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
  • A far nicer solution, as the DNO  will have to re-cable the street anyway, if the predicted EV and heat pump loads come to pass, will be to go continental, not perhaps in the sense of coffee and croissants, but to make 3 phase supplies to the house more common.  3 phase boards with a decent current rating are already available.

    In the meanwhile the load cutting car chargers and so on are a good stop-gap and the same idea may extend to other loads so the heating goes off during peaks of cooking or something.

    For now the risk is very much on paper. A CU that is really good for 100A all day in all possible domestic installation situations, probably does not exist right now, and if it did then it would probably also be good for 110A in all but the warmest.

    Mike,

Reply
  • A far nicer solution, as the DNO  will have to re-cable the street anyway, if the predicted EV and heat pump loads come to pass, will be to go continental, not perhaps in the sense of coffee and croissants, but to make 3 phase supplies to the house more common.  3 phase boards with a decent current rating are already available.

    In the meanwhile the load cutting car chargers and so on are a good stop-gap and the same idea may extend to other loads so the heating goes off during peaks of cooking or something.

    For now the risk is very much on paper. A CU that is really good for 100A all day in all possible domestic installation situations, probably does not exist right now, and if it did then it would probably also be good for 110A in all but the warmest.

    Mike,

Children
  • A far nicer solution, as the DNO  will have to re-cable the street anyway, if the predicted EV and heat pump loads come to pass, will be to go continental, not perhaps in the sense of coffee and croissants, but to make 3 phase supplies to the house more common.  3 phase boards with a decent current rating are already available.

    I wouldn't disagree with that, but perhaps a consideration is that it would be recommended to use three-phase inverters in domestic situations ... or over-rate the Neutral [in the supply main and upstream of the connection points of inverters] by roughly the sum of all inverters on the phase with the largest current output from inverters. The reason being, if the inverters are exporting on a phase with little load, and the other two phases have a lot of load, consider the vector sum of the Neutral currents

  • yes the other advantage is the 16A/phase limit allows for a more sensibly balanced export - perhaps also making lost PEN detection easier and a few other things into the bargain.  I cant see it happening under the current funding arrangements though  - at least round here the not quite enough DNO folk already have their work cut out just keeping the system running as it is. (I almost said ' on a fire brigade basis' but that is not quite the right image is it ... )

    mike

  • (I almost said ' on a fire brigade basis' but that is not quite the right image is it ... )

    Definitely not ... Joy

  • Wow. I must admit I`d never considered that. In simple terms if you had three single phases at 100A pull each on a three phase supply but one of those 100A was in fact on export then you`d need to redraw the vectors and say make the export vector minus 100A which would skew the N vector well off course? if I understand properly