At a 75°C termination rating, a 100 A overcurrent device needs at least 3 AWG copper. That is the minimum on ampacity grounds - a long run may need more for voltage drop.
Minimum conductor
By material and termination rating
NEC 2023Every conductor size in this table is transcribed from the 2023 NEC (NFPA 70, 2023). The code is republished on a three-year cycle and adoption varies by jurisdiction - confirm against your locally adopted edition and your authority having jurisdiction (AHJ) before you build.
Smallest conductor whose published ampacity - capped by the small-conductor overcurrent rule where one applies - reaches 100 A, at the table's basis conditions of 30°C ambient and no more than three current-carrying conductors. Derating for a hotter location or a fuller raceway pushes these up.
Material
60°C
75°C
90°C
Copper
1 AWG
3 AWG
3 AWG
Aluminum
1/0 AWG
1 AWG
2 AWG
Read the 90°C column carefully
The 90°C column above is almost never the answer at the terminals. Breakers, panels, and most equipment are listed for 60°C or 75°C conductor terminations, and the conductor must be sized at the temperature the termination is listed for, not the temperature the insulation can survive. The 90°C column exists mainly as a starting point for derating: you may correct down from it for ambient and conductor count, but the result still cannot exceed what the termination allows.
This trips people up constantly. Your THHN is 90°C-rated wire; your breaker is a 75°C termination. The 75°C column governs.
Equipment grounding conductor
NEC 2023Both conductor sizes below is transcribed from the 2023 NEC (NFPA 70, 2023). The code is republished on a three-year cycle and adoption varies by jurisdiction - confirm against your locally adopted edition and your authority having jurisdiction (AHJ) before you build.
Copper
8 AWG
Aluminum
6 AWG
Sized by
Rating of the overcurrent device ahead of the equipment
The equipment grounding conductor is sized from the overcurrent device, not from the circuit conductors. Where the circuit conductors are upsized - for voltage drop, for instance - the grounding conductor must generally be increased proportionally, a rule this reference does not compute.
Distance
How far 3 AWG copper runs at 100 A
NEC 2023Every distance in this table is transcribed from the 2023 NEC (NFPA 70, 2023). The code is republished on a three-year cycle and adoption varies by jurisdiction - confirm against your locally adopted edition and your authority having jurisdiction (AHJ) before you build.
One-way distance at which the circular-mil voltage-drop estimate reaches 3% of nominal, carrying the full 100 A. Beyond this the conductor needs to go up a size for voltage drop even though its ampacity is still fine.
System voltage
Max run at 3% drop
120 V single phase
70 ft
240 V single phase
145 ft
480 V single phase
290 ft
FAQ
Frequently asked questions
What size wire do I need for a 100 amp breaker?
3 AWG copper at a 75°C termination rating, the smallest conductor whose 2023 NEC ampacity, capped by the small-conductor rule where one applies, reaches 100 A. In aluminum the same column calls for 1 AWG. That is the minimum at a 30°C ambient with no more than three current-carrying conductors; derating or a long run pushes it up.
What size ground wire does a 100 amp breaker need?
8 AWG copper or 6 AWG aluminum, from the 2023 NEC equipment grounding conductor table. The grounding conductor is sized from the rating of the overcurrent device, not from the circuit conductors, and generally must be increased proportionally when the circuit conductors are upsized for voltage drop.
Can I use the 90°C column to put smaller wire on a 100 amp breaker?
Usually not. Breakers, panels, and most equipment are listed for 60°C or 75°C conductor terminations, and the conductor must be sized at the temperature the termination is listed for, not what the insulation can survive. The 90°C column of 2023 NEC Table 310.16 mainly serves as the starting point for derating; the result still cannot exceed what the termination allows.
How far can a 100 amp circuit run on 3 AWG copper?
Carrying the full 100 A, the circular-mil estimate this page uses keeps 3 AWG copper inside a 3% voltage drop out to about 70 ft one way at 120 V and 145 ft at 240 V. Beyond that the conductor goes up a size for voltage drop even though its ampacity is still fine. The 2023 NEC treats 3% as a recommendation for efficient operation, not a general requirement.
Source:NFPA 70, National Electrical Code, 240.6(A); NFPA 70, National Electrical Code, Table 310.16; NFPA 70, National Electrical Code, Table 250.122. Published by NFPA.NEC 2023Transcribed from the 2023 NEC (NFPA 70, 2023). Confirm against your locally adopted edition and your authority having jurisdiction (AHJ).