Wire Size Calculator

Written byOleh Soloninupdated 25 September 2026

What you know

Load, kW

The total of the appliances on the circuit.

Supply

Conductor

Installation

Advanced options

Power factor cos φ

1 for heaters and incandescent lamps; motors and power supplies are lower.

Result

Cross-section, mm²2.5
Design current, A16.7
Rated current under the PUE, A27
Breaker20 A
This size carriesup to 5.6 kW at 220 V and power factor 0.95
Current headroom, A10.3
Table usedPUE table 1.3.6, two-core cable in air

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 wire in free air is cooled by it; the same wire in conduit runs hotter, so the tables rate it lower.

Load, breaker and cable are chosen together, so that the design current ≤ the breaker rating ≤ the cable's current rating. If the breaker the load needs is above what a section carries, the section goes up: 16.7 A needs a 20 A breaker, and 1.5 mm² at 19 A is then too small. A breaker rated above the cable lets an overloaded cable overheat before the protection notices.

How to use

Choose what you know: the total load in kilowatts or the current in amps.
Choose the supply: 220 V single-phase or 380 V three-phase.
Set the conductor material and how it is laid — cable in air or in the ground, single-core wires in free air or in a conduit.
Read the cross-section, its rated current from the tables, the breaker and how much power that size can carry.

Cross-sections for common loads

The ratings come from Russia's Electrical Installation Rules (PUE), tables 1.3.4–1.3.7, for conductors at +65 °C, air at +25 °C and soil at +15 °C. Other countries rate cables under their own wiring rules, so read these as the Russian figures.

All rows come from the calculator on this page, with a power factor of 0.95 where the load is in kilowatts.
Load	Laying	Current	Breaker	Size	Rated current
10 A, copper	two-core cable in air	10 A	10 A	1.5 mm²	19 A
16 A, copper	two-core cable in air	16 A	16 A	1.5 mm²	19 A
3.5 kW at 220 V, copper	two-core cable in air	16.7 A	20 A	2.5 mm²	27 A
25 A, copper	two-core cable in air	25 A	25 A	2.5 mm²	27 A
25 A, aluminium	two-core cable in air	25 A	25 A	4 mm²	29 A
25 A, copper	two wires in a conduit	25 A	25 A	2.5 mm²	27 A
7 kW at 220 V, copper	two-core cable in air	33.5 A	40 A	6 mm²	50 A
15 kW at 380 V, copper	three-core cable in air	24 A	25 A	2.5 mm²	25 A

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 tables 1.3.4–1.3.7 of the Russian wiring code (PUE) for rubber- and plastic-insulated conductors, at a conductor temperature of +65 °C with air at +25 °C and ground at +15 °C; other temperatures and bundled cables need correction factors.
Ratings come from PUE tables 1.3.4–1.3.7 (Russian wiring code) for conductors at +65 °C in air at +25 °C; aluminium has its own tables.

Where the numbers come from

Checked against ampacity tables 1.3.4–1.3.7 of the Russian PUE wiring code, chapter 1.3 · verified September 2026.

FAQs

What cable does 16 amps need?

By PUE table 1.3.6, a two-core copper cable of 1.5 mm² in air carries 19 A and 2.5 mm² carries 27 A. A 16 A breaker already fits 1.5 mm²; 2.5 mm² leaves room if more appliances go on the circuit.

Why is the rating lower in conduit?

A wire in an enclosed space sheds heat less easily. A 2.5 mm² copper wire is rated 30 A in free air, and three such wires in one conduit 25 A.

Can I fit a breaker larger than the cable rating?

No. The breaker has to trip before the cable overheats, so its rating cannot exceed the cable's: a 1.5 mm² cable in air (19 A) takes a 16 A breaker, not a 20 or a 25.

How do I size a cable from power?

First find the current: watts divided by 220 V and the power factor, or by 1.732 × 380 V and the power factor on three phases. Then pick the size whose rated current is at least that current and the breaker rating. The calculator does both steps.

Should I account for run length?

Yes, separately: these tables deal with heating, and a long run also has to pass a voltage-drop check — there is a calculator for that. Use the larger of the two sizes.

Which breaker ratings does the calculator use?

The preferred series of IEC 60898-1: 6, 8, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100 and 125 A. A 13 A design current gets a 13 A breaker, not a 16 A one.

Worked example

A socket circuit

Input: 3.5 kW, 220 V, copper cable in air

Output: 16.7 A, 2.5 mm², 20 A breaker

Note: 1.5 mm² would carry 19 A, but a 20 A breaker is too big for it, so the calculator moves up to 2.5 mm², rated 27 A.

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