Derating cable factors

Appendix 1 Selection of Power cables 16/531 A 16.1 Introduction 16/531 A16.2 Technical details 16/544 A16.3 Service conditions 16/544 A 16.4 Recommended derating factors 16/544 A 16.5 Voltage drop 16/544... 

The selection process of power cables is almost the same as that of a bus system discussed in Section 28.3. For simplicity we consider only the basic data for selection which would suffice the majority of applications. For accurate calculations a similar approach will be essential as for the bus systems (Chapter 28). For site conditions and laying arrangements which may influence the basic rating of a cable, corresponding derating factors have also been provided. The information covered here will be useful to users to meet their cable requirements, although the data may vary marginally for different manufacturers. For more data on cables, not covered here, reference may be made to the cable manufacturers. 

Step 2. Scale-down the cable ratings to suit the derating factors, prepare a revised table -... 

Manufacturers quote current ratings of their cables laid in air at a particular ambient temperature e.g., 25°C, 30°C and 45°C. They also provide tables of derating factors (/fair) based on the chosen ambient temperature, see the above tables, and Chapter 8 of Reference 4. 

From Table 9.22 the nearest cable rated current equal to or greater than 728.9 amps for cables run in air is 740 amps for a 400 mm 3-core cable. This choice would have a spare capacity in the cable of only 1.5%, which is rather low for a practical design. A 400 mm high voltage cable is also difficult to manipulate during laying. A better choice would be two cables in parallel. The same overall derating factor can be used if the two cables are spaced sufficiently far apart. 

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