Plug in solar

[ebee]

OK John, I will stand corrected, an unfused spur from a ring or radial is part of that ring circuit or radial circuit.
If it contains a fuse (such as an FCU or a SFCU then it becomes a circuit in its own right - that what I meant but you spotted my non-deliberate mistake.

 

[denso13]

We haven't seen approved installation instruction yet, but it seems they are defining a circuit as everything connected to a single breaker in a consumer unit, i.e., everything that a breaker turns off is one circuit.

 

[JohnW2]

We haven't seen approved installation instruction yet, but it seems they are defining a circuit as everything connected to a single breaker in a consumer unit, i.e., everything that a breaker turns off is one circuit.

That's the BS7671 definition of 'a circuit', but as I eluded to it gets a bit more complicated (sort-of ambiguous) when one had two or more overcurrent protective devices (OPDs, like breakers and fuses) in series (an FCU downstream of a breaker in a CU being a very common example).

In that situation, one of the OPDs (the one in the CU) will, indeed kill 'everything', thereby suggesting that everything downstream of that OPD is 'one circuit'. However, an OPD downstream of the first one (e.g.an FCU) only kills whatever is downstream of it, leaving the bits upstream (between OPDs) still energised (only 'killable' by the breaker in the CU), suggesting that the bits before and after the FCU must be different circuits, hence a total of two circuits.

That's why I wrote that when there are two OPDs, by the BS7671 definition one can at one and the same time have bits of wiring which constitute both 'one circuit' and 'two circuits'

 

[denso13]

In that situation, one of the OPDs (the one in the CU) will, indeed kill 'everything', thereby suggesting that everything downstream of that OPD is 'one circuit'.

Is that not what it then means, it seems to say that? Although we haven't seen any actual approved wording.

It's really for a DIY plug in scenario, if someone wants to employ an electrician to confirm things/install then it might be different.

Posters who know a lot more than me on the electrics forum frequently have disagreements on what things mean, most recently this one, https://www.diynot.com/diy/threads/fitting-an-isolator-between-meter-and-consumer-unit.659358/

I'd be gobsmacked if the plug in solar regulations had everyone in agreement on here, rightly or wrongly. Seems like they were always on a hiding to nothing.

 

[ericmark]

We haven't seen approved installation instruction yet, but it seems they are defining a circuit as everything connected to a single breaker in a consumer unit, i.e., everything that a breaker turns off is one circuit.

I have seen consumer units put in sheds, and the whole thing was supplied from a fused spur, from a ring final circuit, and a fused spur can't form a new circuit, as the ring is the final circuit.

This English question has been raised many times, a RCD is an overcurrent device, OK not line to neutral or line to line, but still measures current, not voltage, so no getting around it, it is an overcurrent trip.

So the incoming supply uses many overcurrent devices before it reaches a 13 amp socket, from the DNO fuse, to RCD, MCB, RCBO, and fuse so we have main and sub circuits, radial does not appear in my old copy of BS7671 definitions, appendix 15 calls both ring and radial a final circuit but only the Ring final circuit. A final circuit arranged in the form of a ring and connected to a single point of supply. Is in the definitions, and that word "final" is important, as you can't in English have another circuit from a final circuit, and that is final!

So what does it matter? Well if a circuit is fed from two current limiting devices, the current draw is the sum of those current, so taking my own house, the DNO fuse is 60 amps, the inverter is 5 kW (21.7 amp) so the total feed into the consumer unit is 81.7 amps, the CU is rated at 100 amps, so all is OK.

So a 16 amp radial with an 800 watt (3.5 amp) inverter, can deliver 19.5 amps, and 2.5 mm² is rated 20 amps or more with most installation methods, but with a 20 amp radial looking at 23.5 amps, now not quite so cut and dried, Reference Method C* (Clipped direct) is rated 27 amps, but most others rated 21 amp or less, and with a 25 amp overload no real option, 28.5 is over its rating.

So with a ring, the cable must be rated 20 amps or more, and the extra 3.5 amp is spread between both legs, but now it depends on where the extra 3.5 amp is inserted, centre of the ring so 1.75 amp per leg extra maybe not so bad, but the ring final assumes 20 amp draw is central, and 12 amp even spread, but this does not need to be the case, so to reduce the chance of one leg being overloaded for a significant time, appendix 15 tells us we should run any non portable item over 2 kW from a dedicated circuit.

We have always done this with the immersion heater, next is likely the tumble drier, a condenser drier unless heat pump type, using over the 2 kW limit for an extended time, so should have a dedicated supply, a vented type often has the option of 1 kW, and the heat pump 750 watt, but the condenser drier can run for over an hour at over 2 KW so should have a dedicated circuit.

The washing machine clothes or dishes, only uses the high current for a short time. But the tumble drier is another story.

So we are looking at possible overload which can result with a plug in solar, be it radial or ring, and I see no way this can be avoided.

To look at a circuit in isolation, and say for example that circuit only feeds the immersion heater so maximum of 13 amps, and it uses 2.5 mm² cable rated at 20 amps, so there is no problem adding 3.5 amps to the circuit, is fine. But to say you can add 3.5 amps to any random circuit is completely different.

Tasked with the job of writing an instruction set, I simply can't see how it can be done. Looking at an EV, we use current transformers (CT) to see what rest of house is using, but we have nothing like that with plug in solar.

So the government thinks it's OK, so up to them to write the instruction set, as I can't see how it can safely work.

 

[denso13]

I have seen consumer units put in sheds, and the whole thing was supplied from a fused spur, from a ring final circuit, and a fused spur can't form a new circuit, as the ring is the final circuit.

This English question has been raised many times, a RCD is an overcurrent device, OK not line to neutral or line to line, but still measures current, not voltage, so no getting around it, it is an overcurrent trip.

So the incoming supply uses many overcurrent devices before it reaches a 13 amp socket, from the DNO fuse, to RCD, MCB, RCBO, and fuse so we have main and sub circuits, radial does not appear in my old copy of BS7671 definitions, appendix 15 calls both ring and radial a final circuit but only the Ring final circuit. A final circuit arranged in the form of a ring and connected to a single point of supply. Is in the definitions, and that word "final" is important, as you can't in English have another circuit from a final circuit, and that is final!

So what does it matter? Well if a circuit is fed from two current limiting devices, the current draw is the sum of those current, so taking my own house, the DNO fuse is 60 amps, the inverter is 5 kW (21.7 amp) so the total feed into the consumer unit is 81.7 amps, the CU is rated at 100 amps, so all is OK.

So a 16 amp radial with an 800 watt (3.5 amp) inverter, can deliver 19.5 amps, and 2.5 mm² is rated 20 amps or more with most installation methods, but with a 20 amp radial looking at 23.5 amps, now not quite so cut and dried, Reference Method C* (Clipped direct) is rated 27 amps, but most others rated 21 amp or less, and with a 25 amp overload no real option, 28.5 is over its rating.

So with a ring, the cable must be rated 20 amps or more, and the extra 3.5 amp is spread between both legs, but now it depends on where the extra 3.5 amp is inserted, centre of the ring so 1.75 amp per leg extra maybe not so bad, but the ring final assumes 20 amp draw is central, and 12 amp even spread, but this does not need to be the case, so to reduce the chance of one leg being overloaded for a significant time, appendix 15 tells us we should run any non portable item over 2 kW from a dedicated circuit.

We have always done this with the immersion heater, next is likely the tumble drier, a condenser drier unless heat pump type, using over the 2 kW limit for an extended time, so should have a dedicated supply, a vented type often has the option of 1 kW, and the heat pump 750 watt, but the condenser drier can run for over an hour at over 2 KW so should have a dedicated circuit.

The washing machine clothes or dishes, only uses the high current for a short time. But the tumble drier is another story.

So we are looking at possible overload which can result with a plug in solar, be it radial or ring, and I see no way this can be avoided.

To look at a circuit in isolation, and say for example that circuit only feeds the immersion heater so maximum of 13 amps, and it uses 2.5 mm² cable rated at 20 amps, so there is no problem adding 3.5 amps to the circuit, is fine. But to say you can add 3.5 amps to any random circuit is completely different.

Tasked with the job of writing an instruction set, I simply can't see how it can be done. Looking at an EV, we use current transformers (CT) to see what rest of house is using, but we have nothing like that with plug in solar.

So the government thinks it's OK, so up to them to write the instruction set, as I can't see how it can safely work.

Are Joe public going to know any of that though? A circuit is everything a breaker shuts off from the consumer unit and if in doubt get an electrician.

The manufacturer's write the instructions (which hasn't been done yet) which will get G98 approval, or not. If it gets type approval then Joe public buys it, follows the instructions and plugs it in.

 

[ebee]

I have seen consumer units put in sheds, and the whole thing was supplied from a fused spur, from a ring final circuit, and a fused spur can't form a new circuit, as the ring is the final circuit.

Yes Eric the Ring Final is a final circuit in its own right and supplis sockets etc but as far as fused spur is concerned it is the distribution circuit for that fused spur and if you put another fused spur fed from that fused spur (lower fuse value of course) then that fused spur is fed from a fused spur fed from the ring final circuit so both the ring final and the first spur are distribution circuits to that last spur! Strange innit? but that is the way it is.
A bit like the Monarch is a kingg or Queen over other kings and nobles but still all over us lot (the Serfs) LOL

 

[denso13]

A bit like the Monarch is a kingg or Queen over other kings and nobles but still all over us lot (the Serfs) LOL

Yes, but there is an ultimate Monarch, not equal Monarchs...

 

[ebee]

yes but a king could be the monarch but not all kings or queens are monarchs just the one - you think final circuits being also distribution circuits to some other final circuits make being monarchs or kings and queens feel easy

 

[denso13]

you think final circuits being also distribution circuits to some other final circuits

Yes, but is there, normally, a breaker is at the beginning of the chain?

 

[pcaouolte]

Joe public buys it, follows the instructions and plugs it in.

Some Joe public will buy it, ignore the instructions, and plug it in, much the way that they do with a fridge or any other electrical gadget with a plug on it.

Some Joe public will buy more than one, ignore the instructions and plug more than one in.

Some Joe public will read the instructions and be unable to understand why the instructions don't allow them to do what they originally intended to do when the bought more than one, so they will crack on regardless.

No one will police it and we end up with properties with several of the things connected to whatever socket is nearby, if that socket needs a four way extension to allow several solar units to be plugged in, that's what will happen.

 

[denso13]

Some Joe public will buy it, ignore the instructions, and plug it in, much the way that they do with a fridge or any other electrical gadget with a plug on it.

Some Joe public will buy more than one, ignore the instructions and plug more than one in.

Some Joe public will read the instructions and be unable to understand why the instructions don't allow them to do what they originally intended to do when the bought more than one, so they will crack on regardless.

Well yes, such is life, and no different to pre-plug in solar. Some people drive cars too fast as well.

 

[ericmark]

Are Joe public going to know any of that though?

It should all be in the manufacturers instructions, but I bet it's not. As electricians, we are taught how to arrange things so with an overload, some device will auto disconnect.

There was one odd one out, when a system was designed near the end of World War II to reduce the amount of copper required for the rebuild. So we had a ring of 7/029 cable, which is between the new 2.5 mm² and 4 mm² we also had solid live pins on the plugs, which could transfer heat away from the fuse in the plug better, and the 4% permitted volt drop was increased to 5% which allows more cable to be used in a ring final from around 80 meters to 106 meters, all these have reduced the current carrying capacity of the ring final, but we have increased the 30 amp fuse to a 32 amp MCB/RCBO most to do with harmonising with Europe, so over the years the ring final has been degraded, and we just hope that having multi-ring finals, and reducing what items use, will stop the ring final from being overloaded, and the regulations suggest that non-portable items over 2 kW should have a dedicated circuit, this will it hopes mean items which use high power for long periods will not be overheating the ring final.

We used to split the ring side to side of the house, but to keep the split same as lighting, so as long as we have table lamps, etc, an RCD tripping will not plunge a room into darkness, but this means the power is not even distributed, with some homes we have rings just for the kitchen, but there is no standard layout, so each home needs assessing there is no common formula to say if you do XYZ then it is safe to use solar.

So the instruction set must lay all this out before anyone buys the kit, once bought it is too late, so we need some rapid reader, so people know before they buy if the units are suitable for their home.

And after all that, no battery, so we have two methods of getting non electrician installed solar, and I for one, could not say which is the best value for money. There are loads of videos about people who have fitted solar, but many already had solar, so were paid for export, or they fitted batteries.

Efixx have already done a video, and the house where he lives, is littered with solar panels and batteries. I would love to hear how battery without connection to grid compares with no battery but connected to grid compares.

 

[denso13]

It should all be in the manufacturers instructions, but I bet it's not.

I agree, but there is no need for that much detail. Let's see what comes out.

 

[JohnW2]

Is that not what it then means, it seems to say that?

I'm sure it does, IF there is only one OPD involved - but, as I said, it becomes more complicated, and potentially ambiguous, if there is more than one ...

Given two OPDs [say an MCB in the CU followed by an FCU (with fuse) somewhere along the length of the 'physical' circuit run (i.e.'all the wiring') ], the upstream device (MCB in CU) will, indeed (per BS7671) define the entirety of that 'physical circuit' (including any branches or unfused 'spurs'). However, the other OPD (fuse in FCU) will define everything downstream of the OPD as being the entirety of one circuit, thereby leaving eveything upstream of the FCU as being a separate, second, 'circuit'!!
So, depending upon how one looks at it, such a situation can satisfy the BS7671 definition of one circuit OR of two circuits

Although we haven't seen any actual approved wording.

I doubt that you'll ever see any 'actual approved wording' which is not EITHER silent on this matter OR which refers (explicitly or implicitly) to the BS7671 definition