UK Wiring Regulations 551.7.2 help - Understanding cable sizing when inverters are in parallel with a grid connection

[ashak]

Hi,

First time poster, so please be nice!

I've been pondering the feasibility of installing a pair of 8kW Sunsynk inverters in parallel with some battery storage in my UK property. For the time being, it would make things much simpler if all of this lived in my garage rather than the house. So I set about trying to plan how this might work and trying to be certain that what I was planning met the UK wiring regulations. However, i've been stumped by regulation 551.7.2 (which I ended up looking at after the IET Electrical Energy Storage Systems code of practice pointed it out in relation to cable sizing) and wonder if anyone can help me understand why the regulation is this way.

Anyhow, as I said, with the above potential system, I initially worked out that the inverters have a max output of 8800W or at 230V that would be 38.26A. That seems easy enough, I could connect these into my garage consumer unit with MCBs rated around 40-50A.

The two inverters in parallel would be 17.6kW or at 230V that would be 76.5A. The chargers on these inverters seem to be rated at the same power as the maximum output. Therefore I concluded that over the link between my garage and the house, I would never see more than 76.5A in either direction (since either the inverters will be charging batteries or they will be outputting power). I investigated the SWA cable between the garage and the house and it's 2x 16mm2 cable, using the tables in BS7671, I concluded that it would be possible for this cable, which is buried in the ground and seemed to be the thing that had the largest effect on the correctional factors, to carry 91A.

Great, so from the wiring regs course/exam I was taught that Ib <= In <= Iz <= It
Where Ib = Design current, In = Rating of protective device, Iz = Current carrying capacity of the cable, It = Tabulated current carrying capacity of the cable

So I think I have Ib = 76.5A and It = 91A. Since In needs to be in between those values, I could say In = 80A. I can't remember how to get from It to Iz, but that's not too important right now... It seems like this might work.... 76.5A <= 80A <= Iz <= 91A.. great

So them I remembered that with multiple sources you need to take into account the rating of the consumer units. The equation being InA >= In + Ig
Where InA = the rated current of the assembly, In = the rated current of the protective device, Ig = the rated current of the generating set
So... if my protective device is the 80A MCB I mention above and my inverters output 76.5A my consumer unit needs to be rated at 156.5A

Ok, whatever, i've not looked into whether that's feasible with a standard residential style UK consumer unit, and again, it's not too important in relation to my actual question. But reminding myself of this stuff in the code of practice also pointed out something else within regulation 551.7.2, which is relating to conductors of the final circuit when a generating set is installed on the load side of an overcurrent protective device. Which leads to the equation:
Iz >= In + Ig
Where Iz = current carrying capacity of the final circuits, In = the rated current of the protective device, Ig = the rated current of the generating set

The code of practice has a bunch of diagrams pointing all of these things out, comparing them to what I am trying to achieve, Iz would be the current carrying capacity of the SWA cable from the house, which i've already said is 91A (or thereabouts, again not 100% important to my actual question), but that then means
91A >= 80A + 76.5A is no longer true. Suggesting my cable is too small.

If that's the case, fine, that's how it should be. But for the cable I don't understand how this applies, which makes me think that i'm clearly misunderstanding something here. For the consumer unit in the garage it makes perfect sense. If by some miracle I had some things in my garage drawing 156A of current, in theory that should be possible with up to 80A coming from the grid and up to 76.5A coming from the inverters. But what I can't understand is how the same applies to the cable. No matter how I think about it, I would either be drawing up to 80A from the grid over to the garage, anything more would trip the breaker or the inverters would be feeding up to 76.5A back towards the house (or the grid) from the garage. At no point should that cable be carrying more than 80A. But if the code of practice and wiring regulations are to be believed, then the cable needs to be able to carry up to 156A...

So (at long last) the question is, why is this the case?

Thanks

 

[fourtytwo]

Have you got your G99 permission from your DNO yet ?
If not cable sizing is the least of your problems.
I suggest you try this UK based forum as they are pretty knowledgeable on UK regs
UK solar

 

[ashak]

Have you got your G99 permission from your DNO yet ?
If not cable sizing is the least of your problems.
I suggest you try this UK based forum as they are pretty knowledgeable on UK regs
UK solar

My main goal is to understand whether I can achieve what I want via this existing cable whilst staying compliant with the regulations. How the regulations state this cable should be rated just don't make sense to me so I wanted to try to understand why the regulations seem to suggest this.

I don't know whether the DNO would even entertain what I am hoping to achieve, nor have I made a G99 application before to give me some idea of what they will or won't allow, but before tackling that process I wanted to work out if I could do what I wanted to and remain within the regulations. Obviously I could replace the cable if I needed to do that, but there are reasons beyond the scope of my question that why I don't want to do that, which would likely change my plans.

But as suggested my main goal was to understand this specific part of the regs and why that cable would need to be rated in that way because it doesn't seem right to me.

Thanks for the suggestion of the UK solar forum, I will enquire there if I don't find anyone here that can shed any light on the matter

 

[kommando]

What is the DNO fuse rating on your supply before the meter. They are typically 60A or 100A.

If you are planning a 16kw system you may want to consider a 3 phase supply which will increase the network connection by a factor of 3. But then you would need to replace the line to the garage to 4 core.

 

[ashak]

What is the DNO fuse rating on your supply before the meter. They are typically 60A or 100A.

If you are planning a 16kw system you may want to consider a 3 phase supply which will increase the network connection by a factor of 3. But then you would need to replace the line to the garage to 4 core.

Thanks for the reply, It's 100A. I had thought about moving to a 3 phase supply especially if I couldn't get approval from the DNO to put 16kW of inverters on a single phase supply, but again there are reasons beyond the scope of my question for why I don't really want to do that right now if possible, which also relate to now wanting to replace the SWA cable between the house and the garage right now to support this.

Perhaps I should have included less information in my post, I had hoped that it would allow people to understand what i'm trying to do and why I am confused about rating the cable between my house and garage based on regulation 551.7.2 and the examples showing where and how that rating is applied from the code of practice.

 

[kommando]

Well if your fuse is 100A and the regs say you need a 156A wiring then you are overloading the 100A by a significant margin. Unless someone knowing the regs can get you back to under 100A then your scheme is dead on single phase and you need 3 phase.

 

[Quattrohead]

Long time expat so I am not up to date on UK regulations, but here in the USA as long as you size the breakers to protect the cable, that is the only thing they actually care about.
If you are thinking of selling back to your provider, that is a whole different ball game. If you can self-consume without a connection to the outside that is your easiest option.

 

[ashak]

Well if your fuse is 100A and the regs say you need a 156A wiring then you are overloading the 100A by a significant margin. Unless someone knowing the regs can get you back to under 100A then your scheme is dead on single phase and you need 3 phase.

I think you're misunderstanding what i'm getting at.

The 156A comes from an equation specifically for rating a cable when you have generating sets in parallel with with another source and you're attaching those generating sets within an existing consumer unit.

So in my case the generating sets are my inverters and the other source is the grid.

In relation to your response, the 156A would never traverse the 100A fuse. I would only be generating at maximum of 76.5A

Perhaps it's hard to understand without a diagram, so here's a terrible ascii art representation:

<grid>==<100A fuse>==<CU in house DB1>--<80A MCB in DB1>==_SWA_==<CU in garage DB2>--<40A MCB in DB2>--<8kW inverter>

Obviously there would be a second 40A MCB in DB2 and a second 8kW inverter based on my original

I am trying to check whether the rating of the cable above marked as ==_SWA_==, which I believe to to have a tabulated rating of 91A, is suitable based on regulation 551.7.2 in BS7671.

Both equations in that regulation pretty much say that the consumer unit i've marked as <CU in garage DB2> and the cable i've marked as ==_SWA_== should be rated at the combined current of the over current protective device (device marked <80A MCB in DB1> above) and the combined output current of the generating sets (so the 2x 8kW inverters)... which i'm saying is a combined maximum of 76.5A.

So 80 + 76.5A = 156.5A

Now, from the point of view of the consumer unit that i've marked as <CU in garage DB2>, this makes perfect sense. If I had loads hanging off of that consumer unit trying to consumer 156A, then in theory 76.5A from the two inverters and 80A from the grid (via the MCB marked 80A MCB in DB1 and cable marked ==_SWA_==) is entirely possible. So if that consumer unit were rated over 156.5A, that would be entirely possible... Unlikely, but possible.

But from the point of view of the cable marked ==_SWA_== it just doesn't make sense.
If we take the above scenario of loads connected to DB2 totalling 156A, the cable marked ==_SWA_== would only ever be carrying 80A of current or the MCB would trip.
If we reverse that entirely and say that there are no loads at all in the garage but I am consuming 156A of current in the house from loads connected to DB1 (again unlikely, but possible) the cable marked ==_SWA_== would only ever see 76.5A of that current, from the inverters. The remainder would come from the grid and never traverse the cable marked ==_SWA_==
Another alternative would be I am exporting 76.5A to the grid (very unlikely) and I have no other loads anywhere, the cable marked ==_SWA_== would only ever carry 76.5A from the inverters.

I can't think of any scenario where the cable marked ==_SWA_== could ever be carrying 156A unless it were under some fault condition. Even then, the 80A MCB would trip and all would be well. That's no different to putting an over current protective device in to protect any cable.

So... why does regulation 551.7.2 in BS7671 suggest that the cable I have marked as ==_SWA_== be rated in this way. It makes no sense to me when I don't think it could ever be carrying more than 80A

 

[ashak]

Long time expat so I am not up to date on UK regulations, but here in the USA as long as you size the breakers to protect the cable, that is the only thing they actually care about.
If you are thinking of selling back to your provider, that is a whole different ball game. If you can self-consume without a connection to the outside that is your easiest option.

Sizing the breakers to protect the cable was where I started out in my first post. Until I got to this regulation 551.7.2 in BS7671 and the associated stuff from the code of practice. At which point everything fell apart for a reason that I just can't understand

 

[kommando]

I don't know the regs and do not plan to study them either, but purely from logic if your interpretation of the regs is correct and a 156A wire is needed then your 100a fuse and 100a breaker will blow. So either your interpretation is wrong or your scheme is too powerful for the 100A fuse.

 

[SeaGal]

I will try and read through this and give some thoughts... in the meantime, can you clarify what you mean when you say...

If I had loads hanging off of that consumer unit trying to consumer 156A, then in theory 76.5A from the two inverters and 80A from the grid

That scenario doesn't seem to fit with BS7671 regulation 712.411.3.2.1.1 where it states "The PV Supply cable (on the AC side) shall be connected to the supply side of the protective device for automatic disconnection of circuits supplying current-using equipment."

 

[SeaGal]

OK... The 'description' of the regulation is "A new requirement in Regulation 551.7.2 relates to where a generating set is connected via an LV assembly e.g. a consumer or distribution board. The rated current of the LV assembly must take into account the additional supply from any generating set(s) . It requires the rated current of the assembly to be greater than, or equal to, the rated current or current setting of the incoming circuit overcurrent protective device either incorporated within the LV assembly or upstream of it, plus the rated output current of the generating set or sets."

I'm not a Sparky, but I'd read that as it would be the CU that needs to handle the load from the inverters as well as the incoming grid. Makes total sense and is in line with a previous posting I made here.

UK Hybrid ESS / paper work / approval

I've been so focused on what to buy that I hadn't looked too much at the paperwork etc. I've already ordered part of it and now full of regrets fully expecting that by the time I have negotiated sign off of my installation I'm going to have spent so much money it will never be worth it...

diysolarforum.com

In your diagram CU2 would therefore have to support the possible 80A coming from CU1 plus the generation from the inverters - that could happen if, as you say, it is possible to draw 156A in the garage.

Also, CU1 would have to support the 100A from the grid plus the max of 76A coming from inverters - that could happen if your house load was 176A.

But, with regard to..

why does regulation 551.7.2 in BS7671 suggest that the cable I have marked as ==_SWA_== be rated in this way

I don't see anything in the regulations that suggests that. As you point out, the max current that can be carried through the SWA is 80A.

With regard to your comment...

I had thought about moving to a 3 phase supply especially if I couldn't get approval from the DNO to put 16kW of inverters on a single phase supply,

I can't see any reason why your DNO would not let you put 16kW of ENA Type Tested inverters on your supply, but it is very likely (depending on your local grid network topology and number of other users) that they will either restrict your export without you paying for network re-enforcement - likely to be in the 5 figure bracket - or simply restrict your export with a G100 compliant export limitation feature.

 

[ashak]

Also, CU1 would have to support the 100A from the grid plus the max of 76A coming from inverters - that could happen if your house load was 176A.

Yes, I had thought about this too, but realised that people were already getting confused by my question, so didn't want to add to that confusion

I don't see anything in the regulations that suggests that. As you point out, the max current that can be carried through the SWA is 80A.

551.7.2 - A generating set used as an additional source of supply in parallel with another source shall be installed:
- on the supply side of all overcurrent protective devices for the final circuits of the installation, or
- on the load side of all the overcurrent protective devices for the final circuits of the installation, but in this case all the following additional requirements shall be fulfilled:
(i) The conductors of the final circuit shall meet the following requirement:
Iz >= In + Ig
Where:
Iz is the current carrying capacity of the final circuit conductors
In is the rated current of current setting of the protective device of the final circuit
Ig is the rated output current of the generating set
... it goes on

Quite ambiguous in my opinion, but then when considered with the examples in the code of practice,this particular example almost exactly matches my theoretical setup:


In1 i'm saying would be 80A, Ig i'm saying would be 76.5A (there are other example with multiple other sources on the same CU and they show Ig as Ig1 + Ig2, so I think i'm correct. So with those values, Iz must be >= 156.5A, but nothing explains why that is the case.

I can't see any reason why your DNO would not let you put 16kW of ENA Type Tested inverters on your supply, but it is very likely (depending on your local grid network topology and number of other users) that they will either restrict your export without you paying for network re-enforcement - likely to be in the 5 figure bracket - or simply restrict your export with a G100 compliant export limitation feature.

I had wondered about this and i'm hoping that's the case, right now at least I have no intention of exporting since it'll just be batteries charged overnight at a cheap rate and then used throughout the remainder of the day. Again, I didn't mention it due to further potential confusion.

Thanks for taking the time to get back to me.

 

[SeaGal]

Great, thanks for that clarification. I did some more research and there are some interesting discussion on this matter from about half-way down this IET forum thread...

Domestic consumer unit rating with PV and battery storage. - Wiring and the Regulations BS 7671 - IET EngX - IET EngX

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

engx.theiet.org

I can totally understand the need to make sure the CU's are rated according to the sum of grid + generation (as I mentioned above).

With regard to the interconnecting cable (i.e. both the one you ringed and the one from CU2 to ESS), it would appear that the wording of 551.7.2 does not take into account the fact that the ESS cannot be a load and a generator at the same time, which is perplexing.

However, the wording of BS7671 does not make that clear and, apparently has been in place since 2017.

The _only_ situation I could imagine (and this is a very theoretical situation) is if there was a fault within your SWA, could power be driven from both ends of the wire and overload the wire... but, no, thinking it through, if there was a fault causing a short half way along the SWA, current would only travel from each direction to that fault, in which case no part of the SWA would handle more than 80A (in your example) - so ignore that.

In my mind - and, as I said, I'm not a sparky... just a mere electronic engineer - the wording and example in 551.7.2 is wrong. I loved the comment in the thread that said "Messers Ohm and Kirchhoff would be spinning in their graves"

Ironically, and as briefly reference in the IET thread, the connection to a Solis 3.6kW inverter is via a 3 pin Wieland RST type connector which is rated at 20A. So it is technically impossible to 'double rate' the connection from the CU to the inverter. Which would mean that virtually all hybrid PV installations in the UK will have been installed without meeting regulation 551.7.2. Just plain crazy.

 

[ashak]

Thanks for the IET link, i'd failed at finding that, very interesting. I'm glad it's not just me that thinks this is a bit odd.

I may just rethink my plans and think about hooking the inverters up to the supply side of all overcurrent protective devices for the final circuits.

I still don't get how that would be possible though with a hybrid inverter, since the definition of a final circuit is: 'A circuit connected directly to current using equipment, or to ...'. So I think in this instance, because the inverters are also chargers and don't just produce, this becomes a final circuit (at least when charging)?
Gah, that little bit from that IET link about being a load and a generator at the same time makes alot more sense now too.

I guess if this was just PV with no batteries and therefore no charging circuit in the inverter, it wouldn't be an issue connecting it on the supply side of all overcurrent protective devices for final circuits since the circuit to the PV inverter wouldn't then be a final circuit.

Stupid regs! How annoying.

Thanks for your input SeaGal, it's much appreciated

 

[SeaGal]

I still don't get how that would be possible though with a hybrid inverter, since the definition of a final circuit is: 'A circuit connected directly to current using equipment, or to ...'. So I think in this instance, because the inverters are also chargers and don't just produce, this becomes a final circuit (at least when charging)?

Yes.

I guess if this was just PV with no batteries and therefore no charging circuit in the inverter, it wouldn't be an issue connecting it on the supply side of all overcurrent protective devices for final circuits since the circuit to the PV inverter wouldn't then be a final circuit.

Agree. To me it looks like there is an oversight in the wording of the regs.

Stupid regs! How annoying.

To be honest... I can't see many if any hybrid installations that would comply with the regs as they stand right now if the inverter has a plug-type connection, for the reasons mentioned above.

I'd just go with your existing configuration, assuming your CU's are suitably rated and make sure that the MCB as either end of the SWA are rated greater than the inverter's charge/discharge current but less than the current the SWA will handle.

... there are probably worse faults on most household ring mains that worrying about this technicality / oversight.

Thanks for your input SeaGal, it's much appreciated

YW.