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.

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.