Domestic consumer unit rating with PV and battery storage.

Having read the COP on  Electrical Energy Storage Systems and  completed the IET course on the same subject I had a query regarding the rating of domestic consumer units and switch gear which I addressed by email to NICEIC technical. I also came across an older discussion on this forum but am still no closer to a definitive answer. I've included my findings and would welcome constructive input. 

Post by GKenyon in previous thread

Because an EESS charges the battery as well as as discharging it, you will need to check the rating of the CU is not exceeded. For example, if the CU is rated for 100 A, and there's a 100 A service fuse, and a 16 A output battery storage system - by feeding 16 A in at one end through an OCPD, because that OCPD gets hot it contributes to the total heat load in the CU - therefore the CU should be rated for 116 A.

My question to NICEIC.

Hello
Please can you help with the following.
Domestic installations with PV and/or battery storage.
551.7.2 Where the generating set is connected to either the main consumer unit or via a separate consumer unit via Henley blocks the rating of the consumer units shall be protected by a OCPD InA≥In+Ig(s).
Where In = 100A DNO fuse and Ig(s) = 16A MCB or 2 x 16A MCB's which would be 116A or 132A, what inspection code should be given on an EICR where a standard domestic consumer unit is fitted which has a rating of 100A. Can any allowance be given on connected load being less than 100A or as the regulation relates to the rated current of the assembly and is a "shall" requirement does the load have no influence on the code assigned. 
Answer from Certsure

The Certsure Technical Helpline provides general information and guidance for compliance with the British Standard BS 7671, the Requirements for Electrical Installations, and matters concerning electrical safety within electrical installations designed, constructed, inspected, and tested to BS 7671. Without detailed knowledge of your installation, we cannot offer advice specific to your installation and can only generically provide comments based on the information you have provided.

The intent of the regulation is to ensure that the assembly is not overloaded with the additional generating set, as the main fuse may not protect the assembly if for example the internal busbar is pulling 116A.

Regulation 536.4.202 states: see regulation

From the viewpoint of an EICR, we would be looking for evidence that the assembly is being overloaded, such as burning, distorting and the likes.

The above regulation allows for diversity to be taken into account, so we can exercise our engineering judgement in declaring whether or not the assembly is suitably protected.

We trust that we have answered your current question; however if you require any further information or clarification, then please do not hesitate to contact us either by e-mail to helpline@certsure.com or by telephone on 0333 015 6628

I've read 536.4.202 and am interested on your views on the last paragraph with the shall requirement and how this ties in with the answer given by certsure. 536.4.3.2 is also relevant but has not been mentioned in the reply.

Thanks for your time.

Parents
  • If the unit can charge the battery using grid power, then in my view no special consideration is needed. The battery charger is simply another load like say a couple of fan heaters. A 100 amp service fuse will limit the sustained total current to 100 amps.

    If the equipment can supply the load by use of the solar power AND grid power at the same time, then it seems to me that the total load could be 116 amps and that the consumer unit should be rated at 116 amps or more.

    If the equipment can supply the load from solar power AND the battery AND from grid power, then it seems to me that the total load could reach 132 amps and that the consumer unit should therefore be rated at 132 amps or more.

    In practice I doubt that it matters much for several reasons. Firstly the overload resulting from use of solar AND/OR battery power as well as grid power will be fairly brief as the sun will go in or the battery run down. No one worries about a 100 amp consumer unit loaded short term to 116 amps, which the 100 amp cut out fuse will pass for hours.

    Also, if the backfeeding MCBs are installed at the end of the busbar distant from the main switch, then nothing will be overloaded in any case. Neither the main switch, nor any part of the busbar can be loaded to more than 100 amps, so why worry.

    However my view that it probably does not much matter counts for very little. A STRICT reading of regulations and guidance thereon would seem to require a consumer unit rated at more than 100 amps as described above.

Reply
  • If the unit can charge the battery using grid power, then in my view no special consideration is needed. The battery charger is simply another load like say a couple of fan heaters. A 100 amp service fuse will limit the sustained total current to 100 amps.

    If the equipment can supply the load by use of the solar power AND grid power at the same time, then it seems to me that the total load could be 116 amps and that the consumer unit should be rated at 116 amps or more.

    If the equipment can supply the load from solar power AND the battery AND from grid power, then it seems to me that the total load could reach 132 amps and that the consumer unit should therefore be rated at 132 amps or more.

    In practice I doubt that it matters much for several reasons. Firstly the overload resulting from use of solar AND/OR battery power as well as grid power will be fairly brief as the sun will go in or the battery run down. No one worries about a 100 amp consumer unit loaded short term to 116 amps, which the 100 amp cut out fuse will pass for hours.

    Also, if the backfeeding MCBs are installed at the end of the busbar distant from the main switch, then nothing will be overloaded in any case. Neither the main switch, nor any part of the busbar can be loaded to more than 100 amps, so why worry.

    However my view that it probably does not much matter counts for very little. A STRICT reading of regulations and guidance thereon would seem to require a consumer unit rated at more than 100 amps as described above.

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