The maximum amount of current that could be transferred under normal conditions is called current rating. Similarly, the maximum amount of power a cable can transfer under normal conditions is said to as cable rating. It usually depends on temperature, insulation and the conductor being used. Cable rating is classified as follows:
- Normal Current Rating
Normal current rating refers to how much current can pass through the cable safely. One of the major factors that affect the cable rating is heat. Mostly, the heat is produced due to the resistance the current faces in the core of the cable. However, if the cable is installed in an environment where it is persistently exposed to heat it also affects the current capacity of the cable.
- Over-Current Rating
Over-current rating refers to the flow of current higher than the normal current. The continuous flow of over-current causes severe damage to electrical appliances. To tackle this over-current damages many protection devices are present. But the two most commonly used over current protection devices are fuses and circuit breakers.
In a fuse, a wire is encapsulated between its two terminals. The material of the wire sets the amount of current that passes through the fuse. When a fuse is subjected to over-current condition the wire within the fuse gets heated and breaks its connection.
Circuit breaker works more like a switch when a circuit breaker comes in contact with abnormal current change it changes its state from ON to OFF. So, that when the current becomes normal again and is flowing according to the cable ratings the switch can be turned ON again.
- Short-Circuit Rating
Current has the tendency to moves through the easiest path through the circuit. It means that if a circuit is composed of two paths in which one path possess impedance relevant to the other. The current will move through the path of less impedance neglecting the other path. This phenomenon is called short circuiting and it certainly affects the cable rating. But it usually occurs due to the carelessness of connections, improper installation of cable, damaged insulation, unfused small conductors.