When designing an electrical system, whether domestic or industrial, it is good to take into account the quality, characteristics and capacity of the electric cables to be used.
The wide variety of sizes and materials can make it difficult even for an expert installer to determine which type of cable is best suited.
It must be said that when a system is designed, it is unlikely to use the same type of electric cable for the whole of the circuit, since the cable terminations will each perform a different function and will be in equally different places within the ‘homes.
Therefore, the electric cable used to install an intercom will be different from that used to bring the light into the rooms or, in the case of large establishments using high consumption machinery, to power certain equipment, varying composition, size and type according to voltage and amperage required.
How an electric cable is made: differences and properties
A cable is presented as follows:
- an internal conductor, generally in bare or tinned copper or – in rare cases – in aluminium, whose structure can be smooth and circular or woven with string and there may be more than one according to function;
- an insulator, a dielectric material that surrounds the conductor;
- a filler (in the case of multipolar cables), whose purpose is to ‘fill’ the empty spaces between the cores (ie, several wires of conductor wrapped in the insulator) inside the sheath;
- a shielding (also called a protective screen or armour), composed of aluminium and polyester or PVC or polyethylene tape, with the aim of providing mechanical protection able to withstand any tearing, bending and micro-abrasion caused or accidental;
- a sheath, to further strengthen the cable against shocks and protect it from dust and debris.
To understand which type of cable is most suitable according to the circumstance, the classification according to the guidelines of the Standard CEI 20-29 is a help. It establishes four classes of cables differentiated according to the conformation of the conductor that provide all the information necessary to understand the intended use.
The classes are divided into:
- insulated cable and rigid single-conductor wire with smooth circular section;
- insulated cable and rigid conductor with braided cord;
- mobile cable and flexible conductor;
- mobile cable and very flexible conductor.
To further establish all the properties, the cables are marked with an abbreviation composed of letters and numbers and each of them indicates a precise value.
The characteristics of a cable (even if not all of them are always shown) are represented in the following order: harmonization status, voltage, insulation, sheath, construction peculiarities and conductor structure.
A cable that shows, for example, the initials ” N07V-K ” is the classic type used for homes and whose characteristics allow easy adaptation to various contexts:
- N stands for ” national type cable ”
- 07 indicates the ” nominal voltage 450/750 V ”
- V designates the presence of ‘polyvinyl chloride (PVC)’ for the sheath
- K indicates conductor ” flexible for fixed laying ”.
Last but not least, there is a distinction also for the colour of the insulation that surrounds the conducting cores of a cable, distinct from each other according to the function they perform: the yellow-green soul is used only for the protective conductor, the blue colour represents the neutral conductor, the brown colour (or black or grey) the phase conductor.
The flow rate refers to the maximum current intensity that passes through an electric cable in optimal laying conditions: it significantly affects the installation of an electrical system, given that the success of the whole project depends on this feature.
If the flow rate of the cables is too small compared to the amount of current that must pass through them, it would incur the joule effect, where the amount of excess energy present inside the cables would be transformed into heat with consequent risk of overheating, short circuit and in the most serious cases, fire.
Determining it is not a simple operation, because it depends on many variables, including the different types of cables used, their size and installation methods, but to overcome this problem, they come to the aid of calculation formulas that help to:
- avoid the oversizing of a line so as to avoid further costs due to the purchase of extra cable meters;
- do not make too abundant calculations and simplify the electrical circuit making it more efficient at the same time.
Example of calculation of the electrical flow rate
Assuming a voltage drop of 2%, in a DC system the calculation is made by dividing the voltage drop by twice the power and then multiplying this result by the voltage. Thus obtained this resistance value, it must be multiplied by 0.0178 after having divided it by the length of the cable.
For an alternating single-phase system, the calculation is the same but related to the single-phase. As for the alternating three-phase, on the other hand, the voltage drop must be divided by the power (and not by the double one) and proceed as described above.