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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.

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.
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.

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).

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

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?
A common design target is about 3% on a branch circuit and 5% total, but applicable codes and equipment requirements control.
Does power factor matter?
In AC circuits it changes how resistance and reactance contribute to impedance and therefore voltage drop.

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