hi all,
Outside summer house for small gym. Couple sockets, light.
Need power, 40/50m approx cable run from back of socket, 100m to cu.
At that length 2.5mm² would need to be doubled up the maximum length of a ring final is 106 meters. It would need careful measurements before one could actually work out minimum size, but likely 6mm² is the smallest cable you could consider.
1st option is fcu off back of socket via armoured but limited to 13a, borderline acceptable.
Right on the maximum for 2.5mm² would have to be at least 4mm²
What's the probable max load you could run off back of socket? As technically a spur? I.e basicly not fusing down?
Regards
The idea of not fusing down and using 6mm² and then back to a 2.5mm² ring in the shed is what is called a figure of 8. We debated this in another post there is nothing in the regulations to say you should not use a figure of 8 but the guidance books for inspecting and testing say one should test for a figure of 8 and correct if found.
As I see it the problem arises in ensuring the two parallel conductors of the ring final actually load share.
433.4.2 Unequal current sharing between parallel conductors
Where the use of a single conductor is impractical and the currents in the parallel conductors are unequal, the design
current and requirements for overload protection for each conductor shall be considered individually.
NOTE: Currents in parallel conductors are considered to be unequal if the difference between the currents is more than 10% design current for each conductor. Refer to paragraph 2 of Appendix 10.
Appendix 10 gives all the calculations required so not impossible, but not really for DIY and I am not convinced it would be possible to calculate the maximum current in each leg of the ring when you have a variable load?
The Appendix 12 puts forward the view that if you limit and fixed device and the shed is a fixed device to 2kW then it is unlikely any overload would happen for long enough to cause a problem. Once you exceed the 2kW again you need to do the maths to work out if an overload is likely.
As to the Appendix being part of the regulation or not is also debated. Since we are tested on the Appendix content when we take our exams I would say yes it is part of the regulations. Back in the days of 14th edition the regulations actually contained advice for example distance socket to sink. But by the time of the 16th edition all the advice was moved into the guides and the regulations became more a book of rules.
However the regulations are not law but can be used in a court of law. What happens if they are not complied with varies as to what not complying resulted in. We tend to consider insurance if work is done in the house which is considered sub-standard insurance companies can decide not to pay out. Although I have not know of a case. I know when I had a water leak the standard of workmanship was questioned. However I was able to show it was due to a faulty component so it was not investigated further.
Clearly if nothing goes wrong then there is no problem, however when things do go wrong then to be under a cloud not knowing if insurance will pay or worse if you will be prosecuted is the last thing anyone wants.
We often call it jobs worth, but it really is down to is it worth taking the risk rather than wire back to the consumer unit. Personally I would wire back to consumer unit.
