Voltage Drop Calculator

About this calculator

Why voltage drop matters

Every conductor has resistance, so current flowing through a run turns a little electrical energy into heat — and the voltage at the load ends up lower than at the source. Too much drop and motors run hot and weak, lights dim, and sensitive electronics misbehave. Sizing conductors for acceptable drop is one of the most common field calculations in electrical work.

The NEC K-method

This calculator uses the resistance-based K-method built on NEC Chapter 9, Table 8 conductor properties:

Single-phase (2-wire): VD = 2 × K × I × L ÷ CM
Three-phase: VD = √3 × K × I × L ÷ CM

K = 12.9 for copper or 21.2 for aluminum (ohms per circular-mil foot at 75 °C), I = load current in amps, L = one-way run length in feet, CM = conductor area in circular mils. Percent drop is VD ÷ source voltage × 100.

What the Code says

The NEC sets no hard voltage-drop limit, but 210.19(A)(1) Informational Note No. 4 and 215.2(A)(1) Informational Note No. 2 recommend sizing feeders and branch circuits so drop stays within 3%, with no more than 5% total from service to the farthest outlet. Treat these as design targets, not inspection pass/fail criteria — the enforceable requirement is that connected equipment operates within its voltage rating.

Worked example

A 3-phase, 480 V feeder carries 100 A over 200 ft in 3/0 AWG copper (167,800 cmil):

VD = 1.732 × 12.9 × 100 × 200 ÷ 167,800 = 2.66 V → 2.66 ÷ 480 = 0.55%

Well under the 3% guideline — 3/0 is plenty for this run.

Field notes

• When drop is too high, the usual fix is upsizing one or two conductor sizes — copper's lower K wins over aluminum at the same size.
• The K-method is a resistance approximation; on long runs or large conductors, AC reactance (NEC Chapter 9, Table 9) adds a correction worth checking on critical feeders.
• K assumes 75 °C conductor properties — hot environments still need separate ampacity derating per 310.15.

Common questions

What voltage drop does the NEC require?

The NEC does not set a hard limit. Informational notes to 210.19(A)(1) and 215.2(A)(1) recommend 3 percent on the branch or feeder and 5 percent total. The enforceable point is that the equipment still sees a voltage inside its rating.

For design, installation, or safety decisions, verify assumptions against the applicable code edition, equipment documentation, and site requirements.