16mm tails 100A fuse - EV & ESS

vantech said:

The maximum potential simultaneous load on these tails would be approximately 74A (5kW charge + 5kW discharge + 7kW EV).

I'm not sure I follow that ... although RDF might lead you there for the consumer unit.

Hi, 

It’s a single phase install. 100A cut out, residential supply. 

DNO has approved a 5kW feed-in. 

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). 

This new ESS combined with a 7kW EV charger, will connect into a new dedicated external CU. The plan is for the ESS to sit on a 63A MCB, and the EV a 32A RCBO (2 pole). This new CU is then connected via 16mm tails to a Henley block. 

The Henley block is fed from a DP isolator (from the back of the meter). The main house CU (25mm tails) are connected back in, with the new 16mm tails connected to the new CU. 

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

Thanks 

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.

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.

Thanks, that’s a really useful breakdown.

Just one correction on CU2 — the ESS branch is on a 50A MCB, not 63A.

So on that basis, the maximum non-fault downstream loading of CU2 would be 32A for the car charger (40A MCB) and 45A for the ESS branch (50A MCB), giving a total of 77A.

That strengthens the case that CU2 itself is not at risk of overload, because the maximum downstream loading is bounded by the connected equipment and their protective devices, rather than simply adding incoming supply and generation capability.

On CU1, I agree that an 80 A fuse upstream is the sensible approach given the PV on the board and possible ESS contribution. This would be 100A, 16A, 22A = 138A without the upstream 80A fuse.

Your point on the tails is also fair. If the CU2 tails are 16 mm² in conduit, then the real question is whether the downstream arrangement and inverter limits genuinely ensure they can never be overloaded. In my case, the ESS output/import is inherently limited by the inverter, but I agree that this needs to be treated as a hard documented limit rather than just an assumption.

I also agree on car charger load curtailment — I wouldn’t rely on that as the sole means of overload protection.

So I think the position becomes:

CU1 protected conservatively with an upstream fuse. Fed from the DNO cupboard to the switched fused, and then onto CU1 using 25mm tails. 
CU2 maximum downstream non-fault load is 77A. I will go for 25mm tails. 

I was not factoring in the actual realistic loading on CU2 before, in that it cannot discharge and charge at the same time and the maximum possible load is set by the device and can only be reduced through settings and not increased. 

Thanks.