Protecting 16mm tails

[NickD]

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]

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]

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]

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

 

[telectrix]

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

 

[Octopus]

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]

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]

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]

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

 

[polo1]

Talking about tail sizing, what do you think of these....
Yes, 2.5mm doubled up so, 5mm tails!!!

 

[PEG]

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

 

[NDG Elecs]

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]

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?

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]

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]

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

 

[telectrix]

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?

of course you should change that 16A for a 20A. what about when you add the 10kW shower to that radial?

 

[NickD]

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

I can't think of a reason, but that doesn't mean there isn't one. Big organisations move in mysterious ways. I would be happier with some kind of assurance that I am perfectly within my rights to rely on the DNO fuse to protect the tails.

20 is a rounder number than 16 if you tend to think in decimal?

Are you sure you're not ill advised when you advise me that I'm being ill advised?

 

[trev]

It's already got one Tel, the 2.5 feeding it doubles as a heater

 

[Leesparkykent]

of course you should change that 16A for a 20A. what about when you add the 10kW shower to that radial?

Thats when you remove the circuit from the rcbo due to nuisance tripping and dab it in with the tails lol.

 

[Knobhead]

As an experiment I ran my flat on a 16A MCB for two months. Never tripped once.

If you look for the diversity figures UKPN use they’re unbelievably low.

 

[trev]

Thats when you remove the circuit from the rcbo due to nuisance tripping and dab it in with the tails lol.

Funny you should mention that, I did an EICR on a dog grooming place the other week, 8.5k shower fed in 4mm, no MCB, supply to it shoved in with the tails then through an 80 amp RCD

 

[NickD]

Why do you need a shower? You've got us.

 

[Knobhead]

Nick, just admit, you’re milking this job by adding totally unnecessary work.

Someone had to say it, it may as well be a grumpy git like me.

 

[Leesparkykent]

Funny you should mention that, I did an EICR on a dog grooming place the other week, 8.5k shower fed in 4mm, no MCB, supply to it shoved in with the tails then through an 80 amp RCD

I see something similar in a kebab shop for the fryers once, ironically the place was called blaze kebabs lol.

 

[NickD]

Nick, just admit, you’re milking this job by adding totally unnecessary work.

Someone had to say it, it may as well be a grumpy git like me.

If I was going to milk the job, I would have just got on and milked it without asking about it. Actually I just want to do an engineeringly-correct job, if that's a word. If it suits you to believe that milking it is my motivation then that's your prerogative. I could think of a darn sight more lucrative ways to milk this job than bunging in an OCPD, if that was my inclination.

 

[Knobhead]

Non of your arguments have any credence yet you persist in them.

Why?

 

[Octopus]

Non of your arguments have any credence yet you persist in them.

Why?

Because the OSG has a pretty picture showing tails to be 25mm

 

[NickD]

Client at a job the other day had agreed a board change in order to extend the socket ring circuit. However with the fusebox cover off it became apparent that the standalone RCD next to it was protecting the socket circuit in question, not the shower circuit as advised. RCD was in good order and so was the socket ring circuit. Did I milk it by proceeding with the board change? No.

 

[NickD]

Non of your arguments have any credence yet you persist in them.

Why?

Forget any arguments you think I'm making then and please just answer my question; do you consider it is entirely justified and good engineering practice to rely on the present rating of the DNO's fuse to protect the consumer's tails?

 

[Octopus]

Forget any arguments you think I'm making then and please just answer my question; do you consider it is entirely justified and good engineering practice to rely on the present rating of the DNO's fuse to protect the consumer's tails?

and common sense applied.

 

[cprfenom]

Im not getting why you ask a question, then argue to ---- about the replies.

In this situation it really does seem that Tony has hit the nail on the head. Its as if you want someone to justify your point of view. If there is no evidence that the current tails are under any pressure, why don't you leave them be, or when you ask for an isolator to be put in in order to change the consumer unit, ask the engineer to add a 60/80 and not the standard 100A they always seem to put in.

Don't ask questions if your point of view is set in stone...

 

[Knobhead]

Forget any arguments you think I'm making then and please just answer my question; do you consider it is entirely justified and good engineering practice to rely on the present rating of the DNO's fuse to protect the consumer's tails?

One of the first questions you were asked was “what size is the DNO fuse?”
You don’t know.

Will the DNO suddenly decide to up the fuse?
You don’t know.

Will the load dramatically increase?
You don’t know.

About the only thing you do know is that there has been no problem to date.

 

[NickD]

Im not getting why you ask a question, then argue to ---- about the replies.

In this situation it really does seem that Tony has hit the nail on the head. Its as if you want someone to justify your point of view...

No, absolutely not, not at all. I don't actually have a point of view on the answer to this. I would be very happy indeed to walk away from this confident that leaving the 16mm tails in situ, relying long-term on the rating of the DNO fuse for their protection, was a totally reasonable and defensible engineering proposition.

Let's try again...

The stated purpose of the DNO fuse is to protect the DNO's cable upstream of it, and any protection it gives to the consumer's system downstream of it is mere happy accident. It is unclear (to me, anyway) what the duty of care of the DNO is if it goes to uprate the fuse when the consumer's system in fact relies on the present rating of the fuse for its safety. The DNO is a large complex organisation with which I am not particularly familiar. I can have no real control over what it does with the fuse in the future, and I would be kidding myself if I were to make blanket assumptions about the DNO's operational processes and practices (including about them complying with what may appear to me to be common sense).

However, what makes it legitimate and reasonable for me to rely on the present rating of the DNO fuse to protect the consumer's tails for the long term, is the following: _____________

Please fill in the blank. Thanks.

PS If you believe "The tails have been OK up to now" is a good answer then with respect I think you've missed my point.

 

[dcelectrical]

am I being thick here, I have never known the DNO to come in and change fuses, they generally ask us to change them when a builder pops them and nicks the one from the flat next door before walking away whistling the tune of rawhide.

 

[Knobhead]

The stated purpose of the DNO fuse is to protect the DNO's cable upstream of it, and any protection it gives to the consumer's system downstream of it is mere happy accident.

Pardon?

Would you care to explain how a downstream device protects the supply cable.

 

[NickD]

Would you care to explain how a downstream device protects the supply cable.

On balance, no. (I think it's something to do with it going bang though.)

 

[Des 56]

The problem we have here is this quote from the opening post
quote
Ordinarily I would upgrade the tails & EC to 25/16 as a matter of course


The post then goes on to describe the difficulty in routing the proposed tails

I may be wrong but it seems to me that the question of the supplier fuse (given the above quote)is never taken into account and it is only now mentioned because of routing problems for those tails

It is for this reason that I think the post is looking for justification of the change ( when he knows full well its not required) and he is unfortunately clutching at straws to allay the criticism that has been given

 

[NickD]

I think the post is looking for justification of the change ( when he knows full well its not required)

If you want to think that, that is of course your prerogative; and it would seem the logical next step is to blow me out of the water by filling in the blank with a great answer, which totally convinces with reason and evidence why the tails can be left as they are as long as the present DNO rating protects them.

Alternatively, if I have no such agenda and am genuinely asking the question, the logical next step is to fill in the blank with a great answer, which totally convinces with reason and evidence why the tails can be left as they are as long as the present DNO rating protects them.

 

[Knobhead]

On balance, no. (I think it's something to do with it going bang though.)

The service fuse will be a BS88, about a subtle as a brick under O/L. It won’t limit the fault current.

If you consider that in urban areas the feeder cables may be protected by 400A or even 630A fuses a 35mm[SUP]2[/SUP] concentric doesn’t stand a chance.

That weakness is part of the protection. A fault on a small service cable the cable will act as the fuse and blow the fault clear.

It’s as rough as the proverbial badgers arse, but it works.

 

[NickD]

The service fuse will be a BS88, about a subtle as a brick under O/L. It won’t limit the fault current.

If you consider that in urban areas the feeder cables may be protected by 400A or even 630A fuses a 35mm[SUP]2[/SUP] concentric doesn’t stand a chance.

That weakness is part of the protection. A fault on a small service cable the cable will act as the fuse and blow the fault clear.

It’s as rough as the proverbial badgers arse, but it works.

I bow to your superior knowledge. Could you be so kind as to deploy it in the direction of answering my question?