Many 12-volt guides aim for 3% or less on critical loads such as navigation electronics and up to 10% for lighting. Set your own limit in the max drop box.
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 12V 10 amps at 20 feet?
8 AWG copper for a 3% drop, or 10 AWG if you can accept about 4%.
Should I use the round-trip length?
No, enter the one-way distance. VoltageDrop doubles it for the positive and negative conductors.
Why does 24 V help?
For the same power, doubling the voltage halves the current, so the drop in volts halves and the percentage drop falls to a quarter.
Is VoltageDrop free?
Yes, with no sign-up or ads.
Are my inputs saved?
Yes, in this browser only.