Voltage Drop Calculator
Calculate voltage drop for copper wire runs. Enter system voltage, current, wire gauge, and one-way distance to find voltage drop and NEC compliance.
About the Voltage Drop Calculator
Voltage drop occurs in every electrical conductor due to resistance. Excessive voltage drop reduces efficiency, causes motors to overheat, and can damage sensitive electronics. The NEC (National Electrical Code) recommends a maximum of 3% voltage drop for branch circuits. This calculator uses the formula VD = (2 × K × I × L) / CM, where K = 12.9 for copper, I = current in amps, L = one-way length in feet, and CM = wire cross-sectional area in circular mils. For calculating your electricity costs, try our Electricity Cost Calculator.
To reduce voltage drop, use a larger wire (lower AWG number), shorten the run, or increase the circuit voltage (240V has half the voltage drop of 120V for the same load). Aluminum wire has a K value of about 21.2, so it drops more voltage than copper for the same gauge. For complete electrical circuit analysis, combine this with our Ohm's Law Calculator to solve for voltage, current, resistance, and power simultaneously.
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