Protecting 16mm tails

NickD · Mar 26, 2015

Got a domestic install with client requesting CU upgrade. Tails are 16mm, EC 10mm, and buried in structure between meter and fusebox. Rating of DNO cutout fuse not known. Ordinarily I would upgrade the tails & EC to 25/16 as a matter of course (domestic load includes an outdoor hot tub as well as the usual sockets/cooker/immersion type stuff, don't recall if electric shower present) but routing them through the existing route of the 16mm tails looks like a non starter and I can't see a practical alternative route. The meter box is small and absolutely chocker with no space in there to add a separate fused cutout. Adding a fused cutout next to the replacement CU (in downstairs cloakroom) would probably be doable but cluttered/messy.

Installing a 63A MCB in the new CU, suitably labelled, immediately downstream of the main switch and feeding all RCDs/final circuits...good/stupid idea?

Engineer54 · Mar 26, 2015

Seeing as the vast majority of domestic installations don't need anywhere near a 100A DNO supply and that 16mm tails are generally good to 80A , i can't see any problem. Why would you automatically upgrade tails to 25mm as a matter of course, for what reason??....

NickD · Mar 26, 2015

Engineer54 said:
Seeing as the vast majority of domestic installations don't need anywhere near a 100A DNO supply and that 16mm tails are generally good to 80A , i can't see any problem. Why would you automatically upgrade tails to 25mm as a matter of course, for what reason??....

Because I have no control over what cutout fuse the DNO chooses to fit at a later date and I have no control over the client choosing to run all their final circuits close to full capacity simultaneously (however many times I calculate for diversity).

Leesparkykent · Mar 26, 2015

Have a look at the fuse in the cut out and see what rating it is.

telectrix · Mar 26, 2015

how long have these 16mm tails been in without signs of any damage? sod it, leave well alone.

Octopus · Mar 26, 2015

NickD said:
Because I have no control over what cutout fuse the DNO chooses to fit at a later date and I have no control over the client choosing to run all their final circuits close to full capacity simultaneously (however many times I calculate for diversity).

Which is NOT your problem.

Pull fuse, but I'd leave the 16mm tails in place FWIW

Knobhead · Mar 26, 2015

telectrix said:
how long have these 16mm tails been in without signs of any damage? sod it, leave well alone.

There’s no profit to be made doing nothing.

trev · Mar 26, 2015

NickD said:
Because I have no control over what cutout fuse the DNO chooses to fit at a later date and I have no control over the client choosing to run all their final circuits close to full capacity simultaneously (however many times I calculate for diversity).

So because at some unspecified point the DNO might decide willy nilly to put a 100A fuse in you're going to upgrade?
Shouldn't you also then consider the possibility of a 200A going in? Or a 3 phase head being installed?
How likely is it that this fuse upgrade will come out of the blue?
How likely is it that the client will run all their circuits at or close to the full demand placed on them?

Simon-0116 · Mar 26, 2015

Seen one today with tag on fuse holder saying 60 amp only in a house.

polo1 · Mar 26, 2015

Talking about tail sizing, what do you think of these....

[ElectriciansForums.net] Protecting 16mm tails

Yes, 2.5mm doubled up so, 5mm tails!!!

PEG · Mar 27, 2015

...Nice twisting,and respect due for red zip-ties,having noticed the only potential risk being conductor identity :conehead:

NDG Elecs · Mar 27, 2015

trev said:
So because at some unspecified point the DNO might decide willy nilly to put a 100A fuse in you're going to upgrade?
Shouldn't you also then consider the possibility of a 200A going in? Or a 3 phase head being installed?
How likely is it that this fuse upgrade will come out of the blue?
How likely is it that the client will run all their circuits at or close to the full demand placed on them?

Haway Trev, DNOs don't do willy nilly things. They always check their Ze when replacing a knackered head, and always have their own 16mm cables on their vans. DNOs are many things but never willy nilly!!

Nick, am sure the 16mm will be fine, check the loading obviously. It's no wonder we are running out of copper!!

NickD · Mar 27, 2015

trev said:
So because at some unspecified point the DNO might decide willy nilly to put a 100A fuse in you're going to upgrade?

I guess what it comes down to is this: how legit/reasonable is it to rely on the rating of the presently-installed DNO fuse to protect the tails? As I understand it, the DNO fuse is there to protect their cable/equipment, and any protection it gives to the consumer side of the installation is a side effect. If at a later date they were going to uprate the fuse, such that the 16mm tails would no longer be protected by it, are they under a clear duty to consider this and act accordingly?

trev said:
How likely is it that the client will run all their circuits at or close to the full demand placed on them?

Like any risk that has to be managed based on the likelihood of it happening and impact if it does. Impact of extended overload of the tails could be pretty catastrophic. How likely, well, we have to be thinking over a timescales of decades I guess (imagine a cold Christmas Day with the cooker going full blast and people getting showers, when the boiler packs up and so five electric heaters and the immersion get turned on). Not inconceivably unlikely.

None of this is meant to sound pretentious and up myself by the way, that's just the way I write

rattlehead85 · Mar 27, 2015

Don't send ya head into spasm worrying about 16mm tails. As mentioned above they are good for upto 80a and i wouldn't mind betting that that your main fuse is 60a 1361 type iib anyways so leave alone. You'd be doing well to blow even a 60a fuse from normal household demands.

trev · Mar 27, 2015

Nick the DNO fuse is there to protect their kit so if a 40/60/80 amp fuse has been quietly getting on with the task and not bothering anyone what possible motivation could they have for changing it? I've got a 16 amp RCBO on a radial circuit in my house, it's been doing what it does for about 6 years now, think I should change it for a 20 just for the hell of it?
I don't think you're pretentious by the way, wrong, misinformed, ill advised perhaps but not pretentious