What Size Wire for a 100 Amp Service?

For a single-family dwelling service, use 4 AWG copper or 2 AWG aluminum. A 100 amp subpanel or feeder serving only part of the load needs larger wire: 3 AWG copper or 1 AWG aluminum.

The answer

Dwelling service: 4 AWG copper / 2 AWG aluminum

Subpanel or feeder: 3 AWG copper / 1 AWG aluminum

Wire size for 100 amp service
Case Copper Aluminum
Dwelling service Single-family home, 83% rule (required 83 A) 4 AWG 85 A, 21.2 mm² 2 AWG 90 A, 33.6 mm²
Subpanel / feeder Part of the load or non-dwelling (required 100 A) 3 AWG 100 A, 26.7 mm² 1 AWG 100 A, 42.4 mm²

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How the answer is worked out

For a single-family dwelling, NEC 310.12 lets the service conductors carry 83% of the rating. 100 A × 0.83 = 83.0 A. The smallest copper size rated at least that much at 75 °C is 4 AWG (85 A), and the smallest aluminum size is 2 AWG (90 A).

A subpanel or feeder that serves only part of the load, or any non-dwelling service, takes the full rating with no 83% reduction. For 100 A at 75 °C that is 3 AWG copper or 1 AWG aluminum.

This tool uses the 75 °C column for every service size. The 60 °C column is not used for service conductors here, so the sizes above are the values to compare with your utility and inspector.

The 83% rule for dwelling services

NEC 310.12 lets the service conductors for a single-family dwelling, which carry the entire load of the home, be sized at 83% of the service rating when the rating is 100 to 400 A. For a 100 amp service, 83% is 83 A. 4 AWG copper is rated 85 A at 75 °C, and 2 AWG aluminum is rated 90 A, so each covers it.

Why a 100 amp subpanel is different

The 83% allowance applies only to the conductors that carry the whole dwelling load. A feeder to a subpanel that supplies only part of the load is sized from the ordinary table: a 100 amp breaker needs 3 AWG copper (100 A) or 1 AWG aluminum (100 A) at 75 °C.

Before you buy wire

Your utility and local inspector decide the service entrance requirements, including the size of the conductors and whether the 83% reduction is permitted where you live. The reduction does not apply where hot ambient conditions require a correction, and long runs need a voltage-drop check as well. Confirm everything with your authority having jurisdiction.

How far can you run it? (3% voltage drop)

The distance table uses the same resistance-only model as the calculator, with a 3% limit and the wire at 75 °C. Aluminum has about 1.6 times the resistance of copper for the same size, so an aluminum run of the same length drops more voltage.

Assumes the full 100 A load on a single-phase 240 V circuit, a 75 °C conductor, copper or aluminum, a 3% voltage drop limit and the one-way length in the table. Aluminum shows a dash where that size cannot carry the full current.

Maximum one-way length for 3% drop at 100 A, 240 V, 75 °C conductor
Wire size Copper Aluminum
3 AWG 147.3 ft [44.9 m] —
2 AWG 185.7 ft [56.6 m] —
1 AWG 234.2 ft [71.4 m] 142.2 ft [43.4 m]
1/0 AWG 295.3 ft [90.0 m] 179.3 ft [54.7 m]

Before you buy wire

The table is a starting point. Work through this list before you order conductors, and keep the answer that matches your real installation.

This is general information based on NEC tables, not a permit or an installation plan. Local amendments, the equipment nameplate and an inspector decide what is acceptable. Read the disclaimer.

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