Voltage Drop Calculator

How to use this calculator

1. Choose the conductor metal and wire size (AWG).
2. Pick the system: single-phase, three-phase or DC.
3. Enter the source voltage, load current and one-way length (m or ft).
4. Click Calculate for the voltage drop, % drop and voltage at the load.

How voltage drop is calculated

Voltage drop depends on the conductor resistance, the current, and the round-trip length of the run:

Here k = 2 for single-phase and DC (current flows down and back), and k = √3 (≈1.732) for balanced three-phase. R_/km is the conductor's resistance per kilometre and L is the one-way length in kilometres. Aluminum has about 1.64× the resistance of copper for the same size.

Recommended limits

The US National Electrical Code (NEC) recommends keeping voltage drop to no more than 3% on a branch circuit and 5% total (feeder plus branch). Excessive drop causes dim lights, slow motors and wasted energy.

Copper resistance per AWG (Ω/km)

Aluminum is about 1.64× these values.

Frequently asked questions

What is an acceptable voltage drop?

The NEC recommends no more than 3% on a branch circuit and 5% total (feeder plus branch). Many designers aim for 2–3%.

How do I reduce voltage drop?

Use a larger conductor, shorten the run, raise the system voltage, or split the load across more circuits.

Does voltage drop increase with distance?

Yes — it is directly proportional to the round-trip length of the run, so long cable runs need a larger conductor.