What Gauge Wire for a 100 Amp Subpanel: Sizing, Distance, and NEC Rules

The Core Sizing Rules and Baseline Assumptions

Wire sizing isn't just about picking a number from a chart; it is about matching the conductor's ampacity to the overcurrent device and the termination temperature limits. The most common mistake DIYers make is looking at the 90°C column in NEC Table 310.16 because THHN wire is rated for 90°C. However, per NEC 110.14(C), standard residential breakers and panel lugs are only rated for 75°C. Therefore, you must use the 75°C column for your final ampacity check.

Voltage Drop: When Distance Forces a Larger Wire

Ampacity tables tell you what size wire will prevent a fire. Voltage drop calculations tell you what size wire will actually deliver usable power to your tools and appliances. NEC 215.2 recommends a maximum 3% voltage drop on feeders. On a 240V circuit, 3% is 7.2 volts.

If your subpanel is located 100 feet from the main breaker, 3 AWG copper carrying a full 100A load will experience roughly a 4.9V drop (about 2.04%). This is well within the safe limit. However, if you are running a feeder out to a detached workshop 150 feet away, that same 3 AWG wire will drop 7.35V (3.06%), pushing you over the recommended threshold. At that distance, you must bump up to 2 AWG copper or 1/0 AWG aluminum.

What Changes the Answer? (Derating and Material Swaps)

Why not use 4 AWG copper, which is cheaper and easier to pull? Because 4 AWG copper is only rated for 85 amps in the 75°C column. While NEC 240.4(B) allows you to round up to the next standard breaker size for specific branch circuits, a 100A subpanel feeder is sized to the panel's busbar rating and the main breaker protecting it. You need a true 100A conductor.

Bundling and Derating: If you pull more than three current-carrying conductors through a single conduit (for example, feeding two separate subpanels in the same PVC pipe), NEC 310.15(C)(1) requires you to derate the wire's ampacity. With 4 to 6 conductors, you must multiply the base ampacity by 80%. A 3 AWG copper wire (100A) derated by 80% drops to 80A, meaning you would need to step up to 1 AWG copper just to maintain your 100A capacity.

Aluminum vs. Copper: Never treat these interchangeably. Aluminum expands and contracts more than copper under thermal load, which can loosen terminal screws over time and cause arcing. If your panel lugs are not explicitly marked 'AL/CU', you cannot use aluminum wire. If they are, you must apply an anti-oxidant compound like Noalox to the stripped aluminum conductor before torquing the lug to the manufacturer's specification (typically 20-25 in-lbs for these sizes).

When an Engineer or the AHJ Must Confirm

While the tables above cover 95% of residential workshop and garage subpanels, you must consult a licensed electrical engineer or your local Authority Having Jurisdiction (AHJ) under the following conditions:

• Continuous Loads: If the subpanel will supply loads that run for 3 hours or more continuously (like commercial grow lights, heavy HVAC, or EV chargers), NEC 215.2(A)(1) requires the conductors to be sized at 125% of the continuous load. If your calculated continuous load is 80A, you must size the wire for 100A (3 AWG Cu), but if the continuous load is 90A, you must size for 112.5A, forcing you to use 2 AWG Cu.
• Extreme Ambient Temperatures: If your conduit runs through an unconditioned attic in Arizona where temperatures exceed 110°F (43°C), you must apply temperature correction factors from the bottom of Table 310.16. At 122°F, the 75°C column requires a 0.82 correction factor, meaning your 3 AWG wire is suddenly only good for 82A.
• Local Amendments: Always check with your local building department. Some municipalities in California and New York have localized energy codes that mandate stricter voltage drop limits (e.g., 2% total combined feeder and branch circuit) which will force larger wire sizes regardless of NEC minimums.

Frequently Asked Questions

Can I use 4 AWG wire for a 100 amp subpanel?

No, not for a standard 100A breaker. In the 75°C column of NEC Table 310.16, 4 AWG copper is rated for 85 amps and 4 AWG aluminum is rated for 70 amps. Because the subpanel's main breaker or feeder breaker is 100 amps, the wire must have an ampacity of at least 100 amps. You must use 3 AWG copper or 1 AWG aluminum to safely and legally protect the circuit without nuisance tripping or overheating the lugs.

What size ground wire do I need for a 100 amp subpanel?

According to NEC Table 250.122, the minimum equipment grounding conductor for a 100-amp overcurrent device is 8 AWG copper or 6 AWG aluminum. This ground wire runs alongside your hot and neutral conductors to provide a low-impedance fault path back to the main panel, ensuring the breaker trips instantly in the event of a short circuit. Do not undersize this; a fault on a heavy load like a welder requires a robust ground path to clear safely.

Does a 100 amp subpanel need a neutral wire?

Yes. Even if you only plan to install 240V equipment (like a large air compressor or EV charger), you must run a dedicated neutral wire to a subpanel. Since the 2008 NEC revision (and reinforced in current codes under NEC 250.32), a 4-wire feeder (two hots, one neutral, one ground) is strictly mandatory for all detached structures and subpanels. The neutral and ground buses must remain completely isolated (separated) in the subpanel, and the neutral carries the unbalanced 120V return current back to the source.

How many wires are required for a 100 amp subpanel?

You need exactly four wires for a standard single-phase 120/240V subpanel feeder: two ungrounded (hot) conductors, one grounded (neutral) conductor, and one equipment grounding conductor. If you are pulling individual THHN wires through conduit, you will pull two black (or black/red) wires, one white wire, and one bare or green wire. If you are using cable assembly like SER or MH feeder, ensure it is a 4-conductor cable (e.g., 3-3-3-5 for copper).