new consumer unit 25mm tails

[lobello]

Hello

Had a new consumer unit fitted recently.

At end of day the electrician pointed out the tails from meter to CU were 16mm and the fuse at the supply head may well be 100 amps. We couldn't check this as the supply heads for the flats are in locked room and site manager chap wasn't around. Left it with the sparky that he'd come back and check another time.

Anyways when I googled this subject various stuff came up suggesting that it's obligatory to change the tails to 25mm if not already on a new consumer unit install?

Electrician has suggested swapping supply head fuse for a 60 or 80amp one if it's currently 100amps. Not possible as Managing Agents are adamant fuse cannot be changed.

It's a flat I rent and am keen to have all up to latest regs and specification, covers me insurance wise and are far as possible ensures safety of my tenants. I paid a fair old whack for the new unit install and a few other bits and pieces...

So is swapping 16mm for 25mm tails when changing the consumer unit a law or a regulation or a guideline...?

Is it something that ought to have been covered under the install of the new consumer unit regardless...?

If the fuse at the supply head is currently 60 or 80 amps perhaps all is good and I can forget the 16mm tails, or regardless ought I be getting new tails put in..?

Thanks in advance.

 

[somersetsparks]

Even if you pull the service fuse you may be none the wiser. Often the fuse is riveted in the fuse carrier and there is no way of examining the fuse value. :rolleyes4:

You usually only see that on 'red links' which are solid link fuses carriers. These are usually (but not always) red in colour and found in flats to avoid dual fusing a cct. The fuse is usually in a ryfield unit downstairs with the red link provided for isolation purposes.

 

[lobello]

thanks for all the replies.

fuse appeared to be a 60 amp one along with all the other flats i.e. carrier said 60 amps.

sparky took that as read that it was a 60 fuse. he didn't seem to think there was a fuse at the meter end of tails to the consumer unit...which is odd as from above i;d assumed there was switch fuse supplied by the supplier at this point.

seems ok for now as a 60 amp fuse will protect a 16mm cable.

begs the question why the 'recommendation' to make sure tails from meter to consumer unit are 25mm on consumer unit change?

understood people have run 16mm on 100amp fuses, but if that is 100% safe, why would it be a recommendation...is it a case of 16mm / 100amp *ought* to be ok, but no guarantee?

reassuring to hear folk think that there's no reason for the DNO to swap in a 100 amp fuse if the current one blows at anytime. if that is the case then it'd seem there is no need for me to change tails, but if that's just a likliehood and not guaranteed then surely it'd make sense to upgrade the tails?

reassuring to hear folk think that there's no reason for the DNO to swap in a 100 amp fuse if the current one

another question from me -

at same time as consumer unit upgrade he added in a new socket in the kitchen.

said socket is in a corner piece that houses the waste pipe for the kitchen sink. top 2nd floor flat, all flats below identical so the pipe runs right down to ground level, also runs up to roof - presume this is a breezer pipe. socket is about a foot and a half above floor level.

this doesn't seem ideal to me i.e. if pipe overflowed / blocked / leaked then it could fill the corner housing up with water and reach the back of the socket, the cable running down from loft alongside the pipe would also be hanging in water.

are there are regs about not situating sockets adjacent to/in front of buried pipes, or running cables down next to water pipes?

thanks again

 

[spark 68]

begs the question why the 'recommendation' to make sure tails from meter to consumer unit are 25mm on consumer unit change?......

This is not actually in BS7671, it is only in the OSG... which is a guide to a recommendation.....

It has been commented on in these pages before, and seems to be taken literally for some reason.
Having said that, if I need to replace the tails then of course I fit 25mm, but I won't just replace them just because the OSG says so.

 

[JulianC]

begs the question why the 'recommendation' to make sure tails from meter to consumer unit are 25mm on consumer unit change?

I suspect because people misinterpret the On-Site Guide, which shows a 100A fuse and 25mm tails for a new installation. The same people probably recommend you upgrade the earth to 16mm too.

The inspector gave as an example that 6mm tails to say an additional CU via henley blocks would be acceptable.

I might be missing something here (entirely likely) but how does this square with fault current protection, specifically 434.3 and the omission for devices thereof. Didn't think a 100A fuse would protect anything less than 16mm, 80A for 10mm etc. At least that's how I interpreted it when overlaying the adiabatic line for each cable size on the graph of fuse time/current characteristics in the BGB

 

[lobello]

This is not actually in BS7671, it is only in the OSG... which is a guide to a recommendation.....

It has been commented on in these pages before, and seems to be taken literally for some reason.
Having said that, if I need to replace the tails then of course I fit 25mm, but I won't just replace them just because the OSG says so.

Spark 68 very, very useful reply that really does answers my question! nice one.

 

[spark 68]

I might be missing something here (entirely likely) but how does this square with fault current protection, specifically 434.3 and the omission for devices thereof. Didn't think a 100A fuse would protect anything less than 16mm, 80A for 10mm etc. At least that's how I interpreted it when overlaying the adiabatic line for each cable size on the graph of fuse time/current characteristics in the BGB

Hi Julian,

If you read post #8 again, the poster has two caveats,

1) no signs of overheating or deterioration

2) tail lengths of 3m or less (which they have to be anyway)

Point two is using the forward fusing rules 433.2.2 and 434.2.1 amongst others, and whilst not ideal may be acceptable in the circumstances, especially when the tails may typically be less than 1/2m or so, this then uses the OCPD's in the CU to provide the fault protection of the tails.

 

[JulianC]

Ta for that. 434.2.1 covers it. Must have read it a hundred times. And discounted it every time. 2+2=5 :dunce2: