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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
    Alan B said:
    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?

    If you need 10 mm² copper and add 10 mm² copper, the armour is just a bonus.

    Given that you are nearly there, 4 mm² copper would probably do, but the price differential is not exactly huge, so unless you are in Yorkshire, the customer is unlikely to bat an eyelid.

    Incidentally, are EVCPs particularly bothered about the incoming voltage?

  • 0 in reply to Chris Pearson

    Off topic but I always thought it was just a local joke that the folk from Yorkshire are tight with money. We can’t be that bad; can we? For reference I am from Yorkshire.

  • 0 in reply to Shaun Scholes

    Dont be so down on it, we are just  careful  (and I'm born in Beverley, but I have been away so long my accent identifies me as 'outlander' there as well as here.) Not as profligate as our softer southern cousins, from Lincolnshire and places further below...  
    Have deep pockets, but short arms; my grandparents used to say. I think it goes back to a history of farming and saving and re-using everything that might be useful one day, just to be sure of getting through poor winters.
    Mike.

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  • 0 in reply to Shaun Scholes

    Dont be so down on it, we are just  careful  (and I'm born in Beverley, but I have been away so long my accent identifies me as 'outlander' there as well as here.) Not as profligate as our softer southern cousins, from Lincolnshire and places further below...  
    Have deep pockets, but short arms; my grandparents used to say. I think it goes back to a history of farming and saving and re-using everything that might be useful one day, just to be sure of getting through poor winters.
    Mike.

Children
  • 0 in reply to mapj1
    mapj1 said:
    born in Beverley

    In Westwood Hospital?

    I was brought up in Hull, so I certainly know never to spend a penny when a ha'penny will do.

    Back to wiring: a jewish judge told me a year or two ago about the origins of copper wire. I cannot possibly repeat the tale here, but it involved, inter alia, a Yorkshireman and a penny.

    And back on topic: Alan, have you decided what to do about the installation?

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