Power section

How to select the cable cross-section based on power and current

The cross-section is selected not "by eye", but according to the current that will flow through the conductor. If you choose a thinner one than necessary, the insulation overheats and ages, if thicker, you pay for extra copper. Below are tables for quickly estimating the cross-section, and a formula for non-standard loads.

First, let's measure the current.

The current through the load is considered simple:

I = P / (U × cosφ)

Here P is power in watts, U is voltage (220 V or 380 V), cosφ is power factor. For household loads (heaters, stove, lighting) cosφ is close to 1, so for a house the formula simplifies to I = P / 220.

Example. Oven 3.5 kW: current ≈ 3500 / 220 ≈ 16 A. According to the table below, copper 2.5 mm² wire (up to 27 A) is suitable for such current with a good margin.

Copper cable, concealed wiring

Cross-sectional area, mm²	Current, A (≈)	220 V, 1 phase, kW (≈)	380 V, 3 phases, kW (≈)	Typical application
1,5	19	4,1	12,5	Lighting
2,5	27	5,9	17,8	Socket groups
4	38	8,3	25,0	Power sockets, hob
6	46	10,1	30,3	Apartment entrance, electric stove
10	70	15,4	46,0	House entrance.
16	85	18,7	56,0	Input, power consumers

The power in the table is calculated at cosφ ≈ 1. For a three-phase network with 380 V, the total power is indicated for three phases.

Aluminum cable

Aluminum carries approximately a quarter less current than copper of the same cross-section, and according to modern standards, it is used from 16 mm² and above. Approximately by current rating: 16 mm² - up to 60 A, 25 mm² - up to 75 A, 35 mm² - up to 90 A, 50 mm² - up to 110 A. For household wiring, aluminum is practically not used today - copper is preferred.

The values are approximate and given according to the Electrical Code for typical conditions. The actual permissible current depends on the method of installation (in the wall, in a pipe, in the open air), temperature, and the number of cables laid nearby. Allow for a 20–30% margin, and increase the cross-section on long runs due to voltage drop. Choose a circuit breaker so that its rating does not exceed the cable's permissible current. For critical lines, it is better to entrust the electrical calculations to a professional.

Further on the topic: current load table by cross-section (open and hidden laying) and calculation of cable consumption for an apartment.

Selecting a cable of the required cross-section: VVG cable, PVC wire or all каталог.