Is voltage drop a code requirement or a recommendation?

For most installations, voltage drop is a recommendation in informational notes, not an enforceable rule. There are exceptions: NEC 647.4(D) and several health-care, fire pump, and sensitive electronic load articles do enforce hard drop limits. Your AHJ can also adopt drop limits as local amendments.

Should I size for ampacity or for voltage drop?

Both. Pick the larger of the two. Sizing for ampacity alone passes inspection on short runs but causes equipment problems on long ones; sizing for voltage drop alone may leave you with a wire too small to carry the current safely.

Does the calculator account for power factor?

It uses the simplified formula above, which ignores reactance. That is accurate for resistive loads and short runs. For long feeders carrying inductive loads (large motors), you should use NEC Chapter 9 Table 9 effective Z values for a more accurate result.

Voltage Drop Calculator

NEC 210.19(A) FPN

Calculate voltage drop for any circuit length and wire size. Verify compliance with NEC 3% and 5% recommendations.

Circuit Configuration

Voltage

Current

Phase

Single-PhaseThree-Phase

Conductor Material

CopperAluminum

Wire Run

Wire Gauge

One-Way Distance

Results

Voltage Drop

7.92V

Drop Percentage

3.30%

Receiving End Voltage

232.1V

WARNING - exceeds 3% (branch), within 5% (feeder + branch)

About this calculator

Voltage drop is the loss of voltage across a conductor due to its resistance. The NEC does not mandate a hard limit, but Informational Note 4 to 210.19(A) and 215.2(A)(1) recommend keeping branch circuit drop under 3% and total drop (feeder plus branch) under 5%. Drop matters because it changes how much voltage actually arrives at the load - too much drop and motors run hot, lights dim, and electronics misbehave. This calculator handles single-phase and three-phase circuits with copper or aluminum conductors.

Formula

For single-phase circuits, voltage drop equals two times the one-way length, times current, times the conductor resistance per foot, divided by 1000. Three-phase circuits use a √3 multiplier instead of 2 because the line-to-line return path is shared across phases.

Single-phase: VD = (2 × K × I × L) ÷ CM
Three-phase: VD = (1.732 × K × I × L) ÷ CM
(K = 12.9 copper, 21.2 aluminum; CM = circular mils; L = one-way feet)

Reference: NEC 210.19(A) Informational Note 4, 215.2(A)(1)

How to use

Choose single-phase or three-phase to match your system.
Enter the system voltage, the load current in amps, and the one-way distance from source to load in feet.
Pick conductor material (copper or aluminum) and a starting wire size.
Read the calculated voltage drop in volts and as a percent of source voltage.
If you exceed 3% on a branch or 5% on a feeder, step the wire size up and recalculate.

Worked example

Setup

20-amp, 120-volt single-phase branch circuit feeding a load 150 feet from the panel using #12 copper THHN.

Calculation

#12 copper has 6,530 circular mils. VD = (2 × 12.9 × 20 × 150) ÷ 6,530 = 11.85 volts. As a percent: 11.85 ÷ 120 = 9.9%.

Answer

9.9% is well over the 3% recommendation. Upsizing to #8 copper (16,510 CM) drops it to about 4.7 volts, or 3.9%, and #6 brings it under 3%.

Related study guides

Frequently asked questions

Is voltage drop a code requirement or a recommendation?: For most installations, voltage drop is a recommendation in informational notes, not an enforceable rule. There are exceptions: NEC 647.4(D) and several health-care, fire pump, and sensitive electronic load articles do enforce hard drop limits. Your AHJ can also adopt drop limits as local amendments.
Should I size for ampacity or for voltage drop?: Both. Pick the larger of the two. Sizing for ampacity alone passes inspection on short runs but causes equipment problems on long ones; sizing for voltage drop alone may leave you with a wire too small to carry the current safely.
Does the calculator account for power factor?: It uses the simplified formula above, which ignores reactance. That is accurate for resistive loads and short runs. For long feeders carrying inductive loads (large motors), you should use NEC Chapter 9 Table 9 effective Z values for a more accurate result.