Tail sizes from PV sub-board

CBR600 · Sep 25, 2018

Evening all, just a quick question from a non electrician before you shout! Our systems are all installed using a sub DBB (rather than into the existing board). Normally we would fit using 25mm tails however a contract we are currently on say that 10mm tails can be used............any views?

Octopus · Sep 25, 2018

Crack on .... zero chance of overload

davesparks · Sep 25, 2018

We need a lot of information before we can advise.
What is the nature of the installation? Commercial/domestic
What is the design current here?
What and where is the overload and fault protection relevant to these tails?
What is the installation method for the tails?

CBR600 · Sep 25, 2018

davesparks said:
We need a lot of information before we can advise.
What is the nature of the installation? Commercial/domestic
What is the design current here?
What and where is the overload and fault protection relevant to these tails?
What is the installation method for the tails?

These are domestic single phase installs with maximum 16amps generation. They are connected via a Henley Block to the main house tails and there is normally a 100amp supply head fuse.

Wilko · Sep 25, 2018

CBR600 said:
... are connected via a Henley Block to the main house tails and there is normally a 100amp supply head fuse.

I wouldn't connect 10mm to a 100A fuse without some thought. And then I'd not do it

.

davesparks · Sep 25, 2018

CBR600 said:
These are domestic single phase installs with maximum 16amps generation. They are connected via a Henley Block to the main house tails and there is normally a 100amp supply head fuse.

So what overload protection is there for the tails of you were to install tails less than 25mm.

There are ways of installing this so that tails suitably sized for the current could be installed instead of the oversized 25mm tails.

Octopus · Sep 25, 2018

davesparks said:
So what overload protection is there for the tails of you were to install tails less than 25mm.

There are ways of installing this so that tails suitably sized for the current could be installed instead of the oversized 25mm tails.

Hum ...........tin hat on .......... not sure I agree with you ........ sounds like the installation is head, henley block, then off to 2+ CU's ............ a mini CU with a 32A MCB, say for a shower, won't pull over 65A so whats the problem?

bunker time!

CBR600 · Sep 25, 2018

davesparks said:
So what overload protection is there for the tails of you were to install tails less than 25mm.

There are ways of installing this so that tails suitably sized for the current could be installed instead of the oversized 25mm tails.

As I said I am not an electrician so bear with me. I am assuming you mean that if using the 10mm tails for the PV connection they should be fused down at the point of connection rather than using an HB.

westward10 · Sep 25, 2018

Why are you asking what is your involvement.

CBR600 · Sep 25, 2018

Murdoch said:
Hum ...........tin hat on .......... not sure I agree with you ........ sounds like the installation is head, henley block, then off to 2+ CU's ............ a mini CU with a 32A MCB, say for a shower, won't pull over 65A so whats the problem?

bunker time!

Yes, this is the argument that is being used (well similar). I am sitting between an electrician saying it shouldn't be done and a contract engineer that is says there is no issue.

CBR600 · Sep 25, 2018

westward10 said:
Why are you asking what is your involvement.

I organise the installs......

westward10 · Sep 25, 2018

If there are spare ways why are you not connecting direct to the dist board.

davesparks · Sep 25, 2018

CBR600 said:
Yes, this is the argument that is being used (well similar). I am sitting between an electrician saying it shouldn't be done and a contract engineer that is says there is no issue.

If they know what they are talking about then they will be able to reference specific regulations to support their point (well one of them will at least)

Who is responsible for the electrical design and certifying the electrical installation for compliance with BS7671 and part P?
Whoever it is they are effectively the person who has final say on this matter.

Wilko · Sep 25, 2018

CBR600 said:
Yes, this is the argument that is being used (well similar). I am sitting between an electrician saying it shouldn't be done and a contract engineer that is says there is no issue.

Ha - tell your contract engineer to apply Ohm's Law to Ze = 0.35 and Uo.Cmin = 218.5V. Then look up the 100A fuse disconnection time and plug that into the adiabatic equation with k = 115.
Spoiler Alert - 10mm tails seem too small (in this situation) to melt the fuse without themselves being heat affected.

Ian1981 · Sep 25, 2018

Murdoch said:
Hum ...........tin hat on .......... not sure I agree with you ........ sounds like the installation is head, henley block, then off to 2+ CU's ............ a mini CU with a 32A MCB, say for a shower, won't pull over 65A so whats the problem?

bunker time!

Overload protection would not be the issue however if the 10mm tails are adequately sized for fault protection then it’s acceptable as you stated

davesparks · Sep 25, 2018

CBR600 said:
As I said I am not an electrician so bear with me. I am assuming you mean that if using the 10mm tails for the PV connection they should be fused down at the point of connection rather than using an HB.

That’s not what I mean. Overload protection can be installed after the reduction in conductor size in these situations if certain criteria are met.
If feeding a switchfuse or other 1way CU then it can be made to comply easily. If you are leaving spare ways which could be used to feed something else and increase the load then it may be less easy to comply.

davesparks · Sep 25, 2018

Wilko said:
Ha - tell your contract engineer to apply Ohm's Law to Ze = 0.35 and Uo.Cmin = 218.5V. Then look up the 100A fuse disconnection time and plug that into the adiabatic equation with k = 115.
Spoiler Alert - 10mm tails seem too small (in this situation) to melt the fuse without themselves being heat affected.

See regulations 433.2.2 and 434.2.1, if installed correctly there can be 3metres of reduced size tails between the meter and the CU without upstream fault protection.

davesparks · Sep 25, 2018

Ian1981 said:
Overload protection would not be the issue however if the 10mm tails are adequately sized for fault protection then it’s acceptable as you stated

Or if you apply 434.2.1

CBR600 · Sep 25, 2018

Ok thanks, that makes a bit of sense. The pv board houses an rcbo for the pv and will have a din mounted generation meter. It will have no spare ways.

CBR600 · Sep 25, 2018

westward10 said:
If there are spare ways why are you not connecting direct to the dist board.

To prevent the chances nuisance tripping the main board.

Leesparkykent · Sep 25, 2018

westward10 said:
If there are spare ways why are you not connecting direct to the dist board.

I’ve seen many installed this way that do not comply.

Ian1981 · Sep 25, 2018

davesparks said:
Or if you apply 434.2.1

Installed in galv trunking perhaps as one method?

davesparks · Sep 25, 2018

Ian1981 said:
Installed in galv trunking perhaps as one method?

Yes, or plastic trunking, properly clipped/restrained depending on the exact location and environment.

davesparks · Sep 25, 2018

CBR600 said:
To prevent the chances nuisance tripping the main board.

Tripping what and how?

CBR600 said:
Ok thanks, that makes a bit of sense. The pv board houses an rcbo for the pv and will have a din mounted generation meter. It will have no spare ways.

Is there a main switch in the board?

westward10 · Sep 25, 2018

You must have some design criteria for this want of a better words, lash up.

westward10 · Sep 25, 2018

CBR600 said:
To prevent the chances nuisance tripping the main board.

If you are worried your new install is likely to cause nuisance tripping then you need to reassess what you are doing as the best method is to install directly to the existing dist board providing there are spare ways.

Leesparkykent · Sep 26, 2018

westward10 said:
If you are worried your new install is likely to cause nuisance tripping then you need to reassess what you are doing as the best method is to install directly to the existing dist board providing there are spare ways.

I wouldnt say it’s the best way if there is spare way on a RCD protected way that shares the same side as a socket circuit for example. It would actually be non compliant due to how long the inverter takes to shut down which can be up to 5 seconds under G83.

Octopus · Sep 26, 2018

westward10 said:
If you are worried your new install is likely to cause nuisance tripping then you need to reassess what you are doing as the best method is to install directly to the existing dist board providing there are spare ways.

That's VERY wrong for the reason Lee gave above ........... but is the most common method of install I see ......

CBR600 · Sep 26, 2018

Just a quick thanks to everyone for the contributions on this last night. All very helpful.

westward10 · Sep 26, 2018

Leesparkykent said:
I wouldnt say it’s the best way if there is spare way on a RCD protected way that shares the same side as a socket circuit for example. It would actually be non compliant due to how long the inverter takes to shut down which can be up to 5 seconds under G83.

Where does it state this would be non compliant, not had much to do with these systems so would be interested.

Wilko · Sep 26, 2018

Leesparkykent said:
I wouldnt say it’s the best way if there is spare way on a RCD protected way that shares the same side as a socket circuit for example. It would actually be non compliant due to how long the inverter takes to shut down which can be up to 5 seconds under G83.

Learning every day.
If I've understood correctly - in a dual RCD board if they've attached PV invertor to MCB under one of the RCDs, if that RCD detects a fault and trips, great. But the invertor continues to put 230V onto the common MCB busbar. So ADS has not been achieved (!!!).

davesparks · Sep 26, 2018

westward10 said:
Where does it state this would be non compliant, not had much to do with these systems so would be interested.

I can’t give a regulation, but plain old common sense should be enough.
If a person receives a shock the RCD will trip, but the PV will continue to supply power to all circuits supplied by that RCD for a period of time, therefore defeating the purpose of the RCD and killing the person.

westward10 · Sep 26, 2018

I can see the sense of it now I just wondered if this is quoted anywhere.

Ian1981 · Sep 26, 2018

Regulation 712.411.3.2.1.1 states that on the ac side the pv cable shall be connected to the supply side of a OCPD for automatic disconnection of circuits supplying current carrying equipment

SibertSolar · Oct 19, 2018

Nuisance tripping "might" be a problem when sharing an RCD, but that's the least of the potential issues. More likely (when using transformerless inverters) is that the Type-AC RCD (usually fitted in residential CUs) might become saturated and never trip under fault conditions. That's in addition to the scenario outlined by others relating to the shutdown times of PV inverters and ADS intention of the primary RCD in the first place.....amazing that this topic still causes discussion so many years down the line!! As for the OP's original question, using 10mm2 tails from henley blocks broken into the meter tails (25mm2) is perfectly fine so long as the OCPD on the PV connection is suitably rated. In fact, I'd go so far as to say this is a better method that using a spare way on the non-RCD protected side of the main CU (various reasons including ease of monitoring and avoiding potential issues with RCDs....)

Just my tuppence...