What size wire for 100 amps?
3 AWG copper — at 75 °C terminals, NEC Table 310.16. Aluminum: 1 AWG.
A 100 A circuit needs 3 AWG copper, rated 100 A at 75 °C. At this size the conductors are single conductors in conduit or SER cable; NM-B is not manufactured heavy enough to apply.
Safety first. This is a starting point before derating, not a load calculation. Wire sizing affects fire and shock safety — verify against the current NEC and have circuits designed or reviewed by a licensed electrician for your jurisdiction.
100 amp wire size by material and terminal rating
| Material | 60 °C (NM-B) | 75 °C (typical) | 90 °C |
|---|---|---|---|
| copper | 1 AWG (110 A) | 3 AWG (100 A) | 3 AWG (115 A) |
| aluminum | 1/0 AWG (100 A) | 1 AWG (100 A) | 2 AWG (100 A) |
Sizes are the smallest conductor whose ampacity and NEC 240.4(D) breaker cap both reach 100 A, before any derating. Use the 75 °C column unless every terminal, breaker and device in the circuit is rated otherwise (NEC 110.14(C)).
If the load runs continuously
A load operating for three hours or more is a continuous load, and NEC 210.19(A)(1) requires the conductor to be sized at 125% of it — 100 A becomes 125 A for sizing purposes. That moves this circuit to 1 AWG copper , one or more sizes above the 3 AWG the plain table suggests. EV chargers, electric heat and commercial lighting are the loads this catches most often.
How far can a 100 amp circuit run?
The table above only prevents overheating. On a long run the limit is voltage arriving at the far end, and it bites well before ampacity does. These are the longest one-way runs for 3 AWG copper at 100 A within the 3% drop recommended by the NEC 210.19(A) informational note — and what going up to 2 AWG buys you.
| System | Typical use | 3 AWG | 2 AWG |
|---|---|---|---|
| 120 V | standard receptacle and lighting circuits | 91 ft | 115 ft |
| 240 V | ranges, dryers, EV chargers, feeders | 183 ft | 230 ft |
| 12 V DC | automotive, marine, solar and battery runs | 9 ft | 12 ft |
Single-phase, one-way distance, copper at 20 °C. Aluminum reaches its limit at roughly 60% of these distances. The 3% target is a recommendation, not a code requirement.
What runs on a 100 amp circuit
- subpanels and detached garages
- small dwelling services
- large shop panels
The catch at 100 amps
100 A has two different right answers. A subpanel fed from an existing service uses Table 310.16 (3 AWG copper / 1 AWG aluminum), but a dwelling's MAIN service may use the reduced 310.12 table (4 AWG copper / 2 AWG aluminum). Same amperage, different table, different wire.
Working the other way round? See how many amps 3 AWG carries , including its diameter, resistance and full run-length table.