Meter tails

[ohmy]

Where there isn't a spare way in a domestic CCU and you decide to go from the Henly block to a new garage CCU with an RCD and a single 16A MCB which connects the inverter to the grid. Will 10mm tails be ok, or would they need to be the same as the other tails going out of the Henly (25mm)?

I'd think they'd be ok as there will be no more than 16A going through them at any time, (apart from short circuit current)...they are rated to at 47A as double ins single core?

There is a spare way in the garage unit, which had a 6A MCB which has been removed and blanked. I suppose this opens up the possibility of someone putting in a circuit exceeding the current carrying capacity of the 10mm tails... ?

It's been bugging me because I was told to change them to 25mm tails

 

[JulianC]

Tail size determined by service head fuse size. 100A fuse, 25mm tails. 60A fuse, 16mm tails etc...

 

[ohmy]

http://www.electriciansforums.net/e...ulations/10243-10mm-out-henley-block-rcd.html

confused???

 

[nickblake]

the answer would be no , because there would be no over load protection for those tails as they would be protected by the main supply fuse 60 / 80 / 100 amps , you also need to provide one switch for isolation for the complete installation , which you may have if your one of the lucky ones , if the cut out fuse is labled as 100 amp then the tails must be 25mm

 

[matchedsparky]

This would be alright as Its protected against Overload by the 16A mcb

 

[telectrix]

This would be alright as Its protected against Overload by the 16A mcb

no, because the 16A MCB is at the load end of the tails and only protects it's outgoing cable/s.

 

[matchedsparky]

Hi this makes no difference as the maximum that the 10mm cable will carry will be dictated by the 16A mcb on the load side, the same can be said for a spur from a ring main ie

Cable Max 27A
Breaker / mcb 32A

Cable is protected by the plug top fuse 2 x 13A = 26A

 

[telectrix]

so say, somebody put a spade through the tails, you are saying the 16A MCB it would protect the tails???????

 

[telectrix]

Hi this makes no difference as the maximum that the 10mm cable will carry will be dictated by the 16A mcb on the load side, the same can be said for a spur from a ring main ie

Cable Max 27A
Breaker / mcb 32A

Cable is protected by the plug top fuse 2 x 13A = 26A

no the cable is protected by the 32A MCB at the CU. the reason we are allowed 1 unfused spur is that the total load of the socket will not exceed 26A, but the plug top fuse/s do not protect the cable feeding them

 

[matchedsparky]

Now you are talking about somethig different, I stated that it was protected against overload which it is, you are now talking about short circuit protection even so the answer would be yes the 100A head fuse would blow under short circuit conditions
Thanks

 

[matchedsparky]

Thats exactly what i am saying it is protected by the load

 

[telectrix]

sorry. we will have to agree to disagree. IMO the tails must be sized according to the protective device at their supply end

 

[MCS Renewables]

If you install a new board with new tails from a isco/henly block you would need to install an isolator as well to comply with regulation 537.1.4 .
( Every property should have a single point of isolation) And the tails need sizing as per Julian c above.

 

[johnboy6083]

in this situation then backfusing works. its how distribution panels work, and busbar chambers. the max outgoing load is governed by the OCPD, and shortcircuit protection is provided by the main head.

Im doing a job next week, wehere ive got to tap off the main busbar in a MCC in a treatment works. ive got to tap off in 35mm tails to my switchfuse. the main busbar is rated to 300A. Do i use for example 120mm tails to my switchfuse, which is the same size as the incoming tails into the panel?

 

[matchedsparky]

So if in a commersial premisses where you have 400A head fuses feeding a buz bar chamber, are you saying all cables coming from it require sizing to 400A to feed various distribution board located at various locations around the building..Of cause not because the overload protection is provided upstream. If look at the BS7671 and overload protection it can be installaed upstream, ie the 16A mcb is installed upstream and is therefore protecting the tails from any overload. It is imposisible for them to be overloaded.

 

[matchedsparky]

Exactly correct Johnboy

 

[Des 56]

433.2.2 then 434.3(iv)