Voltage Drop Calculator
Long runs lose voltage. Enter the voltage, load current, one-way length and wire size to see the voltage drop in volts and percent, and the smallest conductor that keeps the drop under your limit. The NEC recommends no more than 3% on a branch circuit and 5% total from service to outlet (informational notes in 210.19 and 215.2).
Your numbers
Result
Enter your numbers to see the result.
Take this to the job site
The same tools, and more, in apps that work offline when there is no signal.
Voltbox: Electrician Toolbox
NICET/ToolsVoltBox has voltage drop, wire size, box fill and conduit calculators built for the job site, with a multimeter guide, and it all works offline.
VoltPrep Electrician Exam Prep
NICET/Tools1,000 practice questions for the journeyman and master electrician exams, with the code-based reasoning behind each answer.
The formula
Single phase: VD = 2 × K × I × L ÷ CM. Three phase: VD = 1.732 × K × I × L ÷ CM. K is the resistivity constant in ohm-circular-mils per foot (12.9 for copper, 21.2 for aluminum at about 75°C), I is amps, L is the one-way length in feet and CM is the circular-mil area of the wire. Percent drop is VD divided by the source voltage.
This is the same direct-current resistance method used in most field tables. It ignores reactance and power factor, so it is accurate for typical branch circuits and slightly optimistic for large conductors on long AC feeders.
Worked example
A 20 A load 150 feet from a 120 V panel on 12 AWG copper (6,530 CM): VD = 2 × 12.9 × 20 × 150 ÷ 6,530 = 11.85 V, or 9.9%. That is far too much; 6 AWG (26,240 CM) brings it to 2.95 V, or 2.5%.
Frequently asked questions
What is an acceptable voltage drop?
The NEC suggests 3% maximum on a branch circuit or feeder and 5% for feeder plus branch combined. These are recommendations (informational notes), not requirements, except where a listing or another rule sets a limit.
Does the length mean one way or round trip?
Enter the one-way distance from the panel to the load. The formula already doubles it for single phase because current travels out and back.
Why use aluminum K = 21.2?
Aluminum is about 64% as conductive as copper, so for the same size it drops about 1.64 times more voltage.
Does temperature matter?
Resistance rises with temperature. The constants used here correspond to about 75°C conductors, the usual assumption for sizing.
Related calculators
This calculator gives an estimate for planning and is not tax, legal or engineering advice. Confirm code-related results against the code edition adopted in your jurisdiction and with your authority having jurisdiction, and follow the manufacturer's instructions.