For a standard 60A sub panel, use 6 AWG copper or 4 AWG aluminum wire with a 60A breaker. For a 100A sub panel, use 3 AWG copper or 1/0 AWG aluminum with a 100A breaker. These sizes assume 75°C terminations, 30°C ambient, and copper THHN in conduit.
- Material: Copper (Aluminum addressed separately with distinct AWG requirements).
- Temperature Column: 75°C (Standard for modern residential breakers and panel lugs).
- Ambient Temperature: 30°C (86°F) or lower.
- Raceway: EMT or PVC conduit with no more than 3 current-carrying conductors.
- Feed Configuration: 4-wire feed (2 Hots, 1 Neutral, 1 Equipment Grounding Conductor) as mandated since the 2008 NEC.
The Core Sizing Matrix for Subpanel Feeders
When wiring sub panel feeders, you must pull wire sizes from the 75°C column of NEC Table 310.16, regardless of the fact that THHN wire insulation is rated for 90°C. The table below maps standard residential subpanel breaker sizes to the minimum required wire gauge and the maximum run length before voltage drop forces an upsizing.
| Breaker Size (OCPD) | Copper AWG (75°C) | Aluminum AWG (75°C) | Max Distance for <3% VD (240V) | Equipment Ground (Cu) |
|---|---|---|---|---|
| 60 Amp | 6 AWG | 4 AWG | 110 ft | 10 AWG |
| 100 Amp | 3 AWG | 1/0 AWG | 130 ft | 8 AWG |
| 125 Amp | 1 AWG | 3/0 AWG | 125 ft | 6 AWG |
| 150 Amp | 1/0 AWG | 4/0 AWG | 120 ft | 6 AWG |
Note: Equipment grounding conductor sizes are based on NEC Table 250.122. If you upsize your current-carrying conductors for voltage drop, you must proportionally upsize your ground wire per NEC 250.122(B).
Why These Exact Sizes? (NEC Rules & Terminal Limits)
A common mistake on the bench or jobsite is looking at the 90°C column for THHN wire and assuming 6 AWG (rated 75A at 90°C) can safely be protected by a 70A breaker. You cannot. NEC 110.14(C) dictates that the ampacity of the wire is limited by the lowest temperature rating of any connected component. Because the lugs inside standard residential load centers and breakers are rated for 75°C, you must use the 75°C column to determine base ampacity.
Why not use one size smaller?
Take the 100A sub panel. The calculated load might only be 92A. Under NEC 240.4(B), you are allowed to use the "next size up" overcurrent protective device (OCPD) if the wire ampacity doesn't perfectly match a standard breaker size. However, the wire's base ampacity must still meet or exceed the actual calculated load. 4 AWG copper is rated 85A at 75°C—it cannot carry a 92A load, nor can it be protected by a 100A breaker. You must step up to 3 AWG copper, which is rated exactly 100A at 75°C.
Variables That Force an Upsize: Length, Bundling, and Aluminum
The matrix above assumes ideal conditions. Real-world jobsites rarely cooperate. Here is the decision framework for when you must abandon the minimum AWG and pull larger wire.
| Variable | Trigger Condition | Required Action |
|---|---|---|
| Run Length | Distance exceeds the "Max Distance" in the matrix above. | Upsize wire one or two steps to maintain <3% voltage drop at full load. |
| Conductor Bundling | More than 3 current-carrying conductors in a single raceway. | Apply NEC 310.15(C)(1) derating factors (e.g., 4-6 conductors = 80% multiplier) using the 90°C column, then verify against 75°C termination limits. |
| Aluminum Wire | Using XHHW-2 or SER aluminum cable to save money on long runs. | Upsize AWG (see table). Apply Noalox anti-oxidant paste. Torque to exact manufacturer specs. |
| High Ambient Heat | Conduit runs across a hot roof or in an attic exceeding 30°C (86°F). | Apply NEC Table 310.15(B)(1) temperature correction factors to the 90°C column before checking termination limits. |
Worked Example: Voltage Drop Math for a 100A Feeder
Suppose you are wiring a 100A sub panel in a detached garage 130 feet from the main panel. You are using 3 AWG copper THHN. According to the Copper Development Association, the formula for single-phase voltage drop is:
VD = (2 × K × I × D) / CM
Where K = 12.9 (copper), I = 100A, D = 130 ft, and CM = 52,620 (circular mils for 3 AWG).
Plugging in the numbers: VD = (2 × 12.9 × 100 × 130) / 52,620 = 6.37V.
Dividing 6.37V by the 240V nominal supply yields a 2.65% voltage drop. This is under the 3% NEC informational note recommendation, meaning 3 AWG copper is perfectly acceptable for this 130-foot run. If the garage were 180 feet away, the drop would exceed 3.6%, forcing you to upsize to 2 AWG copper.
The Aluminum Caveat
Aluminum SER (Service Entrance Rating) cable is significantly cheaper than copper THHN for 100A+ feeds, but it requires strict installation discipline. Aluminum expands and contracts more than copper under thermal cycling, which can cause lugs to loosen over time and create high-resistance arcing faults. If you choose aluminum:
- Brush the exposed conductor strands with an anti-oxidant compound like Noalox before landing them in the lugs.
- Use a calibrated torque screwdriver. NEC 110.14(D) mandates that terminals be tightened to the manufacturer's specified torque. Guessing "tight enough" with aluminum is a leading cause of subpanel fires.
- Ensure the panel knockouts and conduit bodies are large enough; 1/0 aluminum is physically much thicker than 3 AWG copper and will not bend easily in tight LB fittings.
When the AHJ or an Engineer Must Confirm
While the matrix and math above cover 95% of residential and light-commercial subpanel installations, certain scenarios require stamped engineering drawings or explicit AHJ sign-off before you pull a single wire.
- Detached Outbuilding Grounding (NEC 250.32): If your sub panel feeds a detached structure (like a barn or separate garage), you must install a local grounding electrode system (e.g., two 8-foot ground rods) at the outbuilding. The neutral and ground must be isolated in the subpanel, but bonded at the main service disconnect. Local inspectors are highly vigilant about this separation to prevent neutral current from traveling over the earth.
- Feeder Taps and Service Entrance Upgrades: If you are tapping directly from the service entrance conductors (the lines between the meter and the main breaker) rather than from a breaker in the main panel, NEC 240.21 feeder tap rules apply. This is highly regulated and often restricted to licensed contractors.
- Continuous Loads Exceeding 80%: If the sub panel will supply continuous loads (EV chargers, server racks, or baseboard heaters running for 3+ hours), the wire and breaker must be sized at 125% of the continuous load. A 100A continuous load requires wire rated for 125A and a 125A breaker, pushing you out of standard residential breaker frames and into commercial bolt-on or molded-case territory.
Always verify the specific torque values printed on the inside sticker of your load center. Modern panels from Square D, Eaton, and Siemens explicitly list terminal torque requirements (often between 35 and 50 in-lbs for smaller lugs, and up to 250 in-lbs for 1/0+ feeders). Use an inch-pound torque screwdriver, not a foot-pound torque wrench, to avoid shearing the lug screws and ruining the panel busbar.
