Voltage Drop Calculator
Long cable runs lose voltage as heat in the conductor. Enter your circuit and this calculator shows how many volts you lose, what percentage that is, and what voltage actually arrives at the load. Resistance values follow NEC Chapter 9 Table 8 (copper and aluminium, at 75 °C); ampacity figures are the NEC Table 310.16 75 °C column, given for reference.
Formulas: DC/1-phase VD = 2 × I × L × R ÷ 1000 · 3-phase VD = √3 × I × L × R ÷ 1000 (L in feet, R in ohms per 1000 ft)
Worked example: 20 A over a 30 m (98 ft) run of 2.5 mm²-equivalent 12 AWG copper at 230 V: VD = 2 × 20 × 98 × 1.588 ÷ 1000 ≈ 6.2 V, about 2.7% — inside the usual 3% guidance for a branch circuit. If the result exceeds 5% for the combined feeder and branch circuit, the usual fix is a larger conductor, not a higher supply voltage.
As a rule of thumb from the NEC informational notes, aim for no more than 3% drop on a branch circuit or feeder and 5% combined. These are recommendations, not hard code limits — but motors, in particular, start poorly and run hot when the voltage at their terminals sags. For the smallest conductor that satisfies both ampacity and your drop limit, use the wire size calculator.
Frequently asked questions
What voltage drop is acceptable?
The NEC informational notes suggest a maximum of 3% for a branch circuit or feeder and 5% combined. These are design recommendations, not mandatory limits, but exceeding them causes dim lights, slow motor starts and wasted energy as heat.
Is the length one-way or round-trip?
Enter the one-way length from source to load. The formulas already double it for DC and single-phase circuits (out and back); the three-phase formula uses √3 instead.
Why does aluminium drop more voltage than copper?
Aluminium has about 60% higher resistivity than copper, so the same size aluminium conductor has higher resistance and drops more voltage — which is why aluminium runs usually need a larger size for the same job.