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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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    Hi Everyone

    I currently have one swa carrying 2 phases and neutral.

    There is a second SWA, 2 cores unused and one core used for earth.

    My plan is to move this second swa to a second DB that I can use to supply a couple of EV chargers, or just extend the cores through to a second DB.

    Combined SWA armouring gives me plenty of CSA for the 10mm bonding requirement and ADS for a 63A MCB.

    Moving the EV chargers on to their own board and submain alleviates issues with max demand on 63A MCB's and voltage drop

    I guess the second submain was terminated in to the existing DB to supply the earth connection, very expensive way to replace a bit of 10mm copper and leaves me with a cable that's not long enough to get to a second DB. I assume the submain was put in before max demand was calculated, or maybe the owner was considering a heat pump to heat the  house, or solar. Like many domestic renovations from what I can understand the owner has changed things as they have gone along. It's just that in this case its a 7 bedroom mansion in 10 acres of ground.


    The SWA's only use the case of the CU for the armour connection, I will add fly leads to ensure proper earthing and bring them to a common earth bar at both ends.

    As background the third phase is going from the garage to the pool house to power a heat pump for the pool.

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  • 0

    Hi Everyone

    I currently have one swa carrying 2 phases and neutral.

    There is a second SWA, 2 cores unused and one core used for earth.

    My plan is to move this second swa to a second DB that I can use to supply a couple of EV chargers, or just extend the cores through to a second DB.

    Combined SWA armouring gives me plenty of CSA for the 10mm bonding requirement and ADS for a 63A MCB.

    Moving the EV chargers on to their own board and submain alleviates issues with max demand on 63A MCB's and voltage drop

    I guess the second submain was terminated in to the existing DB to supply the earth connection, very expensive way to replace a bit of 10mm copper and leaves me with a cable that's not long enough to get to a second DB. I assume the submain was put in before max demand was calculated, or maybe the owner was considering a heat pump to heat the  house, or solar. Like many domestic renovations from what I can understand the owner has changed things as they have gone along. It's just that in this case its a 7 bedroom mansion in 10 acres of ground.


    The SWA's only use the case of the CU for the armour connection, I will add fly leads to ensure proper earthing and bring them to a common earth bar at both ends.

    As background the third phase is going from the garage to the pool house to power a heat pump for the pool.

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