Tail size

[flukey]

hi all

been having a discussion and would appreciate your views.

100 amp service head , 25mm tails.

if you were to split the tails with a henley to take a feed to say a shower enclosure, would the tails to this still need to be 25mm

 

[pevvers]

I would say no, as long as the length of the tails aren't too long, I think the recommendation is 3 metres unprotected as long as the overcurrent device for the shower is smaller than the current carrying capacity of the tails from Henley to new shower RCBO enclosure.

 

[flukey]

I would say no, as long as the length of the tails aren't too long, I think the recommendation is 3 metres unprotected as long as the overcurrent device for the shower is smaller than the current carrying capacity of the tails from Henley to new shower RCBO enclosure.

so you would put smaller tails in that are supplied by a 100amp main fuse

 

[pevvers]

Think like this...

A busbar chamber has tap offs on the bar that feed isolators around them, this could be a 400A chamber, and you are feeding a 63A switch fuse from it. The tails from the bars to the 63A isolator would not need to be big enough to protect that length from 400A as long as they are big enough for the 63A isolator that it is feeding, without overcurrent protection for the length between the bar and isolator.

If this wasn't the case, then there would be a lot of very big cables inside these busbar chambers!

Does this explain my method of thinking?

 

[flukey]

Think like this...

A busbar chamber has tap offs on the bar that feed isolators around them, this could be a 400A chamber, and you are feeding a 63A switch fuse from it. The tails from the bars to the 63A isolator would not need to be big enough to protect that length from 400A as long as they are big enough for the 63A isolator that it is feeding, without overcurrent protection for the length between the bar and isolator.

If this wasn't the case, then there would be a lot of very big cables inside these busbar chambers!

Does this explain my method of thinking?

it certainly does but my domestic installation certificate would look pretty funny with main fuse size 100amp and tails 10mm
we are talking about a run of less than a metre

 

[pevvers]

Yeah, I kinda agree that it would look weird, but it would comply with BS7671 in my opinion.

Have a look at 433.2.2, page 74 of the BRB

 

[flukey]

7671 states 25mm tails for 100 amp fuse. cant see why putting a henley in line means you can reduce cable size. got a tenner riding on this too

 

[pevvers]

Yes, 25mm to the CU that is supplying the rest of the installation, but for the shower enclosure that is only supplying the one shower circuit at say a maximum of 45A, then there is no need to run 25 to this enclosure from the blocks.

So we're saying 100A cutout with 25mm going to Henley's, then this is splitting to 25mm to the main CU and then a length of as of yet undetermined CSA cable going to a shower enclosure supplying just one piece of current using equipment?

If so, then my reasoning stands that as long as the as of yet undetermined length of cable is less than 3 metres long and it's corrected current carrying capacity is greater than the value of the overcurrent protection in the shower enclosure (hopefully an RCBO) then all is good.

Good luck with that tenner, although I don't know whether you want me to be right or wrong!!!

 

[sparks1234]

What nobody here has thought about is your single point of isolation for an installation, you should no longer be splitting tails in a henly block without installing an isolator before it

 

[flukey]

Yes, 25mm to the CU that is supplying the rest of the installation, but for the shower enclosure that is only supplying the one shower circuit at say a maximum of 45A, then there is no need to run 25 to this enclosure from the blocks.

So we're saying 100A cutout with 25mm going to Henley's, then this is splitting to 25mm to the main CU and then a length of as of yet undetermined CSA cable going to a shower enclosure supplying just one piece of current using equipment?

If so, then my reasoning stands that as long as the as of yet undetermined length of cable is less than 3 metres long and it's corrected current carrying capacity is greater than the value of the overcurrent protection in the shower enclosure (hopefully an RCBO) then all is good.

Good luck with that tenner, although I don't know whether you want me to be right or wrong!!!

i would like you to be wrong but every days a school day thanks for the replies

 

[pevvers]

What nobody here has thought about is your single point of isolation for an installation, you should no longer be splitting tails in a henly block without installing an isolator before it

Ah, but that wasn't the question!