What size wire do I need for 15 amps at 100 feet?
Answer
A 15 amp circuit running 100 feet needs 12 AWG copper. It is sized by voltage drop, and lands at 2.5% voltage drop.
How it is worked out
- Ampacity — does it get too hot?
14 AWG carries 15 A at 75 °C - Voltage drop — does enough arrive?
12 AWG keeps 100 ft inside 3% - The larger of the two wins
12 AWG
drop = 2 × I × R × L single phase, √3 × I × R × L three phase
Worth knowing
- Voltage drop at this size
- 5.94 V — 2.5% of 240 V
- Derated ampacity
- 20 A
- Nearest metric size
- 4 mm²
- Heat lost in the cable
- 89.1 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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