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Single phase sub-main

GA

Evening, I'm seeking some advise pls. I have a 4core 25mm2 SWA and was going to use this for two single phase submains running from and to a common point, where the circuits will then go into separate consumer units. limited loads a garage and stable. The cable will have DP isolation and RCD protection on each circuit (2cores L+N per sub main) at the supply point. I'm also looking to run a separate earth cable of 25mm2 which would be common to both circuits and armouring of the SWA cable is also earthed and common. My question is I believe this to be complaint with 17th Ed; As I can't see anything saying its not. Second question, could I common the neutral for both submains and therefore use the fourth conductor in the SWA as an earth which would save installing the separate earth cable? Again, I can't see anything incorrect as limited loads to a couple of sockets and a few lights.  thanks for assistance. GA

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    Forget the 17th Ed - we're on AMD 1 of the 18th now…

    Two cores each for two circuits - a bit unconventional, but I'm pretty sure entirely compliant. It's rather line running two circuits in the same steel conduit and using the conduit for the c.p.c.

    Sharing a N though is probably a no-no. Firstly there's the lack of overload protection - as it's single phase the N will have to carry the sum total of the two circuits current - which will exceed the N core's capacity if the L cores are used to anything like their potential. Then there is the general requirement for isolation and for the wiring of circuits to be separate - if one of the two circuits were isolated and the N disconnected, the load end of the N conductor would likely be pulled up to full mains voltage by any connected load on the other circuit. Admittedly the main regulations for this (314.4 and 521.8.2) only specifically mention final circuits - but it would seem rather risky not to consider similar risks in distribution circuits, especially if it's not a well controlled environment.

       - Andy.

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  • 0

    Forget the 17th Ed - we're on AMD 1 of the 18th now…

    Two cores each for two circuits - a bit unconventional, but I'm pretty sure entirely compliant. It's rather line running two circuits in the same steel conduit and using the conduit for the c.p.c.

    Sharing a N though is probably a no-no. Firstly there's the lack of overload protection - as it's single phase the N will have to carry the sum total of the two circuits current - which will exceed the N core's capacity if the L cores are used to anything like their potential. Then there is the general requirement for isolation and for the wiring of circuits to be separate - if one of the two circuits were isolated and the N disconnected, the load end of the N conductor would likely be pulled up to full mains voltage by any connected load on the other circuit. Admittedly the main regulations for this (314.4 and 521.8.2) only specifically mention final circuits - but it would seem rather risky not to consider similar risks in distribution circuits, especially if it's not a well controlled environment.

       - Andy.

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