Wire Size Calculator

Choose the smallest wire gauge that keeps voltage drop within your limit for a load and length.

Wire sizes	AWG (US)
Conductor	Copper
Circuit	Single-phase / DC
Supply voltage	230 V
Load current	20 A
One-way cable length	30 m
Maximum voltage drop	3 %

Wire sizes

Conductor

Circuit

Supply voltage

Load current

One-way cable length

Metres for mm², feet for AWG.

Maximum voltage drop

Minimum wire size

14 AWG

Gives 3.68 V (1.6%) drop. 75 °C copper ampacity 20 A (NEC 310.16), subject to the small-conductor limits of 15/20/30 A for 14/12/10 AWG.

Voltage drop by wire size (limit 3%)

14 AWG1.6%
12 AWG1.01%
10 AWG0.63%
8 AWG0.4%

About the Wire Size Calculator

Two things size a cable: it must carry the current without overheating (ampacity, which depends on insulation and installation method) and it must keep voltage drop acceptable. This picks the smallest size meeting your drop limit and, for US copper, the NEC 75 °C ampacity.

The formula

Smallest size with 2 I R L ÷ V ≤ limit (and ampacity ≥ I)

Worked example

Example: Minimum wire size = 12 AWG

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; Maximum voltage drop: 3%.
Formula: Smallest size with 2 I R L ÷ V ≤ limit (and ampacity ≥ I)
Result: Minimum wire size = 12 AWG

Things to keep in mind

Ampacity derating for bundling, temperature and conduit fill isn't applied. A licensed electrician must verify any installation.

Questions people ask

Why is 14 AWG limited to 15 A?

NEC 240.4(D) caps overcurrent protection for small conductors.

Copper or aluminium?

Aluminium needs about two sizes larger for the same current.