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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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  • 0

    Zoomup: 
    It looks like that involves arithmetic. I am not too good at sums so will rely on Table 54.7 for guidance. Armoured cable to the stable and garage will not cost too much.

    Z.

    Absolutely … with multicore SWA it's usually the case that the armour is OK for cpc for smaller csa conductors and this only comes into play with larger CSA. (Of course, that doesn't take into account the requirements in PME installations to achieve the equivalent of main protective bonding where that applies with very small CSA cables.)

    I just wanted to point out that some of the data on the site (e.g. R1+R2) and calculating length of run (for Zs) in TN systems, and also if you did want to do adiabatic for larger CSA cables, you should use an alternative method now.

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  • 0

    Zoomup: 
    It looks like that involves arithmetic. I am not too good at sums so will rely on Table 54.7 for guidance. Armoured cable to the stable and garage will not cost too much.

    Z.

    Absolutely … with multicore SWA it's usually the case that the armour is OK for cpc for smaller csa conductors and this only comes into play with larger CSA. (Of course, that doesn't take into account the requirements in PME installations to achieve the equivalent of main protective bonding where that applies with very small CSA cables.)

    I just wanted to point out that some of the data on the site (e.g. R1+R2) and calculating length of run (for Zs) in TN systems, and also if you did want to do adiabatic for larger CSA cables, you should use an alternative method now.

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