Voltage Drop Calculator (NEC)
Calculate voltage drop for copper or aluminum conductors and find the smallest wire size that keeps you within code. Works for single-phase and three-phase circuits, offline, on any phone.
How to use it
- Pick single- or three-phase and copper or aluminum.
- Enter wire size, load current (amps), one-way run length (feet) and system voltage.
- Read the percent drop and voltage at the load instantly; the tool also flags the smallest wire size within 3%.
Codewire uses the circular-mil method: single-phase VD = 2 × K × I × L ÷ CM, and three-phase VD = 1.732 × K × I × L ÷ CM (K ≈ 12.9 for copper, 21.2 for aluminum). NEC 210.19 and 215 recommend keeping branch-circuit drop at or below 3% and total drop (feeder + branch) at or below 5%.
Frequently asked questions
How do you calculate voltage drop?
For a single-phase circuit, voltage drop = 2 × K × I × L ÷ CM, where K is the conductor constant (~12.9 for copper), I is the load current in amps, L is the one-way length in feet, and CM is the conductor's circular-mil area. For three-phase, multiply by 1.732 instead of 2.
What is the maximum voltage drop allowed by the NEC?
The NEC recommends (in informational notes to 210.19 and 215.2) a maximum 3% voltage drop on a branch circuit and 5% total for feeder plus branch. These are recommendations, not hard requirements, but inspectors and designers commonly enforce them.
How do I fix excessive voltage drop?
Increase the wire size (more circular mils), shorten the run, reduce the load, or raise the system voltage. Codewire's voltage drop calculator suggests the smallest wire size that brings you within 3%.
All Codewire calculators · Free for electricians. Verify against the NEC edition adopted by your local AHJ.