Technical Guidance: Current carrying capacity for cables

When current flows through a conductor, heat will be generated. The amount of heat generated will depend on the magnitude of the current and the resistance of the cable. This can be calculated using this formula: heat in Watts = I2R. If the conductor is insulated, the material used for the insulation will need to withstand the heat generated without degrading or being destroyed.

Appendix 4 of BS7671 provides tables of current carrying capacities for a wide range of cable types and installation methods. For cables that are not buried, the tables assume an ambient temperature of 30°C and a ground temperature of 20°C for buried cables. For PVC insulated conductors, the maximum current carrying rating assumes a conductor temperature of 70°C.

These ratings assume that the maximum current for the cable is continuous for an extended period. 70°C is very hot and at this temperature the PVC insulation will not be damaged but will degrade over a shorter period of time than a cable run at a lower temperature.

If the ambient air temperature, or ground temperature, is lower than 30°C or 20°C respectively then the cable rating can be increased as heat will be more easily radiated from the cable. For higher temperatures, the rating of the cable will be reduced. Appendix 4 of BS7671 provides a range of rating factors (Ca) for various insulation types and temperatures.

If cables are enclosed in conduit or trunking this will reduce the maximum current rating as the ability by the cables to radiate heat is reduced. For instance a 6/2.5mm2 PVC twin and earth cable is rated at 47A when clipped direct to a wall but will be de-rated to 38A if installed in a conduit or trunking. If the same 6/2.5mm2 is installed above a plasterboard ceiling with thermal insulation not exceeding 100mm, the rating would be 34A.

If cables are run together with other cables running at more than a third of their grouped rating then a further de-rating factor will need to be applied (Cg) which also can be found in Appendix 4 of BS7671.

Unless a cable is supplying a fixed load that cannot overload the cable, the fuse or circuit breaker protecting the cable must provide overload protection. The rating of the circuit breaker or fuse, In, must be equal to or less than the current carrying capacity of the cable under the installed conditions (Iz). The load current (Ib) has to be equal to or less than In. Using the example of a 6/2.5mm2, installed in conduit or trunking, the cable will be rated at 38A. For protection against overload the fuse or circuit breaker will need to be rated at not more than 32A.

In addition to the current rating for a particular cable and load, volt drop, the maximum Zs and the thermal constraints for the cable must be achieved.

When designing installations, cable calculations can be determined by carrying out manual calculations using the data in Appendix 4 of BS7671. Alternatively, proprietary software can be used.

Reference guide for current carrying capacities for PVC twin and earth cables

Reference Method

Description

1/1mm2
(A)

1.5/1mm2
(A)

2.5/1.5mm2
(A)

4/1.5mm2
(A)

6/2.5mm2
(A)

10/4mm2
(A)

16/6mm2
(A)

A

Enclosed in conduit in insulated wall

11.5

14.5

20

26

32

44

57

B

Enclosed in conduit or trunking on a wall

13

16.5

23

30

38

52

69

C

Clipped direct

16

20

27

37

47

64

85

100

Above plasterboard ceiling coved by insulation not exceeding 100mm thick

13

16

21

27

34

45

57

101

Above plasterboard ceiling coved by insulation exceeding 100mm thick

10.5

13

17

22

27

36

46

102

In stud wall in with insulation with cable touching inner wall

13

16

21

27

35

47

63

103

In stud wall in with insulation with cable not touching inner wall

8

19

13.5

17.5

23.5

32

42.5


Ratings shown are tabulated values from BS7671 Appendix 4 for 70°C PVC cables. Where appropriate, other rating factors must be applied.

For further information or advice on this technical guidance, please contact our team on 0845 621 11 11.





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