splittting the tails

[sellers]

If the mcb was faulty, the fault current would be high enough to take out the service fuse.

So, a 6mm feeding the cu straight of a henley would have enough fault current (loop readings permiting) to take out the service fuse, and able to omit overload protection due to a maximum possible 26Amps being drawn.

 

[ProDave]

Why does the OP want to use 6mm tails in the first place?

I have not found even a 2 way mini CU that won't accept 16mm tails, so I would never use anything smaller than 16mm

I know the DNO use (I think) 6mm tails direct from the head for economy 7 timeclock switching, so you can argue a precedent has been set saying that is acceptable. But then we know DNO's are a law unto themselves and what they do is sometimes questionable.

 

[soulman]

hi pro dave i always use 25mm tails to an additional ccu because thats all i carry now, it was just something we were debating about yesterday 3 different electricians all had different answers just wanted to check the forum.

 

[sparkytim]

So if you tapped off a 200 Amp Busbar to feed a 63 Amp switch fuse what size feed conductors would you use?

See BGB Pg 83 Reg 433.2.2

 

[ADS]

So if you tapped off a 200 Amp Busbar to feed a 63 Amp switch fuse what size feed conductors would you use?See BGB Pg 83 Reg 433.2.2

You've answered the question yourself.....a 63A switch fuse.............as I said in my earlier post:

The OP might insist that the CU will only ever serve the circuits he has suggested - but the fact is, it's possible for 6mm tails to be overloaded with the setup he describes.Different, of course, if he runs them through a suitably rated, switch-fuse first.

EDIT:

Just to clarify my point - the MCBs in the OPs consumer unit are offering overcurrent protection to the final circuits that are run off them, not the tails that are feeding the CU.

The configuration and rating of these MCBs could be altered at any time.

Running the 'tails' through a suitably rated switch-fuse first will protect them, as you correctly quote Reg 433.2.2, anywhere along the run.

 

[Sidewinder]

ADS,

I think one of us has missed the point insparkytim's post.

Now I understand his post as a 200A bus bar supply, thus protected by 200A fuses, tapped off without protection to a 63A fuse switch, thus at the load end the tails are limited to 63A (allowing for fuse overload).

Thus the "tails" between the 200A supply and the 63A fuse switch could be sized accordingly to the load limiting of the 63A fuse switch.

You can often fit larger fuses than 63A in a 63A rated fuse switch, so do we allow for this?

I would say NO, we limit the load to 63A, thus we size the "tails" to 63A

I can't quite see the difference here between this and the proposed situation with a 20A & 6A breaker in a CU at the end of the 6/10mm tails?

Thus offering protection limited to 26A (allowing for MCB overload). Yes these 20 & 6A MCB's could be changed for 2 x 63A.

Do we "have" to cater for stupidity every time, remember that we must assess the existing supply for being adequate prior to any changes reg.

This situation is common on bus bar chamber installs, and acceptable. It is also linked to the 3m DNO tails thing. Also there is a reg, but as usual my books are not here, where protection can be provided at the load end.

During a PIR, I have recently seen a 2.5mm sq connected to a 1600A supply no source end protection, protected at the load end of run <1m I can't see how I can fail it.

 

[ADS]

I agree with the points you are making, Sidewinder, - and in a lot of other circumstances it is perfectly acceptable and done all the time...........it just doesn't feel right when they're feeding a CU

.....Also there is a reg, but as usual my books are not here, where protection can be provided at the load end.

Yes, overload protection, as Sparkytim said, 433.2.2 - but you have to satisfy 434 (fault current)

 

[Sidewinder]

Wow!

We agree!

Oh dear, where does that lead us... ; )

 

[ADS]

Wow!We agree!

Oh dear, where does that lead us... ; )

I know!

I've promised myself to try and be a good boy from now on.

Also, I could hardly disagree with the points you are making when I was one of the people arguing in favour of the '2.5mm radial spur off a 32A MCB' thread. (I don't know if you remember that one).

It involved similar 'points' and similar 'regs'.

 

[wozz]

ADS,During a PIR, I have recently seen a 2.5mm sq connected to a 1600A supply no source end protection, protected at the load end of run <1m I can't see how I can fail it.

Common in old switch rooms ive been in, usually for the light in there.

 

[sparkytim]

Try 434.2.1

 

[ProDave]

Related to this splitting of tails and what size tails should you use:

I was looking at a commercial install this morning.

3 phase supply. 3 phase dual rate meter into 3 phase DB

BUT there was also a single phase meter. That seemed to take one phase (via 10mm tails) into the meter, then coming out of the meter's L terminal was a bit of blue 2.5mm single insulated that seemed to feed (and meter) a low power supply into the dual rate meter (probably to feed a contactor via a switch contact for the storage heaters)

How on earth is that safe or legal? a single insulated 2.5mm from one meter to another with the only protection being the 100A DNO's fuse. Place your bets which would melt first in the event of a short, the DNO's fuse, or the bit of 2.5mm.

Should I be phoning the DNO to report a dangerous install?

It must have been like this for years and done by the DNO (or of not done by them, approved by them as both meters are sealed)