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Science — Electrical

Voltage Drop Calculator

Pick conductor, wire size, phase, length and current to get the voltage drop, percent and voltage at the load.

Filed by The Statistics Desk  |  Method: Vd = k · I · (R∕1000) · L
Voltage drop
—
Volts lost in the run.
Readout
Percent drop—
Voltage at load—
Conductor resistance—
Within 3% branch limit?—
Show our math
Enter your figures above to see the step-by-step working.
NEC Ch. 9 Table 8 resistances — for reference, verify to codeEverything computed client-side — nothing leaves this page

How the Voltage Drop Calculator Works

Every conductor has resistance, so pushing current down a long wire loses some voltage as heat before it reaches the load. If the drop is too large, motors run hot, LED drivers flicker and heaters underperform. The National Electrical Code recommends keeping the drop under 3% on a branch circuit (and under 5% total including the feeder).

Pick copper or aluminum, the wire size, and whether the circuit is single- or three-phase, then enter the one-way run length and the load current. The tool multiplies the conductor’s resistance per thousand feet (from NEC Chapter 9, Table 8) by the length and current — doubling it for single-phase to account for the return conductor, or using √3 for three-phase — to get the volts lost, the percentage, and the voltage actually delivered.

Vdrop = k × I × (R ÷ 1000) × L  (k = 2 single-phase, √3 three-phase)

Frequently Asked Questions

Do I enter one-way or round-trip length?

Enter the one-way distance from the panel to the load. The formula already accounts for the return path — it doubles the length for single-phase circuits and uses the √3 factor for three-phase — so you do not double it yourself.

What is an acceptable voltage drop?

The NEC recommends no more than 3% on the branch circuit and 5% total from the service to the load. Larger drops waste energy and can stop equipment working correctly; the fix is usually a larger conductor.

Where do the resistance numbers come from?

They are the direct-current resistances of uncoated copper and aluminum conductors from NEC Chapter 9, Table 8, in ohms per 1,000 feet. For the small conductors most branch circuits use, AC and DC resistance are nearly identical.

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