Calculate the voltage drop and percentage drop for single-phase and three-phase electrical circuits. Enter your wire gauge, conductor material (copper or aluminum), one-way run distance and load current to instantly check against NEC recommendations.
Enter circuit details to calculate.
Voltage drop occurs because every conductor has electrical resistance. The longer the wire and the higher the current, the more voltage is lost as heat.
For single-phase circuits:
VD = (2 × K × I × D) ÷ CM
For three-phase circuits:
VD = (√3 × K × I × D) ÷ CM
Where K = resistivity (10.4 for copper, 17.0 for aluminum), I = current in amps, D = one-way distance in feet, and CM = circular mils of the conductor.
The National Electrical Code (NEC) provides recommended (not mandatory) limits for voltage drop to ensure equipment operates efficiently:
Excessive voltage drop causes lights to dim and flicker, motors to run hot and fail prematurely, and sensitive electronics to malfunction or produce errors.
If calculated voltage drop exceeds 3%, increase the wire gauge one or two sizes. Common scenarios where upsize is needed:
Always consult a licensed electrician. NEC compliance is a minimum — good design aims for less than 2% drop on critical circuits.
The NEC recommends no more than 3% on branch circuits and 5% combined from service to load. Most electricians aim for 2% or less on critical runs.
Yes. Voltage drop wastes energy as heat in the wire. While the wasted amount is usually small, consistently high voltage drop indicates undersized wiring that should be corrected.
Aluminum wiring is code-compliant for feeders and service entrance conductors size #8 and larger. It requires special connectors rated for aluminum and anti-oxidant compound. It should not be used for branch circuit wiring smaller than #8.