Q & A of the Day – Guidance Note 1 - are there de-rating factors regarding cable?

Question: In Guidance note 1 - Selection & Erection - Section 7 'Other influences' it is stated that: 'The use of high temperature cable requires careful selection of termination systems and accessories and switchgear which are appropriate to the operating temperature of the cable. Alternatively, where high temperature cable is to be used, it should be de-rated, so as not to allow the conductor temperature to exceed the maximum operating temperature of the equipment to which it is to be connected'.

Most manufactures of miniature circuit breakers (MCBs) state a utilisation temperature of up to 60oC. Are any MCBs rated at higher operating temperatures in order to avoid de-rating of conductor current carrying capacity? What is the benefit of using an XLPE insulated conductor as it should be de-rated? Are there any de-rating factors from cable manufacturers to consider?

Answer: As you indicate, high temperatures can cause problems with both cables and equipment. XLPE cables are more suitable when high temperatures are present. Temperature de-rating factors can de identified from Table 4C1 (BS7671). With regards to electrical equipment, the manufacturer may be the best source of information.

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