Long wire runs waste power as heat and leave less voltage at the far end — lights dim, motors run hot, and 12 V electronics brown out. Pick the conductor, size, and system, enter the current and one-way length, and this calculator shows the drop in volts and percent, the voltage your load actually sees, and whether you are within the 3 % branch and 5 % total guidelines. It covers 4/0 to 30 AWG and IEC sizes from 0.5 to 300 mm², with temperature correction, and runs entirely in your browser.
If the drop is too high, the wire size calculator finds the smallest gauge that meets your limit. For the underlying relationships see the Ohm's law calculator, and for low-voltage lighting runs the LED resistor calculator sizes the current-limiting part.
Frequently asked questions
What is the voltage drop formula?
For DC and single-phase AC: Vd = 2 × L × I × R, where L is the one-way length, I the current, and R the conductor resistance per unit length. The 2 accounts for current flowing out on one wire and back on the other. For three-phase circuits the factor is √3 (about 1.732) and the percentage is taken against the line-to-line voltage. With a power factor below 1, the resistance term becomes R·cos φ + X·sin φ, where X is the conductor reactance. This calculator uses exactly those forms and corrects R for conductor temperature.
What is an acceptable voltage drop?
The widely used guideline, from the informational notes in the US National Electrical Code (210.19 and 215.2), is no more than 3 % on a branch circuit and 5 % total from the service to the farthest outlet. These are recommendations rather than hard rules in most jurisdictions, but equipment runs cooler and more efficiently within them. Low-voltage DC systems such as 12 V solar, RV, and marine wiring are much less forgiving — 3 % of 12 V is only 0.36 V, so many installers target 3 % or less for critical loads like electronics and 10 % for lighting.
Why does conductor temperature matter?
Copper resistance rises about 0.393 % per °C and aluminum about 0.403 %. Tables usually list resistance at 20 °C, but a loaded conductor in conduit runs much warmer. At 75 °C — the termination rating most sizing calculations assume — copper resistance is about 22 % higher than the 20 °C table value, so the drop is 22 % higher too. The default here is 75 °C; set it to 20 °C to match a room-temperature bench measurement or published 20 °C tables.
Where do the wire resistance values come from?
AWG diameters come from the ASTM B258 gauge formula, and their resistance is computed from the cross-section using standard resistivities: 1/58 Ω·mm²/m for annealed copper (100 % IACS) and 0.028264 Ω·mm²/m for 1350 aluminum (61 % IACS). That gives the familiar 3.277 Ω/km (0.999 Ω/1000 ft) for 10 AWG copper. Metric sizes use the maximum resistances from IEC 60228 for class 2 stranded conductors, which are slightly conservative. Stranded AWG wire runs about 2 % higher than the solid figures.
Should I enter the one-way or round-trip length?
Enter the one-way distance from the source to the load — the calculator doubles it for DC and single-phase circuits because current travels out and back. If you enter the round-trip length you will get twice the real drop. For three-phase circuits the √3 factor already accounts for how currents share the three conductors, so the one-way length is correct there too.