What size wire do I need for 70 amps at 250 feet?
Answer
A 70 amp circuit running 250 feet needs 2 AWG copper. It is sized by voltage drop, and lands at 2.8% voltage drop.
How it is worked out
- Ampacity — does it get too hot?
4 AWG carries 70 A at 75 °C - Voltage drop — does enough arrive?
2 AWG keeps 250 ft inside 3% - The larger of the two wins
2 AWG
drop = 2 × I × R × L single phase, √3 × I × R × L three phase
Worth knowing
- Voltage drop at this size
- 6.79 V — 2.8% of 240 V
- Derated ampacity
- 115 A
- Nearest metric size
- 35 mm²
- Heat lost in the cable
- 475.3 W
- Which one governed, and why
- On a short run ampacity decides and the answer is whatever the breaker implies. Past twenty or thirty metres voltage drop takes over and grows without limit — here it is voltage drop.
- Using aluminium instead
- Roughly two sizes up for the same performance, and it has its own small-conductor limits: 12 AWG aluminium is capped at 15 A, not the 20 A a copper 12 gets.
- What this does not cover
- Conduit fill, ambient temperature above 30 °C, more than three current-carrying conductors, and your local code — all of which can push the answer up a size.
The same load at other distances
Other loads at this distance
Do it with your own numbers
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