James asks this question, which is about BS 7671:2008 radial circuit requirements in terms of overcurrent protection devices and the filling in of completion certificates. It has been answered by the NICEIC (Theme – Distribution Equipment including Cable Management):
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Question: A query has been raised as to whether it is possible to use a 16A MCB on a radial circuit serving a number of sockets. If you were to use Appendix 15 of BS 7671:2008, the options given are:
- 20A overcurrent protection device (any type). All live conductors can be wired in 2.5mm2; however you are restricted to serving a floor area of 50m2.
- 30/32A overcurrent protection device (HRC fuse or circuit breaker). All live conductors can be wired in 4mm2; however you are restricted to serving a floor area of 75m2.
There seems to be much discussion online as to whether 16A radial circuits are acceptable – however, as far as I can determine from the regulations, if a 16A overcurrent protection device was to be used, it would need to be treated as a fixed load limiting the circuit to a single socket (13A).
Have you come across this query before, or do you have any further guidance you could share?
Answer: The rating of the device is there to protect the cable. Appendix 15 is informative and not a specific regulation.
If you install a radial circuit with a 2.5mm cable then, effectively, it is capable of taking a load of far greater than 20A.Therefore any device rated at 20A or lower would provide effective protection.
If a 4mm cable is used, a device rated at 32A may adequately protect the 4mm cable if clipped directly, as its loading is above that requirement.
The issue of floor space is purely down to potential volt drop, which would influence the final Zs (earth loop reading at the furthest point on a given circuit). This also affects the size of the cable because of its ability to carry the current effectively.
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