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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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    I would advise against use of a single core as a common neutral, for the reasons already given by others.

    My view is that when a 4 core cable is used as is proposed, that the published current carrying capacity should be reduced by about 15%. I presume that the published ratings are based on three phase, with only 3 cores carrying significant load. Loading all 4 cores produces more heat.

    If however the loading is modest if compared to the cable size, then this is a non issue.

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    I would advise against use of a single core as a common neutral, for the reasons already given by others.

    My view is that when a 4 core cable is used as is proposed, that the published current carrying capacity should be reduced by about 15%. I presume that the published ratings are based on three phase, with only 3 cores carrying significant load. Loading all 4 cores produces more heat.

    If however the loading is modest if compared to the cable size, then this is a non issue.

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