Voltage Drop Calculator
Calculate voltage drop in electrical wire runs based on wire gauge, length, current, and material.
Ensure compliance with NEC 3% and 5% guidelines.
What Is Voltage Drop? Voltage drop is the reduction in voltage as electrical current flows through a wire. All conductors have resistance, and by Ohm’s Law (V = I x R), current flowing through that resistance causes a voltage loss. The longer the wire run and the smaller the wire gauge, the greater the voltage drop.
Why It Matters Excessive voltage drop causes lights to dim, motors to run hot and inefficient, and sensitive electronics to malfunction. The National Electrical Code (NEC) in the United States recommends voltage drop not exceed 3% for branch circuits and 5% for the combined feeder and branch circuit. For a 120V circuit, 3% means no more than 3.6V drop. For a 240V circuit, 3% means no more than 7.2V drop.
The Formula For single-phase circuits: VD = 2 x I x R x L / 1000. Where VD is the voltage drop in volts, I is the current in amps, R is the wire resistance in ohms per 1000 feet (depends on gauge and material), L is the one-way wire run length in feet, and the factor of 2 accounts for the round-trip (supply and return conductors). For three-phase: VD = 1.732 x I x R x L / 1000.
Wire Resistance by Gauge (Copper, per 1000 ft) 14 AWG: 3.14 ohms. 12 AWG: 1.98 ohms. 10 AWG: 1.24 ohms. 8 AWG: 0.778 ohms. 6 AWG: 0.491 ohms. 4 AWG: 0.308 ohms. 2 AWG: 0.194 ohms. 1/0 AWG: 0.122 ohms. Aluminum wire has approximately 1.6 times the resistance of copper for the same gauge.
Practical Approach When the calculated voltage drop exceeds 3%, the solution is to use a larger wire gauge. Increasing the wire size by two gauge numbers roughly halves the resistance and voltage drop. For very long runs (over 100 feet), it is common to upsize wire one or two gauges beyond the minimum required for ampacity.