Wire size for 40 amps at 100 feet
The smallest wire that carries 40 amps safely, then checked so no more than 3% of the voltage is lost over 100 feet at 240 volts.
NM-B house cable (Romex)
8 AWG
2.55% voltage drop
Copper wires in conduit (THHN)
8 AWG
2.55% voltage drop
Aluminum (SER or XHHW)
6 AWG
8 AWG carries the amps; larger keeps the drop to 2.69%
Every size at 100 feet and 40 amps
| Size | Carries, NM-B (60°C) | Carries, conduit (75°C) | Drop, copper | Drop, aluminum |
|---|---|---|---|---|
| 14 AWG | 15 A | 15 A | 10.2% | - |
| 12 AWG | 20 A | 20 A | 6.43% | 10.6% |
| 10 AWG | 30 A | 30 A | 4.03% | 6.67% |
| 8 AWG | 40 A | 50 A | 2.55% | 4.20% |
| 6 AWG | 55 A | 65 A | 1.64% | 2.69% |
| 4 AWG | 70 A | 85 A | 1.03% | 1.69% |
| 3 AWG | 85 A | 100 A | 0.82% | 1.34% |
| 2 AWG | 95 A | 115 A | 0.65% | 1.06% |
| 1 AWG | 110 A | 130 A | 0.51% | 0.84% |
| 1/0 AWG | 125 A | 150 A | 0.41% | 0.67% |
| 2/0 AWG | 145 A | 175 A | 0.32% | 0.53% |
| 3/0 AWG | 165 A | 200 A | 0.26% | 0.42% |
| 4/0 AWG | 195 A | 230 A | 0.20% | 0.33% |
| 250 kcmil | 215 A | 255 A | 0.17% | 0.28% |
| 300 kcmil | 240 A | 285 A | 0.14% | 0.24% |
| 350 kcmil | 260 A | 310 A | 0.12% | 0.20% |
How the size is picked
- Amps first. The wire has to carry the breaker's amps: NEC Table 310.16, with NM-B house cable held to the 60°C column and wires in conduit to the 75°C column. Small wires are capped at 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG copper.
- Then distance. Long runs lose voltage. The size goes up until no more than 3% is lost, the figure the NEC recommends for a branch circuit, using the wire resistances in NEC Chapter 9, Table 8.
- Distance is one way. Measure from the panel to the load; the math counts the trip out and back.
Motors, EV chargers and anything that runs for three hours or more are sized at 125% of their load. Local codes can be stricter; your inspector has the last word.