Voltage Drop Calculator
Select a wire gauge and enter the current, one-way distance, and source voltage to find the voltage drop.
Resistance values are for solid copper wire at room temperature.
How voltage drop is calculated
Voltage drop is found by multiplying the wire's resistance per foot by the current and by twice the one-way distance (since current travels out and back through the circuit). This calculator uses standard resistance values for solid copper wire at common AWG sizes.
Most electrical codes recommend keeping voltage drop under 3% for branch circuits and under 5% total from source to the farthest outlet — excessive voltage drop can cause dimming lights, reduced motor performance, and inefficient power delivery.
Frequently asked questions
Why is the distance doubled in the calculation?
Current has to travel from the source to the load and then back again to complete the circuit, so the total length of wire the current passes through is twice the one-way distance you measure.
What percentage voltage drop is acceptable?
Most electrical codes recommend no more than 3% voltage drop on a branch circuit and no more than 5% total for the combined feeder and branch circuit — higher drops can cause equipment to underperform or overheat.
How do I reduce voltage drop on a long wire run?
Using a thicker wire (a lower AWG number) reduces resistance per foot and therefore voltage drop for the same current and distance. Shortening the distance or reducing the current drawn also reduces voltage drop.