Wire Size Calculator

Written byOleh Soloninupdated 29 August 2026

Design current, A

Conductor

Installation

Circuit type

Result

Cross-section, mm²2.5
Rated current, A30
Headroom, A14
Headroom, %87.5%
Breaker16 A
Conductorcopper
Code minimum2.5 mm² minimum under the Russian PUE tables

The breaker protects the cable, not the appliance

Picks a cable cross-section for a given current, allowing for conductor material and installation method.

Cross-section follows the current the circuit will carry and whether the cable can shed the heat it generates. A cable in free air is cooled by it; one in conduit is barely cooled at all, so its rating falls by roughly a fifth.

The order of decisions runs from cable to breaker, never the reverse. Choose the section that carries the load, then fit a breaker rated no higher than the cable allows. Get it backwards and an overloaded cable melts before the protection ever notices.

What this calculator shows

The smallest suitable cross-section and its rated current
The headroom in amps and as a percentage
A matching breaker rating

What to keep in mind

Current ratings follow the Russian wiring code table 1.3.4 for rubber and plastic insulated conductors at 25 °C.
Aluminium is rated 23 percent below copper at the same cross-section.

Where the numbers come from

Checked against the Russian PUE wiring code ampacity tables (7th edition).

FAQs

What cable does 16 amps need?

2.5 mm² copper in free air, and 2.5 mm² in conduit too, still with margin. It is the standard for domestic socket circuits.

Why is the rating lower in conduit?

An enclosed cable sheds heat poorly and runs hotter at the same current. The code derates it by about twenty percent.

Can I fit a breaker larger than the cable rating?

Never. The breaker exists to disconnect before the cable overheats. A 25 A breaker on 1.5 mm² is a fire waiting to happen.

Should I account for run length?

Yes, but separately. This table covers heating; on long runs the deciding factor is usually voltage drop, which has its own calculator.

Worked example

A socket circuit

Input: 16 A, copper, clipped in free air

Output: 2.5 mm², rated 30 A

Note: Almost double the headroom, which is correct rather than wasteful. It is why 2.5 mm² became the default for socket circuits on a 16 A breaker.

Useful links

All tools in Electrical

Wire Size Calculator

Result

Related tools

Voltage Drop Calculator

Watts Amps Volts Calculator

Three-Phase Power Calculator

Useful links