For a standard 60A detached garage sub panel wiring diagram, use 6 AWG copper THWN-2 wire protected by a 60A double-pole breaker. If upgrading to a 100A feed, step up to 3 AWG copper or 1 AWG aluminum. This assumes a 50-foot run, 75°C terminations, and 30°C ambient temperature.
The Baseline: 4-Wire Feed and Disconnect Rules
Any modern detached garage sub panel wiring diagram must follow NEC Article 250.32, which mandates a 4-wire feeder: two ungrounded conductors (hots), one grounded conductor (neutral), and one equipment grounding conductor (EGC). The days of using the neutral as a ground for outbuildings are over. At the subpanel, the neutral bar must be isolated (floated) from the metal enclosure, while the ground bar must be bonded directly to it.
Furthermore, NEC 225.32 requires a disconnecting means for a detached structure. While you could technically use a main-lug panel and rely on the breaker back at the main house if there are six or fewer breakers, the cleanest, most inspector-friendly method is installing a subpanel with a built-in main breaker. This acts as your local disconnect and allows you to expand the garage's circuit count beyond six without violating code.
Sizing Assumptions & Ampacity Data
Wire sizing is not a guessing game; it is a strict lookup based on physical properties and installation conditions. Before pulling any wire, verify that your installation matches the baseline assumptions below. If your conditions differ, the required wire gauge will change.
- Conductor Material: Copper (unless aluminum is explicitly selected in the decision tree).
- Temperature Column: 75°C. Per NEC 110.14(C), even if your wire insulation is rated for 90°C (like THHN), you must size the wire using the 75°C column because standard residential breaker and panel lugs are rated for 75°C.
- Ambient Temperature: 30°C (86°F). If your conduit runs through an attic that exceeds this, temperature correction factors apply.
- Conduit Type: PVC Schedule 80 underground. Underground conduit is classified as a 'wet location' by the NEC, meaning you must use THWN-2 or XHHW-2 insulation. Standard THHN is for dry locations only and will degrade in wet conduit.
Below is the relevant data extracted from NEC Table 310.16 for our target amperages:
| Target Breaker | Wire Size (Copper) | Wire Size (Aluminum) | 75°C Ampacity (Cu) | 75°C Ampacity (Al) |
|---|---|---|---|---|
| 60A | 6 AWG | 4 AWG | 65A | 65A |
| 100A | 3 AWG | 1 AWG | 100A | 100A |
Decision Tree: Picking Your Feed Wire and Breaker
Use this decision path to lock in your exact materials list. Do not mix and match; follow the logic to its terminal conclusion.
| If your garage load profile is... | Then select this panel size... | And buy this exact wire/breaker combo... |
|---|---|---|
| Basic lighting, standard outlets, and a 120V workbench (Total continuous load < 48A) | 60A Main Breaker Subpanel | 6 AWG Cu THWN-2 (4 wires) + 60A 2-pole breaker at main panel |
| Above, PLUS a 240V welder, air compressor, or single EV charger (Total load > 48A) | 100A Main Breaker Subpanel | 3 AWG Cu THWN-2 (4 wires) + 100A 2-pole breaker at main panel |
| Heavy workshop, multiple EV chargers, or minimizing voltage drop over 100+ feet | 100A Main Breaker Subpanel | 1 AWG Al XHHW-2 (4 wires) + 100A 2-pole breaker at main panel (Aluminum is cheaper for long, thick runs) |
Voltage Drop Check: When Distance Forces an Upsize
Ampacity tells you what the wire can handle before the insulation melts. Voltage drop tells you if your tools will actually run properly at the other end. The NEC recommends keeping voltage drop under 3% for branch circuits and feeders combined (5% total).
Let's run the math for a 60A load on 6 AWG copper at 240V. The circular mils (CM) for 6 AWG is 26,240. Using the standard single-phase voltage drop formula: VD = (2 × K × I × L) / CM, where K is 12.9 for copper.
- At 50 feet: VD = (2 × 12.9 × 60 × 50) / 26,240 = 2.95V (1.2%). This is excellent. 6 AWG is perfectly fine.
- At 150 feet: VD = (2 × 12.9 × 60 × 150) / 26,240 = 8.85V (3.6%). This exceeds the 3% recommendation. Your air compressor might struggle to start on cold mornings.
The Fix: If your trench from the main house to the garage is 150 feet, you must upsize the 60A feed from 6 AWG to 4 AWG copper. The 4 AWG wire drops the loss to 4.6V (1.9%), bringing it back well within acceptable limits. You can still protect 4 AWG wire with a 60A breaker; the breaker only protects against overcurrent, not voltage drop. Use the Southwire Voltage Drop Calculator to verify your exact trench length.
Why Not One Size Smaller? (And What Changes the Math)
A common question on the bench is why we can't use 8 AWG copper for a 60A breaker, since 8 AWG is rated for 50A at 75°C. NEC 240.4(B) allows you to round up to the next standard breaker size if the wire ampacity doesn't match a standard breaker—but only if the required ampacity isn't already a standard breaker size. Since 60A is a standard breaker size, you cannot use 50A-rated wire and round up. You must use 6 AWG (65A) to legally protect a 60A circuit.
Several jobsite realities will force you to deviate from the baseline table above:
- Bundling (Derating): If you pull more than three current-carrying conductors through a single conduit, NEC 310.15(C)(1) requires you to derate the ampacity. If you run two 240V circuits (4 hots, 2 neutrals) in one pipe to the garage, you must apply an 80% derating factor. 6 AWG copper (75A at 90°C for derating purposes) drops to 60A. This leaves zero margin. Upsize to 4 AWG if bundling.
- Aluminum Terminations: If you choose 1 AWG aluminum for a 100A feed to save money on long runs, you must use lugs rated for aluminum and apply an anti-oxidant compound (like Noalox) to the terminations. Never mix aluminum wire with lugs explicitly marked 'Cu' only.
- Grounding Electrodes: The wire size dictates the feed, but NEC 250.32 requires a grounding electrode system at the detached structure. Drive two 5/8-inch copper ground rods at least 6 feet apart, and connect them to the subpanel's ground bar using a solid 6 AWG bare copper grounding electrode conductor.
When an Engineer or AHJ Must Confirm
While this guide covers 95% of residential detached garage builds, specific scenarios require a stamped engineering drawing or explicit approval from your local Authority Having Jurisdiction (AHJ):
- Feeds exceeding 200A: If you are running a 400A service to a massive detached shop with 3-phase equipment, parallel conductor rules (NEC 310.10(G)) and complex magnetic heating interactions in conduit require professional engineering.
- High Thermal Resistivity Soil: If you are direct-burying UF-B or USE-2 cable instead of using conduit, and your soil is unusually dry, sandy, or rocky, the soil cannot dissipate heat. The AHJ may require you to upsize the wire beyond standard ampacity tables.
- Local Amendments: Some municipalities in high-fire-risk zones (like parts of California) mandate rigid metal conduit (RMC) instead of PVC for underground feeds, or require AFCI protection on the main feeder breaker. Always pull the permit and review the local inspector's specific checklist before digging your trench.
For standard 60A or 100A builds using the Eaton BR or Square D Homeline loadcenters, sticking to the 4-wire THWN-2 copper sizes and torque specifications listed on the panel's interior label will get you a clean inspection and a safe, reliable power supply to your garage.
