garage sub-feeders is governed by NEC 215, 225, and 250.32. A typical 60A detached garage sub-feed uses 6 AWG copper THHN (4-wire: 2 hots, neutral, EGC) in 1-inch PVC conduit. A 100A sub-feed uses 3 AWG copper in 1-1/4 inch PVC. The garage requires its own grounding electrode system (driven rod or Ufer) per NEC 250.32(A). Voltage drop is the dominant sizing factor on long runs.
Load characteristics
A detached garage typically combines lighting, receptacles, and one or two larger loads:
| Use case | Calculated load | Typical sub-panel |
|---|---|---|
| Lighting + few outlets | 15-25A | 30-60A |
| Workshop tools, shop vac, small welder | 30-50A | 60-100A |
| EV charging + workshop | 50-80A | 100-125A |
| Apartment / ADU above garage | per NEC 220 | 100-200A |
Most modern garages get 60A or 100A panels for headroom — even small shops accumulate loads (compressor, table saw, dust collector).
NEC code references
- NEC 215 — feeder requirements
- NEC 225 — outside branch circuits and feeders
- NEC 225.30 — number of supplies to a building (generally one feeder per structure)
- NEC 250.32 — grounding at separate buildings — 4-wire feeder + separate GES
- NEC 300.5 — burial depths
- NEC 210.8(A)(2) — GFCI required for garage receptacles
- NEC 210.52(G) — at least one receptacle in attached and detached garages
Conductor sizing
Use 75°C NEC Table 310.16. Voltage drop on long runs may force you up one or two sizes:
| Sub-panel rating | Copper THHN/THWN-2 | Aluminum XHHW-2 |
|---|---|---|
| 30A | 10 AWG | 8 AWG |
| 60A | 6 AWG | 4 AWG |
| 100A | 3 AWG | 1 AWG |
| 125A | 2 AWG | 1/0 |
For runs over 75 ft, calculate voltage drop. Rule of thumb: bump up one gauge per 100 ft for 60A and larger feeders.
| Sub-panel | Run length | Recommended copper |
|---|---|---|
| 60A | up to 75 ft | 6 AWG |
| 60A | 75-150 ft | 4 AWG |
| 100A | up to 75 ft | 3 AWG |
| 100A | 75-150 ft | 2 AWG |
| 100A | 150-250 ft | 1 AWG |
Conduit sizing
4-wire feeder (2 hots + neutral + EGC) in PVC Sch 40:
| Sub-panel | Conductors | Total area | 1" PVC | 1-1/4" PVC |
|---|---|---|---|---|
| 60A | 3 × 6 + 1 × 10 EGC | 0.1733 in² | 20.8% | 11.4% |
| 100A | 3 × 3 + 1 × 8 EGC | 0.3009 in² | over 40% — fails | 19.7% |
| 125A | 3 × 2 + 1 × 6 EGC | 0.3719 in² | over 40% — fails | 24.4% |
For 60A use 1-inch PVC. For 100A and larger, 1-1/4 inch PVC minimum.
For aluminum:
| Sub-panel | Aluminum conductors | 1-1/4" PVC | 1-1/2" PVC |
|---|---|---|---|
| 100A | 3 × 1 AWG + 1 × 6 EGC | 30.0% | 19.5% |
| 200A | 3 × 4/0 + 1 × 4 EGC | over 40% — fails | over 40% — fails (use 2") |
Special requirements
- Separate grounding electrode system: NEC 250.32(A). Drive two 8-ft copper-clad rods 6 ft apart, bond with 6 AWG copper to sub-panel ground bar.
- Neutral isolated from ground at sub-panel: NEC 250.142(B). Remove the bonding screw or strap.
- GFCI: All 125V 15A/20A receptacles in the garage per NEC 210.8(A)(2).
- Disconnect: The sub-panel main breaker (or six-throw rule if MLO) serves as the disconnect for the structure.
- Wet location: Underground portion is a wet location — use THWN-2 or XHHW-2.
- Conduit transitions: Use sweep 90s for underground PVC runs, not standard 90s, for easier pulling.
Worked example — 100A garage sub-panel, 120 ft from main house
- Calculated load: 70A peak (workshop + EV charger)
- Feeder OCPD: 100A 2-pole breaker in main house panel
- Voltage drop check at 120 ft: 100A on 3 AWG copper = 4.4V drop (1.8% on 240V) — pass for 3% feeder limit, but tight. Go to 2 AWG for safety.
- Conductors: 2 AWG copper THHN/THWN-2 × 3 (L1, L2, N) + 8 AWG copper EGC
- Raceway: 1-1/4" PVC Sch 40, buried 18 inches, with Sch 80 risers above grade
- Fill check: 3 × 0.1158 + 0.0366 = 0.3840 in² in 1-1/4" PVC Sch 40 = 25.1%, pass.
- Grounding: Two 8-ft ground rods, 6 AWG copper to ground bar. Bonding screw removed from neutral bar.
- GFCI: 20A 125V GFCI receptacles throughout garage.
Verify with the Conduit Fill Calculator.
Quick reference
| Garage sub-panel | Run length | Copper feeder | EGC | PVC conduit |
|---|---|---|---|---|
| 60A | <75 ft | 6 AWG × 3 | 10 AWG | 1" |
| 60A | 75-150 ft | 4 AWG × 3 | 10 AWG | 1" |
| 100A | <75 ft | 3 AWG × 3 | 8 AWG | 1-1/4" |
| 100A | 75-150 ft | 2 AWG × 3 | 8 AWG | 1-1/4" |
| 100A | 150-250 ft | 1 AWG × 3 | 8 AWG | 1-1/2" |
Related
FAQ
This page's figures are transcribed from NEC 2023 (NFPA 70) Chapter 9, Tables 1, 4 and 5, and checked by a version-controlled automated test suite — they are not signed off by a licensed professional. WireFillChart is an independent reference, not affiliated with or endorsed by NFPA. Always verify against the code edition adopted by your local authority having jurisdiction (AHJ).