How to select a cable with the right cross-section?

When building electrical systems, a question may arise how to select a cable with the right cross-section to transmit the required current. To do this, current calculations must be performed.

It is important to know the following parameters:

• do you use a copper or aluminum cable;
• number of cores to be loaded;
• maximum temperature of the core in the cable circuit;
• ambient temperature;
• installation method;
• soil resistivity.

Temperature

The maximum permissible temperature of the cable does not mean that this is the maximum ambient temperature at which the cable is still operating. The maximum temperature determines the permissible temperature of the conductor given to the combined effect of the environment, current and various other effects. Temperature-limiting factors can be specific to both materials and installation methods.

Copper, aluminum or aluminum alloy are the most common conductive materials used in power cables. As metals have electrical resistance, the cable conductors heat up because of current. The resistance of the conductor depends on the properties of the specific metal and its alloy, and in order to be able to perform electrical calculations and cable installation without measuring the resistance of each wire, the specific wire resistances and the agreed cross-sections assigned to them are standardised. Therefore, it may sometimes appear that the measured physical diameter is smaller than the cross-section value marked on the cable. It is important to understand that from electrical point of view the resistance of cables is important and the cross section of the cable is sooner an informative value.

Depending on the ambient temperature, the permissible load current of the cables also changes. In Estonia, the normal ambient temperature is considered to be 25°C (15°C in the soil), based on which the load currents are also given in the product catalogues of Prysmian Group Baltics. However, it is important to keep in mind that if any part of the cable passes through an environment with higher temperature (such as a boiler room), the maximum permissible current for the entire circuit must be calculated based on the highest ambient temperature. A similar effect occurs also when several cables are installed in parallel, because a loaded cable heats adjacent circuits.

Installation methods 

The load carrying capability of the cables is also affected by how the heat generated in them will be dissipated. The dissipation of heat from cables can be viewed as different installation methods, in which different standard load-currents are provided according to the cross-section of the cable. For example, a cable installed outdoors cools better than a cable installed in a thermal insulation of the building. In the case of a cable installed in the soil, the thermal conductivity of the soil is important.

The load-currents are calculated according to the standard HD 60364-5-52. The load-current values stated in the product sheets of Prysmian Group Baltics are calculated under the conditions outlined in the tables. The load-currents of the cables used must be adjusted to suit the actual conditions. Due to different factors, the load-currents of one cable may differ several times!

Calculation example

In order to find the maximum permissible load-current, it is necessary to know the installation method first. For example, in the case of direct installation in the soil, an installation method D2 is used. For AXPK 4G240 cable, the maximum load-current of 250 A is specified in the standard. Elektrilevi uses other parameters of the installation environment given to which the load-current must be adjusted.

The adjustment should be as follows:

1. Lower soil temperature has been used. This means that according to the standard, a correction factor of 1.04 must be used. This results in 250 x 1.04 = 260 A.

2. In the catalogue a lower thermal resistivity of the soil has been used. This means that the soil drives heat better away from the cable. For a cable installed directly in the soil, a correction factor of 1.5 is stipulated in the standard, resulting in 260 x 1.5 = 390 A. However, when a cable is installed in the pipe, the installation method is D1. As a result, the currents and correction factors are different, and the result is as follows: 218 x 1.04 x 1.18 = 267.5 A. Values may vary by up to +/- 5% due to updated standards, more accurate calculations and rounding-off.

The article was published in the magazine Onninen uudised.