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

    That would involve pulling a cable through 60m of duct with 4 bends and 2 big cables in it. May work but prefer not to try.


    Chris Pearson said:
    Incidentally, are EVCPs particularly bothered about the incoming voltage?

    EVCP's measure supply voltage to hopefully detect open pen issues, most of the time this means they just detect that the DNO are doing a bad job of maintain the 216.2 to 253V range as they should. Added to this voltage drop in the consumers install reduces the voltage further. Guidance had changed recently so that an extra 3% or 4% is now allowed for the installation voltage drop and installers have to keep voltage drop within these limits(different regs quote different numbers). If the EVCP sees a low or high voltage it stops charging and the user wakes up in the morning with a partially charged car.

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

    It's a 3 phase supply to the garage, then  2 phases to the house 70m away. Other to the pool, these are existing supplies and eVCP partially wired.
    Two submains enter the house at the same location, so I can put a new CU nest to the existing one assuming I can find a way to deal with cable bend radiuses.

    tattyinengland said:
    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.

    I only plan to calculate my CPC size based on the armour of the swa being in parallel, but with the swa's going to seperate CU's and a earth cable between the CU's
    Both submains run in to the one CU at the moment,I am quite tempted to leave this as it is and then uses crimps to extend the cores in to a second CU. Insulate the crimps with a couple of layers of heat shrink?

    Sorry these questions probably seem simple, nearly all my work is domestic, so I don't have industrial experience. 

  • 0 in reply to Alan B
    Alan B said:
    If you need 10 mm² copper and add 10 mm² copper, the armour is just a bonus.

    That would involve pulling a cable through 60m of duct with 4 bends and 2 big cables in it. May work but prefer not to try.

    My apologies - I misunderstood your question, which has subsequently been made clearer.

    It seems an odd setup, but you are stuck with it. One length of SWA with 2 lines and a neutral (or even using a 4th core for earthing) might have been more economical.

    Alan B said:
    Both submains run in to the one CU at the moment

    Does that mean that the CU has two isolators? If it has a metal case, the armours are already re-united at the downstream end, which now gives you the 18.5 mm² Cu equivalent CSA.

    I do not see any reason why the armours should not be joined, but I think that would be bonding rather than earthing.

  • 0 in reply to Alan B

    So, Two x SINGLE phase supplies in three core, 35mm SWA run 70m up to the house. Yes?

    Your bonding cable is already installed then - the same 35mm earth that is in the SWA (s)  and used as the earth fault path - can be used as both the earthing and bonding cable; it is allowed to be the same bit of cable in a sub main. I can't for the life of me find the reg - but thanks to (AJ I think?) - about 15 years ago I asked much the same question where I had a new gas supply entering a rather large mansion, on the opposite side of the supply intake - I had a sub DB closer to the gas intake, and I asked if I could use that sub DB and take the bonding to that position instead of all the way to the supply origin.

    There were conditions that had to be met, but certainly that 35mm earth in the SWA, let alone 2 x 35mm would easily satisfy the combined earthing and bonding requirement, if I understand your predicament correctly. 

    Someone will provide a reg shortly I'm sure. 

  • 0 in reply to tattyinengland

    Ah, I'd read it as 3-core SWA carrying L1/L2/N - hence no spare core for PE....

      - Andy.

Reply Children
  • 0 in reply to AJJewsbury

    I had assumed from the question that the armour was providing the PE.

    Just because the cable is 3-core does not mean that all are in use.

  • 0 in reply to AJJewsbury

    maybe....2 phase and neutral in one SWA and a single phase, neutral and earth in the second SWA? Or maybe 2 phases in each SWA and a single neutral? 

    Very unusual surely - but maybe just unusual in my experience....

    So, maybe no earth internal to the SWA at all......

    If the SWA is the ONLY earth - and I'm just thinking out loud here - He'd need to then calculate if the combined SWA earth path is suitable for earthing - Allan B seems able to work out the copper equivalent of the SWA, so I'd suppose from his previous posts he'd be able to work out minimum size of earthing - SWAs combined seems likely. Two SWAs together are suitable for bonding purposes I'd say. 

    I'd probably Create an "MET" in the house and connect that to the SWAs and have another MET in the garage connected to the SWAs again and the Main supply earth, creating two zones, the house and the garage.

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