4.3.4 - Ambient temperature correction factors

The transfer of heat, whether by conduction, convection or radiation, depends on temperature difference - heat flows from hot to cold at a rate which depends on the temperature difference between them. Thus, a cable installed near the roof of a boiler house where the surrounding (ambient) temperature is very high will not dissipate heat so readily as one clipped to the wall of a cold wine cellar.

[Appendix 4] includes two tables giving correction factors to take account of the ability of a cable to shed heat due to the ambient temperature. The Regulations use the symbol Ca to represent this correction factor. The tables assume that the ambient temperature is 30°C and give a factor by which current rating is multiplied for other ambient temperatures.

For example, if a cable has a rating of 24 A and an ambient temperature correction factor of 0.77, the new current rating becomes 24 x 0.77 or 18.5 A. Different values are given depending on whether the circuit in question is protected by a semi-enclosed (rewirable) fuse or some other method of protection. The most useful of the correction factors are given in {Table 4.3}.

In {Table 4.3}, '70°C m.i.' gives data for mineral insulated cables with sheaths covered in p.v.c. or LSF or open to touch, and '105°C m.i.' for mineral insulated cables with bare sheaths which cannot be touched and are not in contact with combustible material. The cable which is p.v.c. sheathed or can be touched must run cooler than if it is bare and not in contact with combustible material, and so has lower correction factors.

Mineral insulated cables must have insulating sleeves in terminations with the same temperature rating as the seals used.

Where a cable is subjected to sunlight, it will not be able to lose heat so easily as one which is shaded. This is taken into account by adding 200°C to the ambient temperature for a cable which is unshaded.

Table 4.3 Correction factors to current rating for ambient temperature -------------- (Ca) (from [Tables 4C1 and 4C2] of BS 7671: 1992)
Ambient temperature	Type of insulation
(°C)	70°C p.v.c	85°C rubber	70°C m.i	105°C m.i
25	1.03 (1.03)	1.02 (1.02)	1.03 (1.03)	1.02 (1.02)
30	1.00 (1.00)	1.00 (1.00)	1.00 (1.00)	1.00 (1.00)
35	0.94 (0.97)	0.95 (0.97)	0.93 (0.96)	0.96 (0.98)
40	0.87 (0.94)	0.90 (0.95)	0.85 (0.93)	0.92 (0.96)
45	0.79 (0.91)	0.85 (0.93)	0.77 (0.89)	0.88 (0.93)
50	0.71 (0.97)	0.80 (0.91)	0.67 (0.86)	0.84 (0.91)
55	0.61 (0.84)	0.74 (0.88)	0.57 (0.79)	0.80 (0.89)
Figures in brackets apply to semi-enclosed fuses used for overload protection