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16mm tails 100A fuse - EV & ESS

vantech

Hi all,

Just looking for a quick sense check.

I currently have job with a 100A DNO fuse with 25mm meter tails feeding the main consumer unit. I’m planning to install an additional external consumer unit via a Henley block to supply:

  • 7kW EV charger
  • 5kW battery storage system (charging) plus 5kW backup circuit (discharge capability) - total 10kW. 

Due to routing constraints, installing 25mm tails to the external CU will be very difficult, so I’m considering using 16mm² tails instead.

The maximum potential simultaneous load on these tails would be approximately 74A (5kW charge + 5kW discharge + 7kW EV). The EV has a 60A load curtailment so the maximum is likely never going to be this high, and the backup circuit on the ESS is supplying sockets (excluding kitchen) and lights, so unlikely ever going to be more than 2-3kW. 

Given that the load is effectively limited by the connected equipment, would 16mm² tails be acceptable on a 100A supply in this scenario, or would 25mm tails be required?

Appreciate any guidance or references to regs / best practice.

Thanks.

  • 0 in reply to gkenyon

    Thanks 

  • 0 in reply to DShutt

    The ESS when it’s providing power to the main CU, will only provide as much as is required. The DNO fuse can in theory provide 100A, with the ESS providing a further 27A. That said, the main CU has never pulled more than 55-60A over the last 2 years with an ammeter installed on the supply. 

    Does the fact the ESS is upstream of the main CU, need to be factored in as a separate source for the main switch rating in this main CU?

    Ps the copex loops out the bottom of the meter box and into the new CU. No insulation, fixed to the wall with saddles. 

  • 0 in reply to vantech

    The tails sound OK to me from a rating perspective.  Will the Copex provide adequate mechanical protection?

    My understanding (and I would recommend that you get opinions from others as well on this as I wouldn't say I was 100% confident) is that you need to ensure that the CU current is limited to its rating (which is probably 100A) and the use of diversity is not an acceptable way of doing that.  Normally in a domestic situation that limitation is provided by the main fuse but the addition of generation in parallel with the main fuse may mean that that is no longer the case, depending on fuse rating and generation rating.

    Look at Wiring Matters from November 2024 (https://electrical.theiet.org/wiring-matters/years/2024/103-november-2024/how-does-the-installation-of-microgeneration-affect-the-rated-current-of-a-consumer-unit/) and the BEAMA guidance on consumer unit ratings (https://www.beama.org.uk/static/uploaded/6861a2e1-d8d4-4d81-b5fa710ba60ca4a7.pdf, which whilst written primarily around RCCDs also applies to the main switch).

    The "simple" fix is to install a 100A switchfuse to protect the old consumer unit (only).

  • 0 in reply to vantech
    DShutt said:
    In your case, the DNO fuse plus the ESS can deliver sufficient current to exceed the main switch rating (assumed to be 100A).
    vantech said:
    the actual demand from the whole property has never gone above 50A in 2 years. 

    In which case, the DNO's fuse could be down-rated.

  • 0 in reply to Chris Pearson

    Down-rating the supply fuse is the simplest solution but the point of this exercise is to add 12.2kW of load to the property (car + ESS charging).  One might expect that charging would occur at a different time to cooking / hot tub use but I think it would be unwise for the electrical design to mandate that.

    Having said that, we are told that the car charger has load curtailment to 60A (but we don't know why), in which case you might get away with a 60A fuse at the cost of car charging being slow for some of the time.  An 80A fuse results in a maximum supply current of 101A to the old CU - arguably not acceptable but one might form a view about the 1A exceedance.

  • 0

    The ESS has a 5kW inverter, and a backup circuit (creates a N-E bond during backup, supplemented by an Earth rod). If the ESS is charging its batteries at 5kW, there is also a possibility that the backup (essential loads) output can also pull 5kW so the total demand from the ESS has a theoretical maximum of 10kW (with the obvious lower loading noted above, socket circuits etc, not the kitchen - gas heating, no large loads, plus lighting). 

    How are the essential loads connected? is there a 3rd CU somewhere? (presumably in that situation the power can't be back-fed into the original CU..)

       - Andy.

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