Voltage Drop Calculator

Estimate voltage drop along a cable run from wire size, length, current and material.

Wire sizes	AWG (US)
Conductor	Copper
Circuit	Single-phase / DC
Supply voltage	230 V
Load current	20 A
One-way cable length	30 m
Wire size	10 AWG

Wire sizes

Conductor

Circuit

Supply voltage

Load current

One-way cable length

Metres for mm², feet for AWG.

Wire size

Voltage drop

1.45 V (0.63%)

Voltage at the load 228.5 V. Within the common 3% guideline for branch circuits.

Other sizes

Size	Drop	%
14 AWG	3.68 V	1.6%
12 AWG	2.32 V	1.01%
10 AWG	1.45 V	0.63%
8 AWG	0.92 V	0.4%
6 AWG	0.59 V	0.26%
4 AWG	0.37 V	0.16%
3 AWG	0.29 V	0.13%
2 AWG	0.23 V	0.1%
1 AWG	0.18 V	0.08%
1/0 AWG	0.15 V	0.06%
2/0 AWG	0.12 V	0.05%
3/0 AWG	0.09 V	0.04%
4/0 AWG	0.07 V	0.03%

About the Voltage Drop Calculator

Current flowing through a cable's resistance loses voltage. For single-phase and DC circuits the current flows out and back (2 × length); three-phase uses √3. US wire resistances are the NEC Chapter 9 Table 8 values at 75 °C; metric sizes use IEC copper and aluminium resistivity at 70 °C. Guidance commonly limits drop to 3% on a branch circuit and 5% overall (BS 7671 allows 3% lighting, 5% other).

The formula

Vd = 2 × I × R × L (single-phase); √3 × I × R × L (three-phase)

Worked example

Example: Voltage drop = 1.76 V (0.7%)

Inputs: Wire sizes: AWG (US); Conductor: Copper; Circuit: Single-phase / DC; Supply voltage: 250 V; Load current: 22 A; One-way cable length: 33 m; Wire size: 10 AWG.
Formula: Vd = 2 × I × R × L (single-phase); √3 × I × R × L (three-phase)
Result: Voltage drop = 1.76 V (0.7%)

Things to keep in mind

Ignores reactance, which matters for large conductors and long AC runs; always follow local wiring codes.

Questions people ask

Why does drop matter?

Motors run hot and lights dim; electronics may misbehave.

Does it waste energy?

Yes: loss = I² R, turned into heat in the cable.