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How best to utilise two submains, achieving 10mm CSA for main bonding

Alan B

I have two 35mm 3 core submains, 70m long, running from a garage to house. on a 63A, C curve MCB. (very big house and grounds)
Armour of each is equivalent to about 9.25mm, just below the 10mm I need for bonding
Supply is 100A 3 phase, with 2 phases going to the house, 3rd to heat a pool.

One submain is going to the main house CU.
I want to take the second submain to a seperate CU for two EV chargers, putting the EV chargers on the main board is likely to take max load over 63A, can't increase the breaker due to voltage drop and I already have voltage drop concerns with the EV chargers. Implementing load management on the EV chargers is challenging because the previous electrician didn't include a data cable in the supply running through a roof and under a path.

To achieve the 10mm minimum CSA for bonding can I connect a 10mm earth between the existing and new EV CU at the house end and then consider the csa of the earth connection to be 9.25 * 2 = 18.5. In my mind it's ok, specially as 9.5 is already very close to the  10mm required?

Anything else I need to think about.

Bit more background

At 63A my voltage drop on the submain is 4.85V, at 32A it would be 2.46V. assuming Max VD for a EV charger is 3%, 6.9V. Running the second submain at 32A gives me far more flexibility. I am also dealing with a 50m cable run to  one of the EV chargers. Will probably use 15m of 6mm already installed followed by 10mm for the remainder of the run.
House is in a rural location but very close to local pole mounted transformer, so hopefully voltage is reasonably stable. But having had a lot of issues with voltage variation in rural locations i am always a bit nervous about it, certainly don't want any opportunity for the DNO to point a finger at me if there are voltage issues.



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  • 0 in reply to Alan B

    What a mess! That BS 1363 socket takes the biscuit.

    It seems to me that there could be two reasons for using the two phases: (1) because the demand is > 63 A and < 126 A; (2) to mitigate voltage drop.

    The loops of SWA appear to give you some room for manoeuvre. I think that you could leave L1/L2/N as it is. Remove the other SWA. Use the 3 cores for L/N/E using the phase of your choice and terminate in a new DB in the usual fashion. Then link the earth bars of the two DBs with a length of 16 mm G/Y. I would also add a notice making it clear that it must not be removed (ETA: a bonding tag might do nicely.)

    That still leaves me with some concern over balancing the phases. Some circuits might need to be moved from L1 to L2.

    Good luck!

  • 0 in reply to Alan B

    whatever you do, keep each phase with its neutral in a given SWA. Because of the steel armour, you dont really want significant current flowing up one cable and down the the other - so its not really 6 cores that can be assigned willy nilly, rather 2 groups of three in separate magnetic containment.
    However L1 L2 N12 in one, and L2L3 N23 in the other will be fine, but don't recombine the neutrals. (where the lines for which the N is the return are explicit.)
    It may be easier to re-gland both into a single large adapatable box or section of metal trunking beneath the CUs and then if need be to extend with some line taps or similar chunky terminals and then come out in two feeds CU 1 and CU 2. Pirahna nuts may be your friend in terms of a high current interconnection of the armours. Banjos are not always the best in these cases. 

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