# Calculation of cable cross-section by current, power, length

Today there is a wide range of cable products, with a cross section of conductors from 0.35 mm.kv. and higher.

If you choose the wrong cable cross-section for household wiring, then the result can have two results:

1. An overly thick core will “hit” your budget, as her running meter will cost more.
2. With an unsuitable conductor diameter (smaller than necessary), the cores will begin to heat up and melt the insulation, which will soon lead to self-ignition wiring and short circuit.

As you know, both of these results are disappointing, therefore, before installation of electrical wiring in the house and the apartment must correctly calculate the cable cross-section depending on the power, current and line length. Now we will consider in detail each of the methods.

## Calculation of the power of electrical appliances

For each cable there is a certain amount of current (power) that it is able to withstand during the operation of electrical appliances. If the current (power) consumed by all devices will exceed the permissible value for the conductive core, then soon an accident can not be avoided.

In order to independently calculate the power of electrical appliances in the house, it is necessary to write down the characteristics of each device separately on a piece of paper (stove, TV, lamps, vacuum cleaner, etc.). After that, all values ​​are summed, and the finished number is used to select a cable with conductors with an optimal cross-sectional area.

The calculation formula is:

Ptot = (P1 + P2 + P3 + ... + Pn) * 0.8,

Where: P1..Pn – power of each device, kW

We draw your attention to the fact that the resulting number must be multiplied by a correction factor - 0.8. This coefficient means that of all electrical appliances, only 80% will work simultaneously. Such a calculation is more logical, because, for example, you definitely will not use a vacuum cleaner or a hairdryer for a long time without a break.

Tables for selecting the cable cross section for power:

These are simple and simplified tables; you can find more exact values ​​in PUE p.1.3.10-1.3.11.

As you can see, tabular values ​​have their own data for each particular type of cable. All you need is to find the nearest power value and see the corresponding core section.

In order for you to clearly understand how to correctly calculate the cable by power, we give a simple example:

We calculated that the total power of all electrical appliances in the apartment is 13 kW. This value must be multiplied by a factor of 0.8, which will result in 10.4 kW of actual load. Next, in the table, we look for the appropriate value in the column. We are satisfied with the figure "10.1" for a single-phase network (voltage 220V) and "10.5" if the network is three-phase.

This means that you need to choose a cross-section of cable conductors that will power all the calculating devices - in an apartment, room or some other room. That is, such a calculation must be carried out for each outlet group, powered from one cable, or for each device, if it is powered directly from the shield.In the example above, we gave a calculation of the cross-sectional area of ​​the wires of the input cable for the entire house or apartment.

Total, we select the cross-section on a 6 mm conductor with a single-phase network or 1.5 mm with a three-phase network. As you can see, everything is quite simple and even a novice electrician will cope with such a task on his own!

The calculation of the cable cross section for current is more accurate, so it is best to use it. The essence is similar, but only in this case it is necessary to determine the current load on the wiring. To begin with, according to the formulas, we consider the current strength for each of the devices.

If the house has a single-phase network, for the calculation it is necessary to use the following formula:For a three-phase network, the formula will look like:Where, P - power of the appliance, kW

cos phi power factor

For more information on formulas related to power calculation, see the article:https://my.electricianexp.com/en/kak-najti-moshhnost-toka.html.

Further, all currents are summed up and according to the table values, it is necessary to select the cable cross section for current.

We draw your attention to the fact that the values ​​of the tabular values ​​will depend on the conditions for laying the conductor. At installation of open wiring permissible current loads and power will be much larger than with pipe wiring.

We repeat, any calculation of the cross section is carried out for a particular device or their group.

Cable selection table for current and power:

## Length calculation

Well, the last method that allows you to calculate the cable cross-section - along the length. The essence of the following calculations is that each conductor has its own resistance, which, with an increase in the length of the line, contributes to voltage loss (the greater the distance, the greater the loss). In the event that the value of losses exceeds the mark of 5%, it is necessary to choose a conductor with larger conductors.

For calculations, the following methodology is used:

• It is necessary to calculate the total power of electrical appliances and current strength (above we have provided the appropriate formulas).
• The calculation of the resistance of the wiring. The formula is as follows: specific resistance of the conductor (p) * length (in meters). The resulting value must be divided by the selected cable cross-section.

R = (p * L) / S, where p is the tabular value

We draw your attention to the fact that the length of the current passage must be doubled, because current initially flows through one core, and then returns back through another.

• The voltage loss is calculated: the current is multiplied by the calculated resistance.

LOSS = (Uof losses/ Unom)*100%

• The amount of losses is determined: voltage losses are divided by the voltage in the network and are multiplied by 100%.
• The total number is being analyzed. If the value is less than 5%, we leave the selected core section. Otherwise, we select a “thicker” conductor.

Suppose we calculated that the resistance of the wires we have is 0.5 Ohms, and the current is 16 Amps, then:

Uof losses= 16 * 0.5 = 8 Volts

LOSS = (8/220) * 100% = 0.03636 * 100% = 3.6%

Which is perfectly acceptable for most cases, according to GOST 29322-14 “Standard voltages”. More details in the article:https://my.electricianexp.com/en/kakoe-otklonenie-napryazheniya-v-seti-schitaetsya-predelnym.html.

Resistivity table:

If you stretch the line for a fairly long distance, be sure to make a calculation taking into account losses along the length, otherwise there will be a high probability of the wrong choice of cable section.

## Video calculation examples

Visual video examples always allow you to better absorb information, so we present them to your attention:

Video instruction: how to calculate the cross section of the wires yourself

Video instruction: how to choose the cable diameter?

Similar materials:

Video instruction: how to calculate the cross section of the wires yourself

Video instruction: how to choose the cable diameter?