Cable Selection and Wiring Systems Flashcards

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

Answer: 0.5

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.

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

Answer: 3% (6.9 V)

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.

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

Answer: Reference Method C

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.

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

Answer: 500 mm

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.

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?

Answer: It must be reduced using a grouping derating factor

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.

Which cable type provides inherent mechanical protection suitable for direct burial or outdoor above-ground runs without requiring additional conduit?

Answer: Steel Wire Armoured (SWA) cable

Steel Wire Armoured (SWA) cable incorporates a layer of steel wires between the insulation and outer sheath, providing mechanical protection that allows it to be buried directly or surface-run outdoors.

What minimum cable cross-sectional area (CSA) is typically required for a domestic cooker circuit supplying a 45A cooker control unit?

Answer: 10.0 mm²

A 10 mm² twin and earth cable is typically required for a 45A cooker circuit based on current-carrying capacity tables (derated for installation method) and voltage drop calculations in BS 7671.