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.
These are the NEC Chapter 9 Table 8 figures for stranded copper at 75 °C, which is the right basis for voltage drop because a loaded conductor runs hot. Work the same 14 AWG out from raw resistivity at room temperature and you get about 2.5 ohms per 1000 ft instead of 3.14. Neither number is wrong; copper gains roughly 0.4% resistance per degree, and 55 degrees of rise accounts for the whole gap. Using the cold figure understates the drop by about 20%, which is exactly the margin you were trying to check.
Practical Approach When the drop comes out above 3%, the fix is a larger wire. AWG is a logarithmic scale: every three gauge numbers doubles the cross-sectional area and therefore halves the resistance, so going from 14 to 11 (or in practice 14 to 10) roughly halves the drop. One step, say 14 to 12, only cuts it by about a third.
On runs over 100 feet it is normal to go one or two gauges beyond what ampacity alone requires. The extra copper costs a few dollars; chasing a dimming-lights complaint through a finished wall costs considerably more.
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