Wire size scenario
For a 15 amps copper circuit at 240 V single-phase, NEC 310.16 ampacity needs 14 AWG at 75 °C. Over 100 feet, voltage drop bumps the recommendation to 12 AWG (about 2.41% drop). Adjust the run below.
Recommended wire size
—Enter a load, length, and voltage to size the conductor.
| Ampacity needs | — |
|---|---|
| Voltage drop needs | — |
| Recommended (larger of the two) | — |
| Ampacity of recommended | — |
| Voltage drop of recommended | — |
At 240 V single-phase over 100 feet, a 15 amps copper load needs 12 AWG (about 2.41% drop). Ampacity alone would allow copper 14 AWG, but over 100 feet voltage drop pushes the recommendation up to 12 AWG to stay within the 3% target (about 2.41%).
This recommendation covers ampacity and voltage drop only, for copper at 75 °C. It does not apply ambient-temperature or conductor-count derating, the 125% factor for continuous loads, conduit fill, or terminal ratings — any of which can push the size up. Aluminum of the same load typically goes up one or two sizes. Always confirm with a licensed electrician and your local code (AHJ).
Ampacity per NEC 310.16; voltage drop per NEC Ch.9 Table 8 (75 °C copper).
Fifteen-amp circuits are common for lighting and general-purpose loads. The familiar small-conductor rule is only the starting point: cable type, terminals, ambient temperature, bundled conductors, and distance can still change the design.
On a short ordinary run, ampacity normally controls. At the 100-foot reference distance used here, voltage drop may justify moving up a size even though the smaller conductor is permitted for overcurrent protection. That upgrade is about performance, not permission to increase the breaker.
Code focus: NEC 240.4(D) limits 14 AWG copper to 15 A overcurrent protection in ordinary applications even when a table column displays a larger ampacity.
| One-way length | Ampacity minimum | Recommended size | Governing check |
|---|---|---|---|
| 25 ft | 14 AWG | 14 AWG | both |
| 50 ft | 14 AWG | 14 AWG | both |
| 100 ft | 14 AWG | 12 AWG | voltage drop |
| 150 ft | 14 AWG | 10 AWG | voltage drop |
For 15 amps of copper at 240 V, NEC 310.16 ampacity at 75 °C needs 14 AWG. Over a 100 feet run, voltage drop calls for 12 AWG, so the recommended size is 12 AWG — the larger of the two. Aluminum, derating, continuous load, and local code may change this; confirm with a licensed electrician.
On a short run where voltage drop isn't a factor, 15 amps of copper needs 14 AWG at 75 °C by ampacity alone (NEC 310.16, with the NEC 240.4(D) small-conductor breaker cap applied). Longer runs need a larger size for voltage drop — this page assumes 100 feet.
Both. The conductor must carry the current (ampacity, NEC Table 310.16) and keep voltage drop within target (NEC Chapter 9 Table 8). The recommended size is the larger of the two. It still doesn't replace ambient or conductor-count derating, the 125% continuous-load factor, conduit fill, or terminal-rating checks — verify with your local code (AHJ).
Source links and page assumptions reviewed .
Estimate only, based on NEC 310.16 ampacity and NEC Chapter 9 Table 8 voltage drop. Not a substitute for a licensed electrician or your local code (AHJ). It does not fully model ambient or conductor-count derating, the 125% continuous-load factor, conduit fill, or terminal ratings. Verify against the current NEC and local code before wiring. Based on NEC 310.16 and NEC Ch.9 Table 8.