Part P Cable Selection and Wiring Systems 2 — Questions and Answers
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?
- 0.25
- 0.5 (Correct answer)
- 0.75
- No derating is required
Correct 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.
Question 2: According to BS 7671 Appendix 12, what is the recommended maximum voltage drop for a lighting final circuit in a domestic installation?
- 1% (2.3 V)
- 3% (6.9 V) (Correct answer)
- 5% (11.5 V)
- 10% (23 V)
Correct 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.
Question 3: Which BS 7671 reference installation method applies when twin and earth cable is clipped directly to a plaster wall surface?
- Reference Method A
- Reference Method B
- Reference Method C (Correct answer)
- Reference Method E
Correct 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.
Question 4: What is the recommended minimum burial depth for a cable installed under a domestic garden or lawn (not subject to vehicle traffic)?
- 150 mm
- 300 mm
- 500 mm (Correct answer)
- 750 mm
Correct 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.
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?
- It increases, as the cables share the generated heat
- It remains unchanged
- It must be reduced using a grouping derating factor (Correct answer)
- It doubles as the circuits share the enclosure
Correct 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.
Question 6: Which cable type provides inherent mechanical protection suitable for direct burial or outdoor above-ground runs without requiring additional conduit?
- Flexible cord (3183Y)
- Steel Wire Armoured (SWA) cable (Correct answer)
- Fire-resistant cable (FP200)
- Standard flat twin and earth cable
Correct 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.
Question 7: What minimum cable cross-sectional area (CSA) is typically required for a domestic cooker circuit supplying a 45A cooker control unit?
- 4.0 mm²
- 6.0 mm²
- 10.0 mm² (Correct answer)
- 16.0 mm²
Correct 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.
Part P Cable Selection and Wiring Systems 2
When a cable is totally enclosed within thermal insulation (e.g., loft insulation), what derating factor must be applied to its current-carrying capacity?