Temp

Temp

Temp:
Condition for installation of cables in Pakistan are often quite different from those in Europe and America.
Thus wiring regulations used in these countries do not give much information or advice on installation of general wiring cables in extreme conditions.
Also the current derating factors used for cables operating at higher than normal temperatures unfortunately differ from country to country thereby causing confusion.
In order to produce a practical guide for given, based on temperature rise, which combine both the English and American recommendations.
The current rating for cables insulated in the four temperature ranges most common in Pakistan are listed in Tables 1 to 20.
From the Tables 1 to 20 it can be seen that hotter the conditions of installation, the less the current carrying of a cables. To understand this effect, two definitions must be given:

Ambient Temp: The temperature of air or material surrounding a cables.

Conductor Temp: The temperature of the conductors of a cables when
current is passed through it.

Thus conductor temp = ambient temperature + temperature rise to current.

All conductor have maximum continuous operating temperature above which the insulation starts to deteriorate, and the increased resistance of the heated conductor causes excessive volt drop.
If a cable has rated full load current passing continuously through it and the imbient temperature is raised, allowable value.
Also, if a fault occurs, resulting in a fault current not quite large enough to operate the protective device, then again the cable will heat up to beyond its maximum temperature.
For these resons the rated current of a cable has to be decreased if the ambinet temperature is high.

Example: 12½ h.p three phase squirrel cage induction motor of continuous rating Full load current stated on nameplate = 18 amps:
Declared supply voltage 440 volts.

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