Voltage Drop Calculator
Calculate the voltage drop in a copper or aluminum cable for DC, single-phase and three-phase circuits – in AWG and feet or mm² and meters – and check it against the 3% guideline.
The formula
For single-phase AC and DC the current flows out and back, so the cable length counts twice: VD = 2 × L × I × R, with R the resistance of one conductor per unit length. For balanced three-phase circuits, VD = √3 × L × I × R.
Worked example
A 120 V, 15 A circuit, 100 ft one way in 12 AWG copper (1.93 Ω per 1,000 ft): VD = 2 × 100 × 15 × 1.93 / 1,000 = 5.79 V, or 4.83%. That is over the 3% guideline; 8 AWG brings it down to 2.33 V (1.94%), and 10 AWG to 3.03% – just over.
How much drop is acceptable?
The NEC recommends no more than 3% on a branch circuit and 5% in total from the service to the farthest outlet. These are informational notes, not requirements, but following them keeps motors from overheating and lights from dimming.
Resistance values
AWG resistances come from NEC Chapter 9, Table 8 (DC resistance at 75 °C; solid wire for 14–10 AWG, stranded above). In metric mode the calculator uses conductivity 56 m/(Ω·mm²) for copper and 35 for aluminum.
Voltage drop is not the only check
The wire must also carry the current without overheating (ampacity, NEC Table 310.16) and match the breaker. A long run may need a bigger wire for voltage drop than ampacity alone would require – never smaller.
Frequently asked questions
How much voltage drop is acceptable?
The NEC recommends at most 3% for a branch circuit and 5% in total.
What is the voltage drop on 100 ft of 12 AWG at 15 A?
5.79 V on 120 V, or 4.83%.
Does aluminum wire have more voltage drop?
Yes – about 1.6 times copper for the same size, so aluminum is usually two sizes larger.