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Voltage Drop Calculator
Enter the conductor, the load and how far the run is. This returns the voltage dropped along the way and what percentage of your supply that is — the number that decides whether you can use the wire in your hand or need to go up a size.
Your numbers
Result
Voltage drop
6.32
%
- Volts dropped
- 7.59V
- Voltage at the load
- 112.4V
- Max run at 3%With this wire and load
- 57ft
- Within 3%?1 = yes, 0 = no
- 0
How this is calculated
- Single phase: VD = (2 × K × I × L) ÷ circular mils.
- Three phase: VD = (1.732 × K × I × L) ÷ circular mils.
- K is the resistivity constant — 12.9 for copper, 21.2 for aluminium.
- L is the ONE-WAY distance. The 2 in the formula accounts for the return conductor.
- Percent drop = volts dropped ÷ system voltage × 100.
Before you order
- The 3% figure is a recommendation in an NEC informational note, not a requirement — but it is what inspectors, engineers and specs use, and 5% total across feeder plus branch circuit is the usual ceiling.
- Voltage drop never replaces ampacity. Size the conductor for the load and the breaker first, then check drop, then upsize if drop fails.
- K = 12.9 assumes about 75°C. A hot conduit run raises resistance and makes real drop slightly worse than this figure.
- Motors are the case that bites: low voltage at start means high current, and that is how windings cook.
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