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

2 AWG copper is the smallest conductor our model estimates at or above 100 A using a 75 °C insulation rating; 1 AWG is required if the terminations are limited to 60 °C. In aluminium the equivalent is 1/0 AWG. Verify against the code edition adopted locally before installing.

Can I use aluminium wire on a 100 amp circuit?

Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 100 A circuit steps up to 1/0 AWG and uses K = 21.2 in the voltage-drop formula instead of 12.9.

How far can a 100 amp circuit run?

On 2 AWG copper the 3% voltage-drop guideline is reached near 92 ft one-way at 120 V and 185 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.

Is a 100 amp breaker sized at 100% of the load?

Only for non-continuous loads. A continuous load is designed at 125%, so 100 A of continuous current becomes 125 A of design current and lands on a 125 A device with conductors sized to match.

What is typically on a 100 amp circuit?

Commonly detached garage, shop and ADU feeders, small service. The load type also drives the receptacle and disconnect ratings, not just the conductor size.

What Size Wire for a 100 Amp Breaker?

Conductor size, aluminium equivalent, derating and maximum run length for a 100 amp circuit — all derived from published conductor geometry and the voltage-drop formula.

Quick answer

A 100 amp circuit needs 2 AWG copper with 75 °C terminations, or 1/0 AWG aluminium. 1 AWG copper applies where equipment terminals are limited to 60 °C. Voltage drop reaches 3% at roughly 185 ft one-way on 240 V and 92 ft on 120 V.

Formula estimate

Copper @ 75 °C

2 AWG

Copper @ 60 °C

1 AWG

Aluminium @ 75 °C

1/0 AWG

3% drop @ 240 V

185ft

Conductor options for 100 A

Estimated ampacity of each candidate size, so you can see the margin you are buying.

Material / rating	Minimum size	Estimated ampacity	Margin over 100 A
Copper, 60 °C	1 AWG	107 A	+7 A
Copper, 75 °C	2 AWG	108 A	+8 A
Copper, 90 °C	3 AWG	105 A	+5 A
Aluminium, 75 °C	1/0 AWG	114 A	+14 A
Aluminium, 90 °C	1 AWG	111 A	+11 A

Ampacity estimated as I = 0.0670 × cmil^0.6505, scaled for insulation rating and material.

Source & method: Conductor areas come from the ASTM B258 relationship d = 5 × 92^((36 − n) / 39). Ampacity is a fitted physical model, not a reproduction of any code table, and runs within roughly ±8% of published values.

Maximum run length on 2 AWG copper

One-way distance before voltage drop passes the 3% and 5% guidelines.

System	3% limit	5% limit	Drop @ 100 ft
120 V 1Ø	92 ft	154 ft	3.24%
240 V 1Ø	185 ft	308 ft	1.62%
208 V 3Ø	185 ft	308 ft	1.62%
480 V 3Ø	427 ft	712 ft	0.7%

Source & method: VD = 2 × K × I × L / cmil single phase, 1.732 × K × I × L / cmil three phase, with K = 12.9 for copper. 2 AWG is 66,371 circular mils and 0.1563 Ω per 1000 ft at 20 °C.

Wire size by run length for 100 A

Ampacity sets the floor; distance is what actually forces the upsize on long runs. Smallest copper conductor that carries the load and still holds 3% drop.

One-way run	120 V 1Ø	240 V 1Ø	480 V 3Ø
50 ft	2 AWG	2 AWG	2 AWG
75 ft	2 AWG	2 AWG	2 AWG
100 ft	1 AWG	2 AWG	2 AWG
150 ft	2/0 AWG	2 AWG	2 AWG
200 ft	3/0 AWG	1 AWG	2 AWG
250 ft	4/0 AWG	1/0 AWG	2 AWG
300 ft	250 kcmil	2/0 AWG	2 AWG

Sized on both criteria at once: estimated 75 C ampacity and 3% voltage drop at the stated one-way distance.

Source & method: A dash means no listed size holds 3% at that distance and voltage — split the load, raise the system voltage, or move the panel closer.

Hot attics, conduit runs and bundling

What a 100 A circuit actually needs once ambient temperature and conductor count are applied.

Ambient	3 conductors	4–6 conductors	7–9 conductors
30 °C	2 AWG	1 AWG	1/0 AWG
40 °C	1 AWG	1/0 AWG	2/0 AWG
50 °C	1/0 AWG	2/0 AWG	3/0 AWG
60 °C	3/0 AWG	4/0 AWG	250 kcmil

Source & method: Ambient correction F = √((Tc − Ta) / (Tc − 30)) with Tc = 75 °C; bundling adjustment uses the standard stepped schedule (1.00 / 0.80 / 0.70). The conductor shown is the smallest copper size whose corrected ampacity still carries 100 A.

Where 100 A circuits are actually used

100 A is the second most common residential service size after 200 A, the standard feed for an accessory dwelling unit or large detached shop, and a frequent commercial subfeeder. On older housing stock it is the existing service that a panel upgrade replaces.

The service and main-feeder allowance is the rule that separates 100 A from ordinary branch-circuit sizing: a dwelling service or main power feeder may be sized at 83% of the rating, which is why 4 AWG copper or 2 AWG aluminium serves a 100 A residential service. That allowance does not extend to the branch circuits or to non-dwelling feeders.

Continuous loads

A continuous load — one expected to run three hours or more, like commercial lighting or an EV charger — is designed at 125% of the load current. 100 A continuous becomes 125 A of design current, which lands on a 125 A overcurrent device and 1 AWG copper.

Frequently asked

What size wire do I need for a 100 amp breaker?: 2 AWG copper is the smallest conductor our model estimates at or above 100 A using a 75 °C insulation rating; 1 AWG is required if the terminations are limited to 60 °C. In aluminium the equivalent is 1/0 AWG. Verify against the code edition adopted locally before installing.
Can I use aluminium wire on a 100 amp circuit?: Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 100 A circuit steps up to 1/0 AWG and uses K = 21.2 in the voltage-drop formula instead of 12.9.
How far can a 100 amp circuit run?: On 2 AWG copper the 3% voltage-drop guideline is reached near 92 ft one-way at 120 V and 185 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.
Is a 100 amp breaker sized at 100% of the load?: Only for non-continuous loads. A continuous load is designed at 125%, so 100 A of continuous current becomes 125 A of design current and lands on a 125 A device with conductors sized to match.
What is typically on a 100 amp circuit?: Commonly detached garage, shop and ADU feeders, small service. The load type also drives the receptacle and disconnect ratings, not just the conductor size.