Skip to content
CalculatorBuddy

Voltage Drop Calculator

Calculate voltage drop for NEC wire data, estimated AWG resistance, or custom wire impedance in DC, single-phase AC, and three-phase AC circuits.

Maintained by CalculatorBuddy TeamEditorial methodology

Add as a preferred source on Google (opens in a new tab)

NEC Data

ExampleSample values — edit any field to see your result.

V
A

Results update as you type.

Voltage drop

0.80V

Estimated result

Voltage drop percentage
0.67%
Voltage at the end
119.20V
Circuit impedance / resistance
0.80422Ω
Method
Based on 75°C, 60 Hz NEC-style AC resistance and reactance data.

Saved calculations stay only in this browser. Share copies a link containing your entered values.

Compare scenarios

Keep a baseline, then change an assumption to see its effect side by side.

Voltage drop0.80 V
Estimate source-to-load voltage loss using NEC-style conductor data, resistance estimated from AWG size, or a manufacturer-provided custom resistance or impedance.

How to use this calculator

Use the length convention and conductor units shown in the form, and select the relevant circuit arrangement. Compare the remaining voltage with the equipment’s operating requirements. This estimate does not replace conductor sizing for current capacity or applicable installation rules.

Examples

Custom 1.2 Ω/km wire

At 120 V, 1 A, and 500 m one way, the calculation is 2 × 1 × 0.5 × 1.2 = 1.20 V, or 1.00%, leaving 118.8 V.

Formula

DC and single phase: Vdrop = 2 × I × L × Z / parallel conductors.

Three phase: Vdrop = √3 × I × L × Z / parallel conductors.

For AC NEC data, effective impedance uses R × PF + X × √(1 − PF²).

This is a planning estimate. Temperature, conductor bundling, terminals, harmonics, installation method, and local electrical code can change conductor sizing.

Data and design limits

The 3% and 5% figures are design guidance reflected in NFPA's published NEC record, not a complete conductor-selection rule. The NEC-style mode uses tabulated resistance and reactance assumptions; the estimated-wire mode uses nominal resistance values; the custom mode is only as accurate as the entered impedance. None checks ampacity, temperature correction, bundling, terminal rating, fault-current duty, overcurrent protection, harmonics, or local amendments. Negative end voltage means the modeled conductor is plainly unsuitable, not that a physical circuit can deliver a negative terminal voltage.

Frequently asked questions

What is voltage drop?
Voltage drop is the loss of electrical potential as current passes through conductor resistance or AC impedance.
Why is one-way distance doubled?
DC and single-phase current travels to the load and back. Three-phase calculations use a square-root-of-three factor instead.
What is an acceptable voltage drop?
NEC informational notes describe 3% at the farthest outlet and 5% total feeder plus branch-circuit drop as a reasonable-efficiency design target in covered contexts. This is not a universal pass/fail rule; applicable code, equipment, and design requirements control.
Does power factor matter?
In AC circuits it changes how resistance and reactance contribute to impedance and therefore voltage drop.

Related calculators