Voltage drop Calculator

Volts lost in the run, the percentage against the NEC 3% guideline, and what actually arrives at your load — DC, single-phase or three-phase.

Circuit details

Distance from panel to load — the round trip is handled for you.

Voltage drop

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Volts lost—
Voltage at load—
Circular mils—
NEC 3% check—
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Method & assumptions

Uses the standard K-factor formula: VD = 2 × K × I × L ÷ CM for DC/single-phase (√3 replaces 2 for three-phase), with K = 12.9 Ω·cmil/ft for copper and 21.2 for aluminum at 75 °C — the values used in NEC exam prep and most trade references. Real-world drop varies slightly with conductor temperature and power factor. The NEC recommends (Informational Note, 210.19) limiting branch-circuit drop to 3% and total feeder + branch to 5% — a design guideline, not a code mandate.

Why voltage drop matters

Long runs act like a resistor in series with your load: lights dim, motors run hot and die early, chargers slow down, and sensitive electronics brown out. The wire size that is legal for the current is not automatically adequate for the distance — a 12 AWG circuit fine at 20 ft can lose over 6% at 100 ft.

The formula, in plain terms

VD = 2 × K × I × L ÷ CM. Current has to travel out and back, hence the 2 (three-phase circuits share return paths, so √3 ≈ 1.732 replaces it). K bundles the conductor's resistivity; CM is the wire's cross-section in circular mils — doubling the copper halves the drop.

Fixing a failing run

Three options, in the order electricians usually try them: upsize the wire (each AWG step down in number cuts drop ~37%), raise the system voltage if the equipment allows (going 120 V → 240 V quarters the percentage drop for the same power), or shorten the run. The calculator suggests the smallest wire that passes 3% automatically.

Low-voltage systems feel it most

At 12 V, losing 0.9 V is already 7.5% — the same 0.9 V at 240 V is 0.4%. This is why solar, RV, boat and LED-strip wiring uses startlingly thick cable for tiny currents, and why our solar sizing pairs naturally with this tool.

Frequently asked questions

What is an acceptable voltage drop?
The NEC's informational note recommends no more than 3% on a branch circuit and 5% combined feeder + branch. It's a design guideline for performance, not an enforceable code limit — but inspectors and engineers treat it as the standard.
Is the length one-way or round trip?
Enter the one-way distance from panel to load. The formula's factor of 2 (or √3 for three-phase) accounts for the return path automatically.
What K value does this use?
12.9 Ω·cmil/ft for copper and 21.2 for aluminum, at the 75 °C terminal rating — the constants used in NEC training material. Actual resistance varies a little with temperature and conductor stranding.
Why is three-phase drop lower?
Balanced three-phase circuits share return current between phases, so the multiplier is √3 (≈1.732) instead of 2 — about 13% less drop for the same wire, current and distance.
Does voltage drop waste electricity?
Yes — the lost volts become heat in the wire. A 6% drop is roughly 6% of that circuit's energy heating your walls, on top of the performance problems at the load.

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