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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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    I assume these are existing supplies? And you're trying to utilise the existing for a separate new requirement - the EVs?

    Two three phase supplies on two 3 core SWAs? Where'd the neutral go? 

    Is the supply intake in the garage and you're supplying the house 70m away with two three phase supplies?

    Do the two sub mains run in the same trench/duct and enter the household in the same location?

    If they do, commoning up the SWAs would be A OK I think - then I'd use a switch fuse or isolator to send the supply onto a second position for the Ev for example.

    If the SWAs enter the property in two different locations, then you could still common up the two SWAs, it'd be a bit less usual, but on a common earth bar at each location - absolutely, why not? This would happen anyway in an industrial setting either deliberately or just due to common metalwork.

    Common away I say.....

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  • 0

    I assume these are existing supplies? And you're trying to utilise the existing for a separate new requirement - the EVs?

    Two three phase supplies on two 3 core SWAs? Where'd the neutral go? 

    Is the supply intake in the garage and you're supplying the house 70m away with two three phase supplies?

    Do the two sub mains run in the same trench/duct and enter the household in the same location?

    If they do, commoning up the SWAs would be A OK I think - then I'd use a switch fuse or isolator to send the supply onto a second position for the Ev for example.

    If the SWAs enter the property in two different locations, then you could still common up the two SWAs, it'd be a bit less usual, but on a common earth bar at each location - absolutely, why not? This would happen anyway in an industrial setting either deliberately or just due to common metalwork.

    Common away I say.....

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