Voltage Drop Calculator
Estimate voltage dropped over a conductor run and percentage voltage loss from wire gauge (AWG), material, supply voltage, distance, and current.
How It's Calculated
Formula
R_{total} = R_{per\,ft} \times L \times k \\ V_{drop} = I \times R_{total} \\ \%_{drop} = \dfrac{V_{drop}}{V_{supply}} \times 100This calculator estimates the voltage dropped across a conductor run and the resulting percentage voltage loss, using a resistive-only model: total resistance is the conductor's per-foot DC resistance times the one-way length times a circuit-topology factor (2x for single-phase/DC 2-wire round trip, sqrt(3) for three-phase). Both copper and aluminum resistance values come directly from NEC Chapter 9 Table 8 (DC resistance, uncoated, 75 deg C reference), per conductor size — not an approximated ratio. This is a resistive estimate under the entered assumptions — it does not model AC reactance, power factor, or conductor spacing, and it is not an NEC or BS 7671 compliance certification. Always verify conductor sizing against your local electrical code before installation.
Worked Examples
120V supply, 20A, 100 ft one-way, 12 AWG copper, single-phase
- R per ft = 1.93 / 1000 = 0.00193 ohm/ft
- R total = 0.00193 x 100 x 2 = 0.386 ohm
- Voltage drop = 20 x 0.386 = 7.72 V
- Percentage drop = 7.72 / 120 x 100 = 6.43%
Frequently Asked Questions
Is the conductor length one-way or round-trip?
One-way — enter the distance from the source to the load, not round-trip. The circuit type setting (single-phase/DC vs three-phase) already applies the correct multiplier to account for the return conductor, so entering a pre-doubled length would double-count it.
Does this account for AC reactance or power factor?
No. This is a resistive-only estimate. It does not model AC reactance, power factor, or conductor spacing, since those inputs are not collected here. For long AC runs where reactance is significant, this estimate may understate the true voltage drop.
Is this result NEC or BS 7671 compliant?
No. This calculator produces an estimated voltage drop under the entered assumptions. It is not a certification of compliance with the NEC, BS 7671, or any other electrical code — verify conductor sizing against your local code and, where required, have the work reviewed by a licensed electrician.
Where does the aluminum resistance value come from?
It comes directly from NEC Chapter 9 Table 8, the same source used for copper, which publishes DC resistance per conductor size for both materials — so aluminum values are not an approximated ratio of the copper values.