Street Lighting cable installation

As far as the regulations are concerned, the important thing is the total resistance of the cable to the end - however if this can be made compliant by increasing the cable size only at one end, or the other, or indeed for a short section in the middle if you need to, this is perfectly acceptable (note that joins do need to be accessible or sealed to be watertight,so a joint at a lamp post sounds sensible.).


I presume from yuor description that the non-compliance is one of Zs at the far lamp versus the current required to trip the protective fuse or breaker, and a lower rated breaker is not a sensible suggestion?.

Rather than the other problem, of voltage drop when the lights are all on - where a lower rated fuse or breaker will not help you, but the section of cable nearer the supply carries more current so is the more sensible to be reinforced anyway.

The other possible solution is an RCD or RCBO for earth fault detection, but that is not ideal as arrangements have to be made for if it false trips in a thunderstorm or something, so perhaps OK for a restaurant car park, where there will be staff around, not for unattended operation.

Thanks much for your reply , is this mentioned in any clause of BS7671 ?

is this mentioned in any clause of BS7671 ? 

Not directly - it's only implied by the requirements for current carrying capacity, Zs, voltage drop etc. and the lack of any demand for a circuit only to consist of a single cable size.

   - Andy.

If it helps there is an example in BS 7671 of a circuit with two different cable sizes - appendix 15  Fig 15B - where 2.5mm² spurs are shown from a 4mm² circuit.


If you really want something to quote - there are bits like 433.3.1 (i) which says that an overload protective device may be omitted where there's a reduction in the value of the current carrying capacity where the upstream protective device already provides overload protection and similarly 434.2.2 for fault protection but those don't really relate to Zs or v.d. requirements.


   - Andy.

You might be able to fit another mcb in line inside the first (or intermediate) lighting column of smaller rating than the one upstream. Might save any digging.