16mm tails 100A fuse - EV & ESS

Thanks 

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. 

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).

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.

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.

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.

There is a 3rd CU. 

ESS + EV 

Main house 

Backup loads 

Hi again, 

Looking for clarification on applying Reg 551.7.2 where both PV and ESS are present on separate consumer units. I have been racking my brain on this and think I have made sense of it. 

Setup:

• DNO supply: 100 A
• Split via service connectors / Henley blocks to CU1 and CU2. 

CU1:

• 125 A rated
• PV on 16 A MCB

CU2:

• 100 A rated 
• ESS + 7 kW EV charger
• ESS: max export 5 kW (~21.7 A)
• Max demand up to 10 kW (based on ~5 kW charging plus ~5 kW backup circuit load)
• ESS cannot charge and discharge at the same time

CU2 assessment:

Using Wiring Matters:

IₙA ≥ Iₙ + I g(s)
→ 100 A + 21.7 A = 121.7 A

But the ESS cannot simultaneously import and export, so this condition cannot occur. It’s impossible. 

Here I think I am good to use a 100A rated CU with no additional upstream fusing. 

CU1 assessment:

CU1 has:

• 100 A available from DNO
• 16 A PV contribution (local MCB) 
• possible backfeed from CU2 ESS of up to 5 kW (~21.7 A)

So applying the same logic strictly:

IₙA ≥ 100 A + 16 A + 21.7 A = 137.7 A

Here I think I need an 80A upstream fuse to protect this board given the contribution from the ESS (CU2) and the solar array.

Question:

Is Reg 551.7.2 intended to be applied strictly using the rated output currents of all generating sources, even where system operation prevents simultaneous contribution?

Or can the designer consider:

• mutually exclusive operating modes (ESS charge vs discharge)
• realistic coincidence of PV and ESS output
• actual maximum demand conditions

when assessing the required CU rating?

Thanks.

I'm still a little confused on a few points, but...

CU1 - if it contains only single "way" and with a 16A MCB on that, then the current through the assembly is limited to 16A - so no worries there (AMD 4 is a bit clearer on that point now).

CU2 -

"But the ESS cannot simultaneously import and export" - agreed. (There was some confusion in some guidance, but I think that's been cleared up now) - I'm still not clear whether the EESS can "backfeed" to CU2 when discharging (without a grid fail situation), or whether it's output is just to the downstream CU3.

I'm presuming non-essential loads (kitchen sockets etc.) are on CU2 too? So total MCBs rating could be >100A?

If the EESS can't backfeed, then the total inputs to the CU could be 100A from the grid plus 16A from CU1 (and with loads not limited to less than that) so the CU should be rated to at least 116A in that case. Adding a 100A fuse between the henleys and CU2 would limit the current to 100A, so the CU then would only need rating at 100A.

If thee EESS can backfeed into CU2, then you've got 100A from the grid, 16A from the PV and 21.74A from the EESS potentially flowing in - so without anything else you'd need a CU rated at over 137A. A fuse before the CU would limit the contribution from the grid and PV but not from the EESS. If it's a 100A CU and 21.74A from the EESS the rest of the input would have to be limited to 78.26A - so probably a 63A fuse. That then begs the question of whether that's sufficient (e.g. when making use of off-peak tariffs, 32A EV load, 21A battery charge, maybe 13A immersion and 10A washing machine...)

Swapping the PV and EESS around might look better - 16A PV contribution on CU2 would mean a 80A fuse would be OK, and a max 10kW/45A (or 63A MCB) load on CU1 wouldn't need overload protection for the 16A tails.

Or I might have pictured this completely wrongly...

   - Andy.

vantech said:

Question:

Is Reg 551.7.2 intended to be applied strictly using the rated output currents of all generating sources, even where system operation prevents simultaneous contribution?

Or can the designer consider:

• mutually exclusive operating modes (ESS charge vs discharge)
• realistic coincidence of PV and ESS output
• actual maximum demand conditions

when assessing the required CU rating?

My view is that:

• Mutually exclusive operating modes - yes, providing they really are mutually exclusive and that is enforced by the design of the inverter or a system of suitably high integrity.
• Realistic coincidence of PV and ESS output - unrealistic cases in what sense - how are you going to prevent those cases occurring?  Probably no unless there is a system of sufficiently high integrity to prevent them.
• Actual maximum demand conditions (I assume you mean after diversity) - not for the purposes of overload protection of the CU(s) (back to 536.4.202).  In my view you can use actual maximum demand if the load is fixed (e.g. only count 13A for an immersion heater on a 16A MCB if it is the only device connected to the breaker, same for 32A for a car charger on a 40A MCB); you'll still be counting 32A for your rings and 6A for your lighting circuits however.

My previous replies missed that the solar in CU1 is being retained and assumed CU1 was 100A rated, although it doesn't change the outcome.

Given the solar on CU1 and the new ESS then despite its higher rating I agree that CU1 will require an 80A fuse (assuming that the potential load sums to greater than 125A).

In the case of the 100A rated CU2, the feed to it can be up to 116A, there is no fuse and there is ~22A of generation attached - we therefore need to look at the maximum downstream loading to see if can be overloaded.  In this case, the maximum downstream loading is 32A (the maximum non-fault load of the car charger) plus 63A (you previously stated this as the MCB for the ESS so it is the worst case no matter whether charging or discharging), a total of 95A so you are OK without a fuse on this CU.  It isn't clear to me what limits the flow to CU3 so I'll stick with 63A as the ESS+CU3 maximum load - if there is some form of limitation to the CU3 supply then the total may be less than 95A, not that it makes any difference from a CU protection perspective.  If there are spare ways then a notice on this CU stating that no further MCBs should be installed without assessing the possibility of overload of both the tails and the CU itself might be a good idea.

CU3 is limited to 63A plus 5kW (22A) from the ESS inverter which is 85A so, assuming that CU3 is 100A rated then it should be OK (and from what you have said, the MCBs in CU3 may limit the load to less anyway).

Note that the proposed 16mm tails for CU2 are rated at 76A in conduit so you need to understand what limits the CU3 load to 5kW to confirm that the proposed CU2 tails will never be overloaded as the MCBs described so far only limit the load on the tails to 95A.  If there isn't a suitable protection performing this function then CU2 will need bigger tails or a 60A fuse on the supply to it.

Do not rely on the car charger load curtailment to limit the tails current - my understanding is that load curtailment in domestic car chargers is a feature intended to avoid the operation of other protective devices which fully protect against overload, not as the sole means of protection against overload.