Part P Cable Selection and Wiring Systems 2

Question 1

When a cable is totally enclosed within thermal insulation (e.g., loft insulation), what derating factor must be applied to its current-carrying capacity?

Accepted Answer

0.5

Answer

BS 7671 requires a derating factor of 0.5 (50% reduction) when cables are completely enclosed in thermal insulation, because the insulation traps heat generated by the cable and significantly reduces its safe current-carrying capacity.

Question 2

According to BS 7671 Appendix 12, what is the recommended maximum voltage drop for a lighting final circuit in a domestic installation?

Accepted Answer

3% (6.9 V)

Answer

BS 7671 Appendix 12 recommends a maximum voltage drop of 3% for lighting circuits (6.9 V on a 230 V supply), which is tighter than the 5% allowance for power circuits, to ensure adequate voltage at luminaires.

Question 3

Which BS 7671 reference installation method applies when twin and earth cable is clipped directly to a plaster wall surface?

Accepted Answer

Reference Method C

Answer

Reference Method C (clipped direct) applies when cables are fixed directly to a surface; this method gives a higher current-carrying capacity than enclosed methods because the surface aids heat dissipation.

Question 4

What is the recommended minimum burial depth for a cable installed under a domestic garden or lawn (not subject to vehicle traffic)?

Accepted Answer

500 mm

Answer

IET guidance recommends a minimum burial depth of 500 mm for cables in garden or lawn areas not subject to vehicles, providing adequate protection from spades, forks, and other garden tools.

Question 5

When two or more circuits share the same conduit, trunking, or cable bundle, what effect does this have on each cable's current-carrying capacity?

Accepted Answer

It must be reduced using a grouping derating factor

Answer

Grouped cables cannot dissipate heat as efficiently as individual cables, so a grouping derating factor from BS 7671 Table 4C1 must be applied, reducing each cable's permissible current-carrying capacity.

Part P Electrical Practice Test

Part P Electrical Practice Test

Part P Cable Selection and Wiring Systems 2