How to Calculate Voltage Drop

Voltage drop is the loss of voltage as current travels along a conductor. On long runs it can leave equipment with too little voltage to operate correctly. This guide covers how electricians calculate it in the field.

The voltage drop formula

The common field method uses the conductor's circular-mil area:

Where K is the resistivity constant (about 12.9 for copper, 21.2 for aluminum), I is the load current in amps, L is the one-way length of the run in feet, and CM is the conductor's circular-mil area (NEC Chapter 9, Table 8).

What is the limit?

The NEC recommends (in the informational notes to 210.19 and 215.2) a maximum 3% voltage drop on a branch circuit and 5% total for feeder plus branch. They are recommendations, but most designers and inspectors treat them as targets.

Worked example

A 120 V, 20 A circuit on 12 AWG copper, 100 feet one-way: VD = (2 × 12.9 × 20 × 100) ÷ 6530 ≈ 7.9 V, or about 6.6% — over the 3% target. Moving up to 8 AWG brings it back within range.

How to fix excessive voltage drop

Increase the wire size, shorten the run, reduce the load, or use a higher system voltage. Upsizing the conductor is the most common fix — and the calculator below suggests the smallest wire size that keeps you within 3%.

Open the free Voltage Drop Calculator →

Frequently asked questions

How do you calculate voltage drop?

For single-phase, voltage drop = 2 × K × I × L ÷ CM (K ≈ 12.9 for copper), where I is amps, L is the one-way length in feet, and CM is the conductor's circular-mil area. For three-phase, use 1.732 instead of 2.

What is the maximum voltage drop allowed by the NEC?

The NEC recommends a 3% maximum on a branch circuit and 5% total for feeder plus branch (informational notes to 210.19 and 215.2).

How do I fix excessive voltage drop?

Increase the wire size, shorten the run, reduce the load, or raise the system voltage.

All Codewire calculators · Verify against the NEC edition adopted by your AHJ.