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Discuss new consumer unit 25mm tails in the Electrical Forum area at ElectriciansForums.net

L

lobello

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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.
 
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No it is a recommendation that on a CU swop you change the tails to 25mm, but at the end of the day the size of the distributor fuse or in your case the main intake fuse will denote the size of your tails. As this seems to be a flat I would guess that there is little chance you will ever need a fuse larger then 60amp, and would hazard a guess that unless the fuse blew at one time, and the only available was a 100amp jobby in the van, then your fuse will be 60 perhaps 80

I would wait till the electrician can access the main intake before worrying too much, as said I doubt your flat would draw anything near 40 amps at any given time, I'm sure he will sort it out.
 
L

lobello

  • Thread Starter Thread Starter
  • #3
No it is a recommendation that on a CU swop you change the tails to 25mm, but at the end of the day the size of the distributor fuse or in your case the main intake fuse will denote the size of your tails. As this seems to be a flat I would guess that there is little chance you will ever need a fuse larger then 60amp, and would hazard a guess that unless the fuse blew at one time, and the only available was a 100amp jobby in the van, then your fuse will be 60 perhaps 80

I would wait till the electrician can access the main intake before worrying too much, as said I doubt your flat would draw anything near 40 amps at any given time, I'm sure he will sort it out.
Thanks Malcolm.

Ok understood it's a recommendation. Does beg the question if the Electric Board/DNO could have put a 100 amp fuse in prior (all be it in extremis) what's to stop then doing that at some point in the future and therefore the switching to 25mm tails begins to make sense.

Not sure on the 40 amps as whole flat is electric - immersion storage water tank, convector/storage heater, electric shower, ceramic hob and electric cooker and of course lights and sockets. So yes a flat but quite possibly more demand on the electrics than a house with gas cooker, gas heating and water...
 
Not sure of your setup regards the flats, but one configuration there will be a main header from the DNO perhaps if it's a large block that will be 3 phase, and from the head will be most likely individual meters to a flat and then a switch fuse for the individual flat, that is the one you need to be checking to make sure of the fuse size. But it could be your meter is in the flat and so the configuration is slightly different

Bottom line the DNO will not upgrade their fuse to 100amp if your tails are 16mm, well they shouldn't put it that way.
 
E

Engineer54

  • Thread Starter Thread Starter
  • #5
Thanks Malcolm.

Ok understood it's a recommendation. Does beg the question if the Electric Board/DNO could have put a 100 amp fuse in prior (all be it in extremis) what's to stop then doing that at some point in the future and therefore the switching to 25mm tails begins to make sense.

Not sure on the 40 amps as whole flat is electric - immersion storage water tank, convector/storage heater, electric shower, ceramic hob and electric cooker and of course lights and sockets. So yes a flat but quite possibly more demand on the electrics than a house with gas cooker, gas heating and water...
Malcolm is correct, there is little to no chance that all of your connected loads will be operating at the same time, and for any period of time. So 40 to 45A is a pretty good call, as to your actual demand....
 
L

lobello

  • Thread Starter Thread Starter
  • #6
Thanks again. Set up is

Locked Room on ground floor with supply heads for all flats in block (3 floors and 3 flats on each floor)
Meter outside my front door in communal area on 2nd floor, other flats meters are also outside their doors, CU is above front door inside the flat.

It's the fuse in the locked room the sparky was talking about checking and even swapping, pretty sure that's the DNO's one..

He didn't mention the switch fuse...if it's a matter of getting this fuse changed then all sounds easier and certainly more guaranteed solution. Is said switch fuse likely to be at the meter end of tails i.e. in communal area or at CU end of tails i.e. in my flat...?
 

telectrix

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Mentor
Arms
Esteemed
generally speaking, the switch fuse should be at the supply end of the tails. or at least within 3meters. this sw.fuse is there solely to protect the tails from an overload/short circuit before the protective devices ( MCBs) in your consumer unit.
 

jaresquire

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Arms
Had my NICEIC annual inspection on the 4th and tail sizes on upgrades were discussed. The view was that outwardley undersized tails were quite acceptable as long as they showed no signs of overheating for the usual system load and that, of course, the CU was within the 3m guideline for downstream protective devices. The inspector gave as an example that 6mm tails to say an additional CU via henley blocks would be acceptable. Not that I would install such tails, but if they are acceptable then they don't have to be changed.

:helmet:
 
I think it's unlikely to be a 100A fuse. Plus, as already said, the DNO won't upgrade it from 60A to 100A if 16mm tails are in place.
 

jaresquire

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Arms
Did my sisters CU upgrade a few weeks ago, 2 bed house , 16mm tails and 100A main fuse ( cause it fell out when I touched it)
 
O

Octopus

  • Thread Starter Thread Starter
  • #11
Out of interst, what does the EIC state the main fuse is?
 
I think it's unlikely to be a 100A fuse. Plus, as already said, the DNO won't upgrade it from 60A to 100A if 16mm tails are in place.
Got to agree, not very likely its going to be bigger than 60amp if its a block of flats. If its the nomal ryefield layout thats all they fit.
 
Size of main fuse is a perfectlty acceptable LIM on an EICR, 16mm tails are O.K even on a 100A fuse in reality in a domestic situation, I wouldn't worry at all. If they are flats, they will more then likely be on 60A fuses in a Ryfield Unit. We usually only fit 80A fuses to all Domestic supplies anyway unless 100A is specificallyu asked for.
 

Taylortwocities

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Arms
Esteemed
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:
 
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.
 
L

lobello

  • Thread Starter Thread Starter
  • #17
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

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Arms
Esteemed
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.
 
J

JulianC

  • Thread Starter Thread Starter
  • #19
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
 
L

lobello

  • Thread Starter Thread Starter
  • #20
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

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Arms
Esteemed
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.
 
J

JulianC

  • Thread Starter Thread Starter
  • #22
Ta for that. 434.2.1 covers it. Must have read it a hundred times. And discounted it every time. 2+2=5 :dunce2:
 
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