cable temperature

Electrical design requires selecting the cable with the correct temperature rating. Designers tend to select the cable with the highest temperature rating, this is not acceptable as the first cost of the project will be affected considerably.

A cable temperature rating is based on the type of material used for its insulation. In this article, we shall be discussing how to correctly select the cable temperature rating for certain installation requirements based on the environmental condition.

North American standards have a different cable insulation designation from IEC. It is always recommended to refer to manufacturers' datasheets for cable parameters. Most commonly used cable insulations are PVC (Polyvinyl chloride), XLPE (cross-linked polyethylene), and EPR (Ethylene propylene rubber).

In critical installations, selecting the proper cable insulation type plays an important part in personnel safety. While PVC is the cheapest among the thermoplastic insulations, in a fire, PVC-insulated cables can form HCl fumes; the chlorine serves to scavenge free radicals and is the source of the material's fire retardance, HCl fumes can pose as a health hazard. Frequently in applications where smoke is a major hazard, PVC-free LSOH (low-smoke, zero-halogen) cable insulation are used. The applicable building code should be consulted to determine the type of electrical wires approved for the intended use.

To determine the cable operating temperature when the load current is known, this formula could be used.

To = ( Io/Ir )2 x ( Tc - Ta ) + Ta

Io = load current, A
Ir = derated cable ampacity, A
Note: Derating factors shall include temperature and grouping factor.
To = cable operating temperature, oC
Ta = ambient temperature, oC
Tc = cable design temperature, oC

Using our example from the previous article.

Load current : Io = 180 A
Cable parameters:
Insulation type = XLPE
Derating factor = 0.85 (combined derating factor)
Ambient temperature = 35oC
Cable temperature rating = 90oC (this value varies with manufacturer)

Though we have selected a 50 mm2, it does not necessarily mean that it has enough ampacity for our load. Checking it with a typical cable ampacity for XLPE cables.
50 mm2 = 125 A
70 mm2 = 160 A
95 mm2 = 195 A
120 mm2 = 225 A

considering the cable ampacity, our logical choice will be a size 95 mm2 or larger cable.

Since a 50 mm2 cable has an ampacity lesser than the load current, it is not possible. We could prove that using our operating temperature formula.

Substituting these values into the formula
Let's try the 50 mm2 values first.>br /> To = ( Io/Ir )2 x ( Tc - Ta )+ Ta
To = ( 185/(125 x 0.85) )2 x ( 90 - 45 ) + 45
To = 116oC (116oC > 90oC, thus a 50 mm2 is not suitable.)

Let's try the 95 mm2 values first.>br /> To = ( Io/Ir )2 x ( Tc - Ta ) + Ta
To = ( 185/(195 x 0.85) )2 x ( 90 - 45 ) + 45
To = 74oC (74oC < 90oC, thus a 95 mm2 is the right cable.)