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.

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 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.

  • 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 ... )


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

    Definitely not ... Joy

  • Although you wouldn't have such an arrangement in the UK on a simple domestic under G83/G98 as there's the 16A/phase limit on embedded generation. If you went down the G59/G99 route you could have more but then you'd have all sorts of extra current sensors and relays that could limit the totals anyway.

    You say you wouldn't have such an arrangement in the UK I feel this is just where things are heading. My original question to the NICEIC was back in April since then I have been scratching my head and getting on with work. I have also applied to the local DNO for the addition of a 3kW battery inverter to an existing 4kWp PV system under G99 and received the go ahead for the addition. The current clamps installed will only limit the exported energy on to the grid not the energy consumed by the installation.


    Although the project details state maximum import capacity of 20kA I know the DNO fuse is 100A. PV inverter is limited to 4kW output and the battery inverter is limited to 3kW output with export limit to be 6kW. The 100A main switch on a domestic consumer unit would usually rely on the DNO main fuse for overcurrent protection as per manufacturer guidance.

    There are many manufacturers supplying these stand alone and combined battery storage systems and social media seems awash with pictures of installed systems that are being added to existing installations.

    With this type of arrangement do I have to consider the additional 7kW as the main switch is protected in import by the 100A DNO fuse and in export by the export limit set in the inverter. If the installation were to consume 24kW+7kW=31kW this energy will be shared across the busbar not through main switch, are we to consider the current carrying capacity of the whole assembly or just the individual switches within the assembly.

    If the installation was split into separate consumer units where generation/battery storage was on one and consumption on another, final circuits totalling more than the100A main switch in the consumption unit would require a OCPD to protect the main switch even if diversity is taken into account.

    This sort of information is not included in the manufacturer's training.

    G99 appears to be set up more to limit export energy than limit generated energy as I see mention of domestic properties with 10kW+ of PV + battery storage on single phase with an export limit of 3.68kW.

    I appreciate the DNO fuse will carry much greater current than its rating before rupture and main switches will carry many amps above their rating before melting. I also appreciate the rare occasion that the solar will produce 4kW and 31kW could be consumed by this domestic installation but times are changing.

  • are we to consider the current carrying capacity of the whole assembly

    551.7.2 says the whole assembly. The main switch of the assembly in general need only to be rated for the maximum of the demands (import or export), but should be selected in accordance with the assembly manufacturer's instructions when it's in an assembly (CU).

  • Last year I phoned the NAPIT technical helpline and asked:

    How would you code an installation with just the DNO supply and a 100 amp fuse and a maximum demand of 175 amps that will quite possibly have a load exceeding 100 amps?

    The same answer applies to an installation that has a steady demand of 116 amps on a supply fuse rated at 100 amps when the batteries in the storage unit are flat so the full 116 amps is being drawn from the network, the demand outstrips the supply, what’s the code on an EICR?

  • If I'm reading the guidance correctly, in simplistic terms.

    A property with a DNO 100A fuse with a consumer unit CU1 rated for 100A  with a 100A main switch where final circuits total more than 100A is compliant as the assembly is protected by the 100A DNO fuse.

    If a generator eg EV back feed, solar PV, wind, battery, CHP etc is added either via a internal MCB in CU1 or a separate consumer unit the installation is no longer compliant unless the DNO fuse rating is reduced or a switch fuse is installed to protect the CU1 assembly.

    Diversity cannot be taken into account to come to a more realistic load value for the assembly.

  • This is effectively what the newly-added bit in Regulation 551.7.2 in BS 7671:2018+A2:2022 says.

  • From my experience the majority of single phase and three phase installations with 100A fused DNO supplies, 100A per phase rated distribution/consumer units with some form of additional supply eg Solar PV etc are not compliant with Regulation 551.7.2 in BS 7671:2018+A2:2022, would a code be given if there is no sign of overheating on a new EICR

    Anyone designing a system in this way would have to detail the arrangement as a departure on the installation certificate, would evidence have to be provided to show the installation is no less safe.

  • would a code be given if there is no sign of overheating on a new EICR

    That is subject to industry guidance. Not sure about Code Breakers, but this particular aspect (Regulation 551.7.2 amended in 2022) is not covered yet in the ESF BPG3, or in  BPG4 ... the nearest could be argued to be lack of overcurrent protection, which leads to C2, but this is not 100 % clear?

    However, back to the question I asked earlier: when carrying out an EICR, do you know the rating of the DNO cutout? Even if it says 100 A on the carrier, it may not be 100 A in there!

    For the coding argument to stand up (noting it would be unsatisfactory if a C2 is recorded) then it's important to take account of the real cutout rating, not assume it.

    The guidance ought to address that also ...

  • I understand the label on the DNO cut out is just the maximum rating not the actual fuse size, wouldn't it be nice if the actual fuse rating was shown.

    I still have more head scratching to do.

    Thanks for your time.