Wire size for 70 amps at 300 feet
The smallest wire that carries 70 amps safely, then checked so no more than 3% of the voltage is lost over 300 feet at 240 volts.
NM-B house cable (Romex)
Not NM-B
Over 55 amps or this far, run wires in conduit or SER cable.
Copper wires in conduit (THHN)
1 AWG
4 AWG carries the amps; one size up keeps the drop to 2.69%
Aluminum (SER or XHHW)
2/0 AWG
3 AWG carries the amps; larger keeps the drop to 2.78%
Every size at 300 feet and 70 amps
| Size | Carries, NM-B (60°C) | Carries, conduit (75°C) | Drop, copper | Drop, aluminum |
|---|---|---|---|---|
| 14 AWG | 15 A | 15 A | 53.7% | - |
| 12 AWG | 20 A | 20 A | 33.8% | 55.6% |
| 10 AWG | 30 A | 30 A | 21.2% | 35.0% |
| 8 AWG | 40 A | 50 A | 13.4% | 22.1% |
| 6 AWG | 55 A | 65 A | 8.59% | 14.1% |
| 4 AWG | 70 A | 85 A | 5.39% | 8.89% |
| 3 AWG | 85 A | 100 A | 4.29% | 7.05% |
| 2 AWG | 95 A | 115 A | 3.40% | 5.58% |
| 1 AWG | 110 A | 130 A | 2.69% | 4.43% |
| 1/0 AWG | 125 A | 150 A | 2.13% | 3.52% |
| 2/0 AWG | 145 A | 175 A | 1.69% | 2.78% |
| 3/0 AWG | 165 A | 200 A | 1.34% | 2.21% |
| 4/0 AWG | 195 A | 230 A | 1.06% | 1.75% |
| 250 kcmil | 215 A | 255 A | 0.90% | 1.48% |
| 300 kcmil | 240 A | 285 A | 0.75% | 1.24% |
| 350 kcmil | 260 A | 310 A | 0.64% | 1.06% |
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.