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Wire Size for 40 Amps
A 40-amp load at 240 V over a 100 ft run needs 8 AWG copper (50 A rating · 6.11 V drop (2.5%)). Change the distance or voltage to re-size.
Wire Size for 40 Amps
Minimum wire size50 A rating · 6.11 V drop (2.5%)
| Size | Ampacity | Voltage drop |
|---|---|---|
| 14 AWG | 20 A | 24.56 V (10.2%) |
| 12 AWG | 25 A | 15.44 V (6.4%) |
| 10 AWG | 35 A | 9.68 V (4%) |
| 8 AWG | 50 A | 6.11 V (2.5%) |
| 6 AWG | 65 A | 3.93 V (1.6%) |
| 4 AWG | 85 A | 2.46 V (1%) |
| 3 AWG | 100 A | 1.96 V (0.8%) |
| 2 AWG | 115 A | 1.55 V (0.6%) |
| 1 AWG | 130 A | 1.23 V (0.5%) |
| 1/0 AWG | 150 A | 0.98 V (0.4%) |
| 2/0 AWG | 175 A | 0.77 V (0.3%) |
| 3/0 AWG | 200 A | 0.61 V (0.3%) |
| 4/0 AWG | 230 A | 0.49 V (0.2%) |
| 250 kcmil | 255 A | 0.41 V (0.2%) |
| 300 kcmil | 285 A | 0.34 V (0.1%) |
| 350 kcmil | 310 A | 0.29 V (0.1%) |
| 500 kcmil | 380 A | 0.21 V (0.1%) |
NEC Table 310.16 (75 °C column), 240.4(D) small-conductor limits and Chapter 9 Table 8 resistance. Apply continuous-load (125%), ambient temperature and bundling corrections, and follow your local code and AHJ.
Planning estimates only. Building codes, loads and material specs vary — have a licensed engineer or contractor confirm quantities and safety requirements. Disclaimer
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