For a 100 amp subpanel, the NEC minimum is 3 AWG copper or 1 AWG aluminum paired with a 100A 2-pole breaker. This assumes THHN/THWN-2 insulation, 75°C terminal ratings, 30°C ambient temperature, and a run under 50 feet.
- Material: Copper (Cu) or Aluminum (Al) — never mix or interchange without specific terminal ratings.
- Insulation: THHN/THWN-2 in conduit, or XHHW-2.
- Temperature Column: 75°C (Standard for modern 100A breakers and panel lugs per NEC 110.14(C)).
- Ambient Temperature: 30°C (86°F) or lower.
- Conduit Type: PVC or EMT with no more than 3 current-carrying conductors.
The Baseline Sizing: Copper vs. Aluminum
Sizing a feeder to a subpanel is fundamentally different from sizing a branch circuit to an outlet. For branch circuits, NEC 240.4(B) allows you to use the "next standard size up" breaker if your wire ampacity falls between standard breaker sizes. This does not apply to feeder sizing when matching a panel's maximum rating. If you are installing a 100A subpanel and intend to utilize its full 100A capacity, the conductor's ampacity must meet or exceed 100A after all derating factors are applied.
According to NEC Table 310.16, looking strictly at the 75°C column (which is dictated by the terminal ratings of standard 100A molded case circuit breakers and panel lugs):
| Conductor Size | Material | 75°C Ampacity | Verdict for 100A Feeder |
|---|---|---|---|
| 4 AWG | Copper | 85A | FAIL (Undersized) |
| 3 AWG | Copper | 100A | PASS (NEC Minimum) |
| 2 AWG | Aluminum | 90A | FAIL (Undersized) |
| 1 AWG | Aluminum | 100A | PASS (NEC Minimum) |
Why not one size smaller? A 4 AWG copper wire is only rated for 85A at 75°C. If your calculated load reaches 90A, a 4 AWG wire will overheat the breaker terminals, potentially causing the breaker to trip prematurely or, worse, degrade the insulation over time. You must hit that 100A threshold.
Variables That Force an Upsize: Distance and Derating
The NEC minimums above assume a short run in a cool environment. In the real world, two main factors will force you to buy thicker, more expensive wire: voltage drop and ambient heat/bundling.
The Voltage Drop Check
NEC 310.15(B) recommends keeping voltage drop on a feeder to 3% or less for reasonable efficiency. Let's run the math for a 100A load on a 240V split-phase system using the standard voltage drop formula: VD = (2 × K × I × D) / CM.
- K (Copper resistivity) = 12.9
- I (Current) = 100A
- CM (Circular Mils for 3 AWG) = 52,620
Scenario A: 100-foot run
VD = (2 × 12.9 × 100 × 100) / 52,620 = 4.9V.
4.9V / 240V = 2.04% drop. This passes the 3% recommendation. 3 AWG copper is fine.
Scenario B: 150-foot run
VD = (2 × 12.9 × 100 × 150) / 52,620 = 7.35V.
7.35V / 240V = 3.06% drop. This fails the 3% recommendation. You must upsize to 2 AWG copper (which yields a 2.4% drop at 150 feet) to maintain efficiency and prevent motor-starting dimming on the subpanel's branch circuits.
Ambient Heat and Conduit Fill
If your conduit runs through a hot attic (e.g., 40°C / 104°F) or you are pulling more than three current-carrying conductors in a single raceway, NEC Table 310.15(B)(1)(1) requires derating. At 40°C ambient, the correction factor for 75°C rated THHN is 0.88.
100A / 0.88 = 113.6A. You now need a wire with a baseline ampacity of at least 114A. 3 AWG (100A) is no longer sufficient; you must upsize to 2 AWG copper (115A) or 1/0 AWG aluminum (120A).
| Installation Condition | Required Copper Size | Required Aluminum Size |
|---|---|---|
| Under 100 ft, 30°C ambient | 3 AWG | 1 AWG |
| 100 ft to 140 ft run | 2 AWG | 1/0 AWG |
| 140 ft to 200 ft run | 1 AWG | 2/0 AWG |
| Attic run (40°C+ ambient) | 2 AWG minimum | 1/0 AWG minimum |
When an Engineer or the AHJ Must Confirm
This guide provides NEC-style guidance for standard residential and light-commercial feeders. Your local Authority Having Jurisdiction (AHJ) has final authority. You must consult a licensed professional engineer (PE) or your local inspector if:
- The subpanel is being fed from a service entrance rather than a downstream distribution panel (service entrance conductors have different sizing rules under NEC Article 230).
- You are installing continuous loads (running 3+ hours) that exceed 80A. Continuous loads require the conductors and breaker to be sized at 125% of the load (e.g., an 80A continuous load requires 100A capacity, but a 90A continuous load requires 112.5A capacity, forcing an upsize).
- Your local municipality has adopted amendments that mandate aluminum-to-copper pigtailing or specific conduit types for underground runs.
Always de-energize the main panel, lock out the breaker, and verify dead with a tested multimeter before terminating any feeder wires. For more on safe termination practices, refer to the Copper Development Association's building wire guidelines.
FAQ: 100 Amp Subpanel Wire Sizing Questions
Can I use 4 AWG wire for a 100 amp subpanel if I only have a 60A load?
Yes, but with a critical caveat. The panel's rating (100A) is simply its maximum capacity. If your actual calculated load is only 60A, you can feed the panel using a 60A 2-pole breaker and 6 AWG copper (or 4 AWG aluminum) wire. The breaker protects the wire, and the wire only needs to handle the breaker's rating, not the panel's physical maximum sticker rating. However, if you are pulling wire to a 100A panel with the intention of utilizing its full 100A capacity now or in the future, 4 AWG copper (85A) is illegal and unsafe for a 100A breaker.
What size ground wire do I need for a 100 amp subpanel?
Per NEC Table 250.122, the minimum equipment grounding conductor (EGC) for a 100A overcurrent protection device is 8 AWG copper or 6 AWG aluminum. This ground wire must be run with the feeder conductors. If you upsized your ungrounded (hot) conductors for voltage drop (e.g., using 1 AWG copper instead of 3 AWG), NEC 250.122(B) requires you to proportionally increase the size of your ground wire as well to maintain the same fault-current clearing impedance.
Does a 100 amp subpanel need a neutral wire?
If you are supplying a standard 120/240V split-phase subpanel that will serve 120V circuits (lights, standard receptacles), yes, you must pull a neutral. The neutral must be insulated (white or gray) and is typically sized identically to the ungrounded hot conductors (3 AWG copper or 1 AWG aluminum). While NEC 220.61 allows you to calculate a reduced neutral size based on maximum unbalanced load, pulling a full-size neutral is the industry standard for subpanel feeders to prevent harmonic overheating and to future-proof the installation. Remember: the neutral and ground must remain strictly separated at the subpanel.
Can I use NM-B (Romex) instead of THHN in conduit for this run?
You can, but the sizing changes. NM-B cable is limited to the 60°C column of NEC Table 310.16, regardless of the 75°C rating of your breaker terminals, due to the thermal limitations of the cable's outer jacket. In the 60°C column, 3 AWG copper is not standardly manufactured in NM-B, and 2 AWG copper is only rated for 95A. Therefore, if you insist on using SER (Service Entrance Cable) or NM-B, you must use 1 AWG copper (rated 110A at 60°C) or 1/0 AWG aluminum (rated 100A at 60°C) to legally feed a 100A subpanel. For most electricians, pulling individual THHN/THWN-2 wires in PVC or EMT conduit is cheaper and allows the use of the 75°C column.
