16th Edition (reference only) – NOW superseded by the 17th Edition IEE Regulations.

chapter 5

chapter 6

Cables, conduits and trunking
  4.1 - Cable insulation materials 4.4 - Cable supports, joints and terminations
  4.2 - Cables 4.5 - Cable enclosures
  4.3 - Cable choice 4.6 - Conductor and cable identification
4.3.6 - Thermal insulation correction factors

4.3.6 - Thermal insulation correction factors

The use of thermal insulation in buildings, in the forms of cavity wall filling, roof space blanketing, and so on. is now standard. Since the purpose of such materials is to limit the transfer of heat, they will clearly affect the ability of a cable to dissipate the heat build up within it when in contact with them,

The cable rating tables of [Appendix 4] allow for the reduced heat loss for a cable which is enclosed in an insulating wall and is assumed to be in contact with the insulation on one side. In all other cases, the cable should be fixed in a position where it is unlikely to be completely covered by the insulation. Where this is not possible and a cable is buried in thermal insulation for 0.5 m (500 mm) or more, a rating factor (the symbol for the thermal insulation factor is Ci) of 0.5 is applied, which means that the current rating is halved.

Table 4.5 - Derating factors (CI) for cables up to 10mm² in cross-sectional
---------------- area buried in thermal insulation
Length in insulation (mm)
Derating factor (CI)
500 or more

If a cable is totally surrounded by thermal insulation for only a short length (for example, where a cable passes through an insulated wall), the heating effect on the insulation will not be so great because heat will be conducted from the short high-temperature length through the cable conductor. Clearly, the longer the length of cable enclosed in the insulation the greater will be the derating effect. {Table 4.5} shows the derating factors for lengths in insulation of up to 400 mm and applies to cables having cross-sectional area up to 10 mm².

Commonly-used cavity wall fillings, such as polystyrene sheets or granules, will have an adverse effect on p.v.c. sheathing, leeching out some of the plasticiser so that the p.v.c. becomes brittle. In such cases, an inert barrier must be provided to separate the cable from the thermal insulation. PVC cable in contact with bitumen may have some of its plasticiser removed: whilst this is unlikely to damage the cable, the bitumen will become fluid and may run.


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Extracted from The Electricians Guide Fifth Edition
by John Whitfield

Published by EPA Press Click Here to order your Copy.

Click here for list of abbreviations