What size wire do I need for a 100 amp breaker?

1 AWG copper or 1/0 AWG aluminum, per NEC Table 310.16 at the 75°C column. The equipment grounding conductor is 8 AWG copper per NEC Table 250.122.

What is the maximum continuous load on a 100 amp breaker?

80 amps. NEC 210.19(A)(1) and 210.20(A) require the circuit to be rated at 125% of a continuous load — one that runs for three hours or more — which works out to 80% of the breaker rating available for continuous use. Non-continuous loads may use the full 100 A.

What size ground wire for a 100 amp breaker?

8 AWG copper or 6 AWG aluminum, from NEC Table 250.122. The table lists maximum overcurrent device ratings, so you round up to the next listed row. If the circuit conductors are upsized for voltage drop, NEC 250.122(B) requires the ground to be upsized proportionally too.

Can I use 1 AWG NM-B cable on a 100 amp breaker?

Yes. NEC 334.80 requires NM-B ampacity to be taken from the 60°C column of Table 310.16, where 1 AWG copper is rated 110 A. That is why NM-B sometimes needs a larger conductor than THHN in conduit for the same breaker.

100 Amp Breaker Wire Size

Copper Wire

1 AWG

Aluminum Wire

1/0 AWG

Ground (Copper)

8 AWG

Double-pole · 240 V · Max continuous load 80 A · NEC Tables 310.16 and 250.122

100 Amp Circuit — Specification Summary

Item	Value
Breaker type	Double-pole, 240 V
Recommended copper conductor	1 AWG
Recommended aluminum conductor	1/0 AWG
Minimum per NEC Table 310.16, 75°C	3 AWG copper (100 A)
Minimum NM-B cable, 60°C column	1 AWG copper (110 A)
Equipment grounding conductor	8 AWG copper / 6 AWG aluminum (NEC Table 250.122)
Maximum continuous load (125% rule)	80 A
Maximum non-continuous load	100 A

Recommended conductor sizes follow the 75°C column of NEC Table 310.16, the column that matches the termination rating of virtually all modern breakers and lugs (NEC 110.14(C)).

Common Uses for 100 A Circuits

Main service for a small home, detached garage and ADU feeders, large workshop sub-panels

What a 100 Amp Breaker Powers

Load	Notes
Detached garage / workshop sub-panel	Very common 100 A feeder
Accessory dwelling unit	Feeder sized from a load calculation
Small home main service	The old residential standard
Large shop equipment panel	Multiple 240 V machines

Always size a circuit from the equipment nameplate or a load calculation, not from a general table. Air conditioners and heat pumps in particular carry a minimum circuit ampacity (MCA) and maximum overcurrent protective device (MOCP) rating that override the general rules.

NEC requirements for this circuit

At 100 A the choice of conductor depends on what the circuit is. A general feeder is sized from Table 310.16, where 3 AWG copper reaches exactly 100 A at 75°C. For a dwelling service, or a feeder carrying the entire load of a dwelling, NEC 310.12 permits 83% sizing — which is why 4 AWG copper or 2 AWG aluminum is accepted for a 100 A house service. 1 AWG copper is the conservative choice that covers either case with margin for voltage drop.

Feeders do not require GFCI. A 100 A feeder to a separate structure must include an equipment grounding conductor and requires a grounding electrode system at the structure (NEC 250.32(A)), with neutral and ground kept separate in the sub-panel.

Voltage Drop by Distance — 1 AWG Copper at 100 A

One-way run	Drop	% of 240V	Verdict
25 ft	0.77 V	0.3%	Within the 3% branch-circuit target
50 ft	1.54 V	0.6%	Within the 3% branch-circuit target
75 ft	2.31 V	1.0%	Within the 3% branch-circuit target
100 ft	3.08 V	1.3%	Within the 3% branch-circuit target
150 ft	4.62 V	1.9%	Within the 3% branch-circuit target
200 ft	6.16 V	2.6%	Within the 3% branch-circuit target

Single-phase, 240V, full 100 A load, 0.154 Ω/1000 ft DC resistance (NEC Chapter 9, Table 8) at 75°C terminations. NEC 210.19(A) Informational Note recommends keeping branch-circuit drop at or below 3%, and 5% total across feeder plus branch circuit. See the voltage drop formula for the arithmetic.

The 80% rule: a continuous load is one expected to run for three hours or more — EV chargers, electric heat, and most commercial lighting. NEC 210.20(A) requires the overcurrent device to be rated at not less than 125% of that continuous load, which leaves 80 A usable on a 100 A breaker. A breaker that trips after an hour or two under a steady load is usually a continuous-load sizing mistake, not a faulty breaker.

Other Breaker Sizes

15A 20A 30A 40A 50A 60A 100A 200A

Wire size for 100 amps — full page → 1 AWG full specifications → NEC Table 250.122 — ground wire size → NEC Table 310.16 — conductor ampacity → Wire size calculator →