Tick continuous load for loads that run 3 hours or more, such as EV chargers, so the wire is sized for 125% of the current.
How the voltage drop calculator works
VoltageDrop uses VD = 2 × I × R × L ÷ 1000 for DC and single-phase circuits and VD = 1.732 × I × R × L ÷ 1000 for three-phase, where I is the current in amps, R the conductor resistance in ohms per 1000 ft and L the one-way length in feet. Percent drop = VD ÷ supply voltage × 100. The suggested wire is the smallest size whose drop is within your limit and whose ampacity covers the current (125% of it for a continuous load).
Wire gauge chart
| Size | mm² | Copper Ω/1000 ft | Aluminum Ω/1000 ft | Copper amps (75 °C) | Aluminum amps (75 °C) |
|---|---|---|---|---|---|
| 18 AWG | 0.823 | 7.95 | — | — | — |
| 16 AWG | 1.31 | 4.99 | — | — | — |
| 14 AWG | 2.08 | 3.14 | — | 20 (15 A breaker) | — |
| 12 AWG | 3.31 | 1.98 | 3.25 | 25 (20 A breaker) | 20 (15 A breaker) |
| 10 AWG | 5.261 | 1.24 | 2.04 | 35 (30 A breaker) | 30 (25 A breaker) |
| 8 AWG | 8.367 | 0.778 | 1.28 | 50 | 40 |
| 6 AWG | 13.3 | 0.491 | 0.808 | 65 | 50 |
| 4 AWG | 21.15 | 0.308 | 0.508 | 85 | 65 |
| 3 AWG | 26.67 | 0.245 | 0.403 | 100 | 75 |
| 2 AWG | 33.62 | 0.194 | 0.319 | 115 | 90 |
| 1 AWG | 42.41 | 0.154 | 0.253 | 130 | 100 |
| 1/0 AWG | 53.49 | 0.122 | 0.201 | 150 | 120 |
| 2/0 AWG | 67.43 | 0.0967 | 0.159 | 175 | 135 |
| 3/0 AWG | 85.01 | 0.0766 | 0.126 | 200 | 155 |
| 4/0 AWG | 107.2 | 0.0608 | 0.1 | 230 | 180 |
| 250 kcmil | 127 | 0.0515 | 0.0847 | 255 | 205 |
| 300 kcmil | 152 | 0.0429 | 0.0707 | 285 | 230 |
| 350 kcmil | 177 | 0.0367 | 0.0605 | 310 | 250 |
| 400 kcmil | 203 | 0.0321 | 0.0529 | 335 | 270 |
| 500 kcmil | 253 | 0.0258 | 0.0424 | 380 | 310 |
Resistance: NEC Chapter 9 Table 8, stranded uncoated copper and aluminum, DC at 75 °C. Ampacity: NEC Table 310.16, 75 °C column (not more than three current-carrying conductors, 30 °C ambient); breaker limits for 14–10 AWG from NEC 240.4(D). 18 and 16 AWG are shown for low-voltage DC only.
Limits of this calculator
Results are estimates for planning. The calculation uses DC resistance and ignores reactance and power factor, conductor temperature below 75 °C, and derating for heat, conduit fill or bundling. Terminal temperature ratings, insulation type, local codes and the equipment instructions can all require a larger wire. Have a licensed electrician design and inspect mains-voltage work.
Frequently asked questions
What size wire for 40 amps at 150 feet?
At 240 V, 6 AWG copper keeps the drop to about 2.5%; 8 AWG would carry the current but drop about 3.9%.
What size wire for a 20 amp circuit?
12 AWG copper for normal runs. Long runs need bigger wire: at 120 V and 20 A, 12 AWG stays within 3% only up to about 45 ft at full load.
Does this replace the electrical code?
No. Ampacity also depends on insulation, terminal ratings, ambient temperature, bundling and local amendments. Have an electrician confirm the final size.
Is VoltageDrop free?
Yes, with no sign-up or ads.
Are my inputs saved?
Yes, in this browser only.