Why does the cable need to be bigger than the current rating suggests?

Because voltage drop is a separate limit. On a long run it binds first, and a cable that will not overheat can still leave the load undervolted.

Why does installation method matter?

Heat. A cable in conduit or buried sheds heat more slowly than one clipped in free air, so it carries less current for the same temperature rise.

Construction & HomeelectricalUnited States only

Wire Gauge Calculator

Conductor size from both current capacity and voltage drop. Long runs are limited by drop rather than by current, which is why a cable rated for the load can still be too small.

Also called: cable sizing calculator, conductor size calculator.

Conductor size required

5.21

5.21 mm² required, so use the next standard size of 6 mm². Current capacity alone would allow 5.21 mm², but the 25 m run needs 3.04 mm² to stay within 3%. Current capacity is the binding constraint here, not voltage drop.

Next standard size

Size on current alone

5.21

Size on voltage drop

3.04

Drop at the standard size

1.52

Installation derating

0.8

Which constraint binds

Current capacity is the binding constraint here, not voltage drop.

On regulation

This is a sizing estimate. The applicable wiring or building regulation governs and overrides any figure here.

How this is calculated

Method and background

How this is calculated

Two constraints apply and the larger governs. Current capacity depends on how the cable sheds heat, so a conductor in conduit or buried carries less than the same conductor clipped in free air. Voltage drop depends on the run length and binds on anything long. On a short run the current rating decides; past twenty or thirty metres the drop usually takes over, and sizing on ampacity alone leaves the load undervolted.

the conductor must satisfy both current capacity and voltage drop, and the larger requirement wins

J: Current density
k: Derating factor

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

25 A over 25 metres

Design current: 25 A
One-way run: 25 m
System voltage: 230 V
Permitted voltage drop: 3%
Conductor: Copper
Installation: In conduit

Conductor size required5.21

25 / (6 x 0.8); 2 x 1.68e-8 x 25 x 25 / (230 x 0.03)

Open this example

a long run makes drop the binding constraint

Design current: 25 A
One-way run: 200 m
System voltage: 230 V
Permitted voltage drop: 3%
Conductor: Copper
Installation: In conduit

Conductor size required24.35

boundary: where the constraint switches

Open this example

Method and limits

What it assumes

A single circuit at 30 degrees Celsius ambient.

What it deliberately does not model

Grouping several cables together derates all of them further.
Ambient temperature above 30 degrees derates capacity.
The applicable wiring regulation sets both the ampacity tables and the permitted drop.

Formula version 1.0.0 · definition 1.0.0 · United States · Report a problem with this calculator

Frequently asked questions

Why does the cable need to be bigger than the current rating suggests?: Because voltage drop is a separate limit. On a long run it binds first, and a cable that will not overheat can still leave the load undervolted.
Why does installation method matter?: Heat. A cable in conduit or buried sheds heat more slowly than one clipped in free air, so it carries less current for the same temperature rise.