Choosing the correct size cable

How to choose the correct size cable?

A standard electrical system is essential for the correct use of all the equipment present in a house, an office or any other building.

When we are building or renovating our electrical system, one of the biggest doubts we can run into is choosing the correct size of electrical cables. Specifically, the electric cables must meet the requirements of the local building regulations and must have the right section to avoid the dangerous overheating of the system. Other aspects involved and factors to be taken into consideration are current absorption, the type of laying, the insulation, the length of the line, and the environmental conditions because cables can be used either for indoor or outdoor installations.

However, when we are dealing with small or medium-sized works, the main factor is certainly the current absorption, expressed in amperes.

In other words, sizing a cable means defining the section based on the intensity of the current flowing through it. 

In the market there are electric cables of different sections, from 1.5 mm² to 35mm²; this data represents the section of an electric cable and indicates the area expressed in square millimetres made up of the copper wires that pass inside it.  

Normally 6 mm² are used for the uprights, 2.5 mm² for the circuits that power the sockets or fixed appliances such as the air conditioner and 1.5 mm² for the light circuits.

Continuing with general information, the larger the section of an electric cable, the greater the current that will pass. Sizing the electrical cables properly means less loss of dissipated energy and therefore optimizing the efficiency of our electrical system.

What do electric cables mean?

Before calculating its section, we need to understand what is meant by electric cables.

The electric cable is an insulated conductor provided with a protective coating. It is made by a metal part, generally copper which is covered with an insulating material. Conductor and insulation form the core of the cable. A cable can be made up of one or more cores. From the outside, it is protected by a sheath.

When we are going to calculate the size of the electrical cables correctly, we can use two different methods. The first is of the maximum allowable voltage drop, the second is called the flow method.

The voltage drop method

The criteria to be considered for this procedure are essentially two. First, it is necessary to know the power used as a consequence we also know the current circulating in the line. To manage the sizing the allowable voltage drop is imposed and the corresponding section is calculated, choosing the section of the cable immediately higher than the calculated one. To make sure that the operation is correct, we must compare the section of the cable to the effective voltage drop: if this value is higher than the allowed one, it is necessary to increase the section and choose a larger cable. 

The flow method

When we use this method we know both the power used and the current circulating in the line, as well as the type of cable, insulation, laying and the number of active cables present in the common conduit is known.

We select the size by a table considering the section of the cable characterized by a capacity immediately higher than the current actually circulating.

Also, the final check is still referred to the table. Note the section, if it is lower than the circulating current, the section of the cable must be increased.

The main sections for correct sizing of electrical cables

If we install cables in outdoor areas, double insulation cables must be used which can consist of one or more conductors such as uni-polar cable, bipolar cable (neutral and phase), tri-polar (neutral, phase, earth), multi-polar cables, up to 4 cores (2 phases, 1 neutral, 1 earth) and one with 5 cores (3 phases, 1 neutral, 1 earth).

In conclusion, the choice of a section of cable that is too small, in fact, would involve a high risk in the safety of both the system itself and, obviously, of the people and things close to it, since we would be in the presence of a high current passing on a section of cable too small.  This could generate high heat along the cable (the so-called Joule effect) and as a consequence short circuits and fires.

Credit to: AB Electrical