Voltage Drop Calculator (Copper & Aluminum)
Long runs lose voltage. Enter the material, wire size, load current, one-way length, and system voltage, and this tool computes the volts dropped, the percent drop, and the voltage actually reaching the load — checked against the common 3% branch-circuit guideline.
Voltage Drop Calculator
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Simplified starting point for planning. Does not apply every NEC adjustment/derating factor. Verify against the current NEC and your local AHJ before installing.
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The voltage drop formula
Voltage drop uses the conductor’s resistance constant (K), the load current, and the round-trip or single-leg distance, divided by the wire’s circular-mil area. K is 12.9 for copper and 21.2 for aluminum.
| Step | Example: 12 AWG copper, 20A, 100 ft, 240V, 1-phase |
|---|---|
| Formula (1-phase) | VD = (2 × K × I × L) ÷ CM |
| Circular mils, 12 AWG | 6,530 CM |
| Volts dropped | (2 × 12.9 × 20 × 100) ÷ 6,530 = 7.90 V |
| Percent drop | 7.90 ÷ 240 × 100 = 3.29% |
3-phase runs use 1.732 (√3) instead of 2 as the multiplier. There is no strict NEC voltage-drop limit for most branch circuits, but 3% for branch circuits (5% combined with the feeder) is the widely used design target for keeping motors, lighting, and electronics happy.
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Frequently asked questions
What voltage drop percentage is acceptable?
There is no hard NEC limit for most branch circuits, but 3% on the branch circuit (5% combined with the feeder) is the common industry design target. Motors, long LED runs, and sensitive electronics benefit from staying under it.
What is the difference between the 1-phase and 3-phase formula?
1-phase voltage drop accounts for the round-trip (out and back) current path, so the formula uses a multiplier of 2. 3-phase uses 1.732 (the square root of 3) instead, reflecting the three-conductor relationship.
Does aluminum wire have more voltage drop than copper?
Yes, for the same wire size. Aluminum’s K constant (21.2) is higher than copper’s (12.9), so aluminum conductors need to be sized larger than copper to hold the same voltage drop over the same distance.
Is this voltage drop calculator free?
Yes — free, no sign-up. Enter your circuit details and read the volts dropped, percent drop, and voltage at the load instantly. Estimates only; confirm against the current NEC and your local AHJ.