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
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