APPENDIX 04: Current Carrying Capacity and Volt Drop for Cables (373)

Question 1

The installation reference for multicore cables clipped direct is (Appendix 4.7.1 p.384, Table 4A2 number 20 p.387)

a) 30
b) C
c) A
d) 13

Answer 1

b) C

Question 2

For a low voltage lighting installation supplied from a public LV distribution system, what is the maximum allowable voltage drop between the origin of the supply and furthest luminaire (Appendix 4 6.4 Table 4Ab p.383)

a) 3%
b) 5%
c) 6%
d) 8%

Answer 2

a) 3%

Question 3

Equipment not likely to cause significant harmonics include (Appendix 4 5.5.1 p379)

a) Variable speed motor drives
b) Fluorescent lighting banks
c) dc power supplies
d) Underfloor heating supplies

Answer 3

d) Underfloor heating supplies

Appendix 4 5.5.1 p379

5.5 Rating factors for triple harmonic currents in four-core and five-core cables with four cores carrying current

5.5.1 Rating factors p.379
Regulation 523.6.3 states that, where the neutral conductor carries current without a corresponding reduction in load of the line conductors, the neutral conductor shall be taken into account in ascertaining the current-carrying capacity of the circuit.

This section is intended to cover the situation where there is current flowing in the neutral of a balanced three-phase system. Such neutral currents are due to the line currents having a harmonic content which does not cancel in the neutral. The most significant harmonic which does not cancel in the neutral is usually the third harmonic. The magnitude of the neutral current due to the third harmonic may exceed the magnitude of the power frequency line current. In such a case the neutral current will have a significant effect on the current-carrying capacity of the cables of
the circuit.

The rating factors given in this appendix apply to balanced three-phase circuits; it is recognized that the situation is more onerous if only two of the three phases are loaded. In this situation, the neutral conductor will carry the harmonic currents in addition to the unbalanced current. Such a situation can lead to overloading of the neutral conductor.

Equipment likely to cause significant harmonic currents includes, for example, variable-speed motor drives,
fluorescent lighting banks and DC power supplies such as those found in computers. Further information on harmonic disturbances can be found in BS EN 61000.