16mm tails with 100A fuse ???

[Wilko]

Job today to change out old fusebox and I'd noted the 16mm tails but on pulling the fuse this morning it was 100A which I wasn't expecting. Fairly recent new smart meter installed by XYZ who are also the DNO ... Shouldn't the fuse be limited to 60A in this case? Thoughts and advice appreciated

 

[Wilko]

I liked this fine old board as soon as I saw it

 

[Des 56]

Wylex plastic front and a wooden back box. The internals look like new

With an older board,I would say that some oxidisation always take place over time.
The board looks to have a extra shiny busbar and fuse contacts

I could very well be wrong and it may be older but I suspect it has not that long come out of the box

 

[Wilko]

I did clean it up a bit, but the shiney bus bar is as it came off the wall

 

[Phil Thompson]

I've had a few wylex boards like that to change, but majority I get are the old mem ones. Last change was one of the wooden backed ones, and have another one to do in next fortnight the same and thenthis week there is one that's the old porcelain type.

 

[Phil Thompson]

Derating was probably the wrong choice of words. What I am saying is if the conductors are not clipped direct throughout the ccc should be based upon them being enclosed even if it is for a short length.

When at college today doing the Periodic course I questioned the tutor on this. He said you were correct

 

[westward10]

When at college today doing the Periodic course I questioned the tutor on this. He said you were correct

Holy Maloney crazy Westy had a point.

 

[Dave OCD]

Wylex plastic front and a wooden back box. The internals look like new

With an older board,I would say that some oxidisation always take place over time.
The board looks to have a extra shiny busbar and fuse contacts

I could very well be wrong and it may be older but I suspect it has not that long come out of the box

I know that the fuses can become really tight after years of not being touched Des, loads of people say 'never had a fuse blow' , not surprised really with the overcurrent a rewirable will put up with.

 

[tom porter]

Well, I did try to move the tails but only got the 20cm I needed for the new CU and they won't budge any more, although there is slack at the meter. So changing the tails will be mission messy. Tails leave the external chest high meterbox in a cavity brick wall, go up and over the front door and drop down from the ceiling. CU mounted high up next to ceiling behind front door. New CU is in the location of the original fuse unit, at Customer's request. But somewhere in that lot the tails go through a beam I reckon and are jamming up. Nothing I can't sort if I rip into the plaster ceiling she's just had done .

In terms of load, I am sure a 60A fuse would be fine. 2BR mid terrace, newish combi for CH and HW. Biggest drain will be an induction hob she's got on order.

There's no insulation anywhere near the tails that I can see looking from both ends. The tails will be less than 3m long.

induction hobs do not have as big of a load as you think

 

[Wilko]

Thanks Tom - I'm planning on clamping it, just for my own education. I think that having a "high" max current, it will reach temp very quickly and wind back. I will report in a couple of weeks when Mr Kitchen catches up.

 

[Lucien Nunes]

One would think reading this thread that none of you had ever picked up a cable 6 inches from the end after soldering a lug on. Copper conducts heat well enough that the temperature near the end of a cable is influenced by what it is connected to. If it's screwed into a chunk of brass that doesn't itself generate heat, then it will sink a fair amount of heat from the cable core and the end will run cooler. If the terminal gets hot - not as in a loose connection but something like a 45A switch terminal where the contact heats in normal operation - then heat might flow into the end of the cable. Then there's the bunching where cables enter an enclosure - heat that can't be effectively dissipated there might be mitigated by the section that's completely in free air as it heads towards its terminal.

What I'm getting at is that there are all sorts of thermal flows near the terminations so applying an installation reference method won't yield a useful answer. Those are designed for average conditions on average cables and as has been mentioned somewhere upthread, it has been proven over decades of experience that where those CCCs are adhered to, no damage is likely under the somewhat different conditions encountered at the end.

As for 16mm² tails: {NOTE FOR THE UNINITIATED: DO NOT DO THIS! IT IS A THOUGHT EXPERIMENT ONLY!} One day when no-one (especially your insurer) is looking, try reducing your tails. If you have GCH, gas cooker and no electric shower, try 2.5mm². Electric cooker or heating, make that 4.0. If you have a shower you will have to go up to 6mm. Ensure your connections are well made. Report your findings... {AGAIN DIYers DO NOT TRY THIS AT HOME!}

 

[Wilko]

Carrying that thought ... 16mm cable has 2.8mV voltage drop per A per m at 70C. So if 16mm tails were running the full 100A of the service fuse rating, they would have a voltage drop of 0.28V in each metre of the run. Since P=V.I that's 28W of heat to be radiated per metre of tails at 100A at 70C. Which isn't a number that seems a big ask (?). Of course this current level should not be allowed as it exceeds the BS7671 current rating for 16mm, just saying.

 

[soms]

When at college today doing the Periodic course I questioned the tutor on this. He said you were correct

As a Trainee I find this thread both very interesting and confusing. I think the more I read here and research things the more things become grey areas. I could no doubt delve into the BYB and be left not much the wiser as many things are open to some level of interpretation. This thread has also brought up more than one topic of technical debate.

The matter of not being able to consider a cable being clipped direct because it enters an enclosure I cannot find words for. As someone learning the trade, all I can think is how terribly confusing it would be in this was the 'correct' thing to do and where would you draw the line? As mentioned, would this apply to a cable entering a patress or junction box, where would it end? Then what alternative rating should you select?
Yes, CCC must be determined on the worst case factor but I had never seen or heard of this specific technicality before.
Next term I will be asking my tutors for their opinion on this point.

I must clarify for myself also the matter of the DNO fuse rating and the absence of providing protection on the customers side of the installation and how the tails should be sized. IIRC the former is mentioned in the BYB.
So does this mean the purpose of the service head fuse to provide some form of overload protection for the service cable or fault protection? What would there be to provide fault protection for?

 

[Wilko]

So does this mean the purpose of the service head fuse to provide some form of overload protection for the service cable or fault protection? What would there be to provide fault protection for?

Yes, the DNO's fuse is for protection of the DNO's equipment.

 

[Phil Thompson]

@soms you obviously need to read the byb a bit more if you are only seeing confusion coming here.

The 16th Edition was the same way, lots of grey areas and areas open to individual interpretation.

Now let's play devils advocate here,

If you actually consider that your tails are say 80cm long and you have 20cm inside the CCU, you have enclosed 25% of them. So if you enclosed 25% of a run of 10mm2 T&E and the rest was clipped direct, would you factor that into your cable rating? Or would you say it's only 25% so I won't factor it in?

 

[westward10]

Hello Soms it is good to think outside the box and get the old grey matter going as opposed to following the sheep blindly. See #54 for further confusion.