Electric Wiring, 3rd Edition

Fuses 21

5 Thermal insulation

If a cable is surrounded by thermal insulation for 0.5 m or more, its rating

must be reduced by factor of 0.5.

Example of cable sizing

To determine the minimum cable size (PVC twin) to supply a load of 28 A

with a cable length of 40 m, protection by BS88 fuses and no correction

factors, we must follow a set procedure:

1 Design current of circuit I

B

= 28 A

2 Select rating of protection I

N

, such that I

N

^ I

B

. Hence I

N

= 30 A

3 Select correction factors (CF) (not applicable) j

4 Determine current carrying capacity of cable I

z

from —

CF

hence I

z

= I

N

= 30 A

5 Choose cable to suit I

z

from tables, i.e. 4.0 mm

2

6 Chec ...