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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
    vantech said:
    I’m planning to install an additional external consumer unit via a Henley block

    I am not sure that I understand the proposal.

    Presumably the battery and backup circuits are there in case of grid failure. So what prevents the battery supply going to the main CU? Is the output of the battery limited to 5 kW?

  • 0 in reply to vantech

    Does the installation method for the new 16mm tails mean they can safety carry 75A?  I don't think you can rely on the car charger for safety functions (i.e. overload protection in this case).

  • 0 in reply to Chris Pearson

    I assume the ESS function has an output which remains live on grid failure and a contactor on the input which disconnects the inverter from the grid (and creates the N-E link) in the event of grid failure (a bit like  a computer UPS).  It is one of the two "normal" arrangements for such things, the other is a separately controlled contactor disconnecting the whole house on grid fail leaving the ESS running but that suffers from being a two box solution and something needs to detect restoration of the grid, either electronics with the contactor or an unswitched feed from before the contactor to the ESS electronics.

    I assume the ESS is something like https://www.victronenergy.com/upload/documents/Datasheet-MultiPlus-II-inverter-charger-EN.pdf which is generally intended to support through connection of loads although it can operate with an external contactor (transfer switch) if required.

  • 0 in reply to DShutt

    The charger cannot pull more than 7kW. The load curtailment on the charger is attached to the main meter tail (before the Henley block) to ensure the total building demand remains below 60A, it isn’t being used to control the load on the 16mm tails and the CU this supplies. 

    The maximum demand will only ever be as high as 74A on this second CU fed by 16mm tails. 

  • 0 in reply to Chris Pearson

    The current install: 

    DNO fuse = 100A 

    Meter > 100A isolator > internal CU 

    New install: 

    Meter > 100A isolator > Henley blocks > internal CU (25mm) and new CU (16mm) 

    New CU: 

    40A EV charger (max 7kW) 

    63A ESS (max 10kW)

    The ESS can charge and discharge at 5kW, and also has a backup circuit (that is a true UPS) that can also supply 5kW. If the charger and backup supplies are both at their maximum, the total demand from the ESS is 10kW. 

    The maximum load on the 16mm tails is therefore 74A max (it’s impossible for for this to be higher, the EV is single phase, 7kW max. The UPS output on the ESS shuts down if the load goes beyond 5kW). 

    The UPS output is TN-S in backup operation (grid disconnect, N-E link, and earth rod supplement). 

    Thanks. 

  • 0 in reply to vantech

    With respect to the tails:

    • if they were in free air then they would be fine (assuming that overall loop impedances are low enough for ADS).
    • if they were wrapped in insulation then they probably wouldn't be fine.
    • reality is probably somewhere between the two.

    The 60A load curtailment is a red herring although I do wonder why it isn't set to something higher given the apparently 100A supply.

    There is a separate discussion as to whether both CUs are now operating within their rating, the odds are that the old one will be outside its rating (sum of circuit breakers in it greater than 100A with a 121A capable supply) arguably the new one is OK.

  • 0 in reply to DShutt

    They will be contained within 1m of copex between the meter cupboard (external) and the new CU. 

    The current install has a kWh and ammeter installed. Even with the current EV, electric cooker, hot tub, and an existing ESS (3kW - to be replaced by the new version) the actual demand from the whole property has never gone above 50A in 2 years. 

  • 0 in reply to vantech

    The current-carrying capacity of 16 mm2 single-core PVC 'tails' using Ref Method C is 87 A single-phase, or 79 A three-phase, according to Table 4D1A of BS 7671.

    Therefore, there is no protection provided against overload (if you think you need that) from the 100 A fuse.

    Alan B said:
    There's a regulation that says anything under 3 meters can be protected by a downstream device.

    Yes, overload protection can be omitted provided the risk of faults is minimised ... that is why we use insulated and sheathed tails.

    In terms of fault protection provided by the distributor's service fuse (cut-out):

    • If the service fuse is assumed to be 100 A BS 88-3 16 mm2 conductors are protected for prospective fault currents exceeding approximately 450 A, and 25 mm2 conductors are protected for prospective fault currents exceeding approximately 340 A

    • If the service fuse is a 100 A BS 1361 fuse, 16 mm2 conductors are protected for prospective fault currents exceeding approximately 520 A, and 25 mm2 conductors are protected for prospective fault currents exceeding approximately 400 A
  • 0 in reply to vantech

    I don't know the rating of 16mm sq tails in Copex so can't comment on their rating.  Hopefully that data is available somewhere.

    The relevant regulation (536.4.202) doesn't permit the use of diversity to protect the CU (specifically the main switch or RCCDs covering several circuits) against overload - either there needs to be an external device (e.g. the DNO fuse or separate switchfuse) or the sum of downstream breakers needs to be less than the switch (unlikely in most domestic CUs).  In your case, the DNO fuse plus the ESS can deliver sufficient current to exceed the main switch rating (assumed to be 100A).

  • 0 in reply to DShutt

    As gkenyon has helpfully referenced the relevant table of the regs, 16mm tails are rated at 76A in conduit but 61A in conduit in a thermally insulating wall.  You are right on the edge of the cable rating - if the conduit is just clipped to the wall then it should be OK, if it has to pass through a wall then it is a lot more debateable.

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