What Size Wire for a Subpanel? (60A, 100A, and the Ground Wire Question)

A subpanel is fed by a breaker in the main panel, and the feeder conductors are sized to that breaker's rating. A 60-amp feeder breaker gets wire rated for 60 amps; a 100-amp feeder breaker gets wire rated for 100 amps. The subpanel's own main breaker (if it has one) doesn't set the wire size — the feeder breaker does.

The common sizes, copper and aluminum, assuming 75°C-rated terminations (standard on modern panels and breakers):

The feeder sizes come from the 75°C column of the NEC ampacity table — 6 AWG copper is good for 65A, 3 AWG copper for 100A, 4 AWG aluminum for 65A, 1 AWG aluminum for 100A. The ground sizes come from NEC Table 250.122, which sizes the equipment ground off the breaker rating: 60A gets 10 AWG copper, 100A gets 8 AWG copper. For the full ampacity table these numbers come from — all three temperature columns, copper and aluminum — the wire gauge chart is the master reference for this article.

One trap to avoid: NM-B (Romex) uses the 60°C column, not the 75°C column. 6 AWG NM-B is only good for 55 amps — not enough for a 60-amp feeder. Subpanel feeders run in SER cable or individual conductors in conduit, which can use the 75°C ratings. Don't run 6/3 Romex on a 60-amp feeder breaker and call it done.

At the main panel (the service entrance), ground and neutral are bonded together — one system, one connection point. At every subpanel downstream, they must be separate: the neutral lands on an isolated neutral bar, the ground lands on a ground bar bonded to the panel enclosure, and no bonding jumper connects the two. Four feeder wires: two hots, one neutral, one ground.

Why it matters: the neutral carries normal return current. The ground should never carry current — it exists only to clear faults. Bond them together at a subpanel and normal neutral current splits across both paths: the ground wire, the conduit, the panel enclosure all become current-carrying. That's energized metal where you don't expect it, and it's exactly what the separation rule exists to prevent (NEC 250.32(B)(1) for feeders to separate buildings; the same separation applies to any subpanel).

In practice, this is the thing that fails inspections most: a subpanel installed with the green bonding screw still in the neutral bar, or the ground and neutral landed on the same bar. Pull the bonding screw, separate the bars, land four wires. If you're buying a used panel or working with an old install that has a 3-wire feeder (two hots + neutral, no ground), that was legal before the 2008 NEC under an exception — new work is four wires, no exceptions.

Copper is the better conductor; aluminum is the cheaper feeder. For a long run to a detached garage, the price gap between 3 AWG copper and 1 AWG aluminum is real money — which is why aluminum SER cable dominates residential subpanel work.

The budget classic is 2-2-2-4 aluminum SER — two 2 AWG hots, a 2 AWG neutral, and a 4 AWG ground, all in one cable. 2 AWG aluminum is good for 90 amps at 75°C, and for a dwelling-unit feeder the Code's 83% sizing rule (310.12) brings a 100-amp dwelling feeder down to an 83-amp requirement — so 2-2-2-4 is the standard economical answer for a 100-amp residential subpanel feeder. Have your electrician confirm the 83% rule applies to your setup; it covers feeders serving the dwelling's load, not every feeder in every building.

Aluminum done right is safe — modern AA-8000 series alloy, properly torqued terminations rated for aluminum, antioxidant where specified. Aluminum done wrong (loose lugs, wrong devices) is where the horror stories come from. If the panel lugs say CU/AL or AL, you're fine; torque to spec with a torque screwdriver, because "tight enough" isn't a torque value.

A subpanel in the same building is one thing. A subpanel in a detached garage, shed, or outbuilding adds three requirements:

1. A disconnect at the building. The Code requires a disconnecting means at a separate structure (225.31). A subpanel with a main breaker satisfies this neatly — it's the disconnect. Size it for the feeder.

2. A grounding electrode at the building. A detached structure with a subpanel needs its own ground rod(s) connected to the subpanel's ground bar (250.32(A)). This is in addition to the equipment ground wire in the feeder — the ground rod handles lightning and voltage stabilization; the feeder ground wire clears faults. You need both.

3. Four wires, still. The separate-building rules don't relax the separation — ground and neutral stay apart at the subpanel, bonded only back at the service. The old 3-wire exception is grandfathered for existing installs; new feeders are four-wire.

And the feeder to that detached building is usually underground — here are the burial depths by wiring method.

Wire size follows the feeder breaker, but the feeder breaker should follow a load calculation — the actual loads the subpanel will serve. A 100-amp subpanel feeding two lighting circuits and a receptacle is wasted capacity and wasted copper; a 60-amp subpanel feeding a welder, a compressor, and an EV charger is undersized on day one. Add up the loads, apply the demand factors, then pick the feeder.

Safety disclaimer, plainly: this article teaches how subpanel sizing works — it doesn't replace the Code or a licensed electrician. Feeder work touches the service, the grounding system, and the inspector's checklist. In the Chicago area, most of this work needs a permit and an inspection. Size it yourself to understand the quote; hire the electrician to do the work.

Electrical contractor with feeder offcuts, partial SER reels, or project surplus from panel upgrades? We buy contractor wire surplus at resale value — anywhere in the country. Fast, free quote.

← Back to the full Wire Guide

• Wire Gauge Chart: What Size Wire for Every Amperage — the master ampacity reference behind these numbers.
• How Deep Do You Bury Electrical Wire? — the feeder to a detached building usually goes underground.
• Stranded vs. Solid Wire — feeder conductors are stranded; here's how to terminate them.