How to use the Voltage Drop Calculator
- 1
Choose single-phase (or DC) or three-phase, and enter the source voltage and the load in amps.
- 2
Enter the one-way length from the panel to the load, the wire size and copper or aluminum.
- 3
Check the drop against your target. If it's over, the calculator shows the smallest wire that brings it back under.
The voltage drop formula
For single-phase and DC circuits: VD = 2 × K × I × L ÷ CM. For three-phase circuits, replace the 2 with √3 (1.732). K is the resistivity of the conductor (12.9 for copper, 21.2 for aluminum), I is the current in amps, L is the one-way length in feet and CM is the wire's area in circular mils from NEC Chapter 9 Table 8.
Example: 16 A on 12 AWG copper (6,530 circular mils) over 100 feet at 120 V gives 2 × 12.9 × 16 × 100 ÷ 6,530 = 6.3 V, or 5.3%. Going up to 10 AWG brings it to 3.3%.
How much voltage drop is acceptable?
The NEC's informational notes recommend no more than 3% on a branch circuit and 5% in total across the feeder and branch circuit. They're recommendations, not requirements, but equipment makers and engineers design to them because low voltage makes motors run hot and electronics misbehave.
Low-voltage DC systems, like 12 V or 24 V lighting and solar, are far more sensitive: the same current loses a much bigger share of a small voltage, so runs need heavier wire.
Frequently asked questions
What is the maximum voltage drop allowed?
The NEC recommends 3% for a branch circuit and 5% total for feeder plus branch. Some jurisdictions and energy codes make these limits mandatory.
Do I use one-way or round-trip distance?
Enter the one-way distance. The formula's factor of 2 (single-phase) or √3 (three-phase) accounts for the return path.
Does aluminum wire have more voltage drop?
Yes, about 64% more than copper of the same size, which is why aluminum feeders are usually one or two sizes larger.
How do I reduce voltage drop?
Use a larger wire, shorten the run, raise the circuit voltage (240 V instead of 120 V) or split the load across circuits.
Is this accurate for long runs?
It uses the standard K-factor method that electricians use for planning. For engineered feeders, use impedance values from NEC Table 9, which include reactance and power factor.