Tail sizes from PV sub-board

[CBR600]

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]

Crack on .... zero chance of overload

 

[davesparks]

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]

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]

... 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]

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]

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]

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]

Why are you asking what is your involvement.

 

[CBR600]

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]

Why are you asking what is your involvement.

I organise the installs......

 

[westward10]

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

 

[davesparks]

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]

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]

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