Wiring a Detached Garage: 60A Subpanel
Design a 60A subpanel feeder to a detached garage — conductor sizing, voltage drop, conduit selection, and grounding.
Scenario Overview
Size and install a 60A, 240V feeder from the main panel to a detached garage 85 feet away. The garage will have general lighting, receptacles, and a future EV charger circuit.
Given Information
- •Main panel: 200A, 120/240V in house
- •Distance: 85 feet from main panel to garage subpanel
- •Garage subpanel: 60A, 120/240V
- •Loads: general lighting, receptacles, future 40A EV charger
- •Burial method: PVC conduit underground
- •Soil: normal conditions
Calculation Steps
Determine Feeder Conductor Size
60A feeder requires conductors rated for minimum 60A at 75°C.
NEC Table 310.16: 6 AWG copper = 65A (adequate) or 4 AWG aluminum = 65A.
Using aluminum to save cost: 4 AWG aluminum THWN-2.
Result: 4 AWG aluminum THWN-2 conductors
Check Voltage Drop
85 ft run, 60A max, 4 AWG aluminum (R = 0.321 Ω/1000ft for AC).
VD = 2 × 85 × 60 × 0.321 / 1000 = 3.27V.
VD% = 3.27 / 240 × 100 = 1.36% — well within 3% limit.
Result: 1.36% voltage drop ✓
Select Conduit
4 conductors: 2× 4 AWG + 1× 6 AWG neutral + 1× 8 AWG EGC (all aluminum except EGC can be copper).
NEC Chapter 9: conductor areas for 4 AWG THWN-2 = 0.0824 in² each. 6 AWG = 0.0507 in². 8 AWG Cu = 0.0366 in².
Total area = 2(0.0824) + 0.0507 + 0.0366 = 0.2521 in².
40% fill: need conduit with area ≥ 0.2521 / 0.40 = 0.630 in².
1" PVC Schedule 40 (area = 0.864 in²) — adequate.
Result: 1" PVC Schedule 40 conduit
Determine Burial Depth
NEC Table 300.5: PVC conduit (not encased in concrete) requires:
Under driveway: 24 inches minimum.
Under yard: 18 inches minimum.
We'll use 24 inches throughout for consistency.
Result: 24 inch burial depth
Grounding Requirements
Detached structure: NEC 250.32 requires a grounding electrode system at the garage.
Install 2 ground rods (8 ft, ≥ 5/8" diameter) at least 6 feet apart.
GEC from garage subpanel to ground rods: #6 AWG copper minimum.
The neutral and ground must be separated at the subpanel (isolated neutral bar).
Result: 2 ground rods + isolated neutral in subpanel
Subpanel Configuration
60A main breaker in subpanel (serves as disconnect for detached structure).
Available circuits: general lighting (15A), receptacles (20A), dedicated EV charger (50A), future spare.
Total available: 60A feeders provide 14,400 VA at 240V — adequate for planned loads.
Result: 60A subpanel with main breaker disconnect
Final Answer
Feed the detached garage with 4 AWG aluminum THWN-2 (2 hots + neutral) and 8 AWG copper EGC in 1" PVC Schedule 40 conduit, buried 24 inches deep. Install a 60A subpanel with main breaker disconnect and 2 ground rods. Voltage drop is 1.36%.
Key Takeaways
- •Detached structures require their own grounding electrode system (NEC 250.32)
- •Neutral and ground must be separated (isolated) at a subpanel — bonding only at the main panel
- •Always check voltage drop for underground runs over 50 feet
- •Aluminum conductors are a cost-effective choice for feeders — just size appropriately
Calculators Used
NEC References
- NEC 225 — Outside Branch Circuits and Feeders
- NEC 250.32 — Detached Structure Grounding
- NEC Table 300.5 — Burial Depth
- NEC 310.16 — Conductor Ampacity