Wire size scenario
For a 40 amps copper circuit at 240 V single-phase, NEC 310.16 ampacity needs 8 AWG at 75 °C. Over 100 feet, voltage drop bumps the recommendation to 8 AWG (about 2.55% 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 40 amps copper load needs 8 AWG (about 2.55% drop). Both checks land on the same size: copper 8 AWG carries 40 amps at 75 °C and stays within the 3% voltage-drop target over 100 feet (about 2.55%).
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).
For 40 amps of copper at 240 V, NEC 310.16 ampacity at 75 °C needs 8 AWG. Over a 100 feet run, voltage drop calls for 8 AWG, so the recommended size is 8 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, 40 amps of copper needs 8 AWG at 75 °C by ampacity alone (NEC 310.16). 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).
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.