25mm tails why..?

[Dustydazzler]

Have you ever come across a melted 16mm tail ? Me , no never ...

why are tails expected to be 25mm , seems utter overkill imo

thoughts

 

[Strima]

I'm in two minds, at the moment there's not a lot of heavy current sing equipment in a domestic situation but in the future we will see more use of EVCP and electric boilers etc so a supply fuse upgrade to 100 amp is quite possible.

However I'm of the opinion that the tails should be sized to the fuse and only upgraded when required.

As the DNO/trained person pulls the fuse when we isolate the supply to change the tails etc then the supply fuse size would be confirmed.

 

[buzzlightyear]

tails etc then the supply fuse size would be confirmed.

Some suppliers send in the clowns to pull a fuse and don't put the fuse size on the it when changing the meter.

 

[Strima]

Some suppliers send in the clowns to pull a fuse and don't put the fuse size on the it when changing the meter.

But sometimes they get the polarity right so that makes it OK...

 

[LastManOnline]

Have you ever come across a melted 16mm tail ? Me , no never ...

why are tails expected to be 25mm , seems utter overkill imo

thoughts

I have actually never come across a melted 10mm tail and still occasionally come across 6mm tails.

 

[Dustydazzler]

I have actually never come across a melted 10mm tail and still occasionally come across 6mm tails.

if I had my way 16mm tails we be the norm for all domestic property.

25mm is total overkill imo...

 

[Grant1987]

if I had my way 16mm tails we be the norm for all domestic property.

25mm is total overkill imo...

Yes I agree however it goes against the regs by using 16mm tails on 80A/100A fuse

 

[7029 dave]

The way I look at 100amp cutout is a fused circuit so cabling has to be suited to that circuit 25mm, as 16mm from memory is only rated at 92amps.

Having said that the fuse is there to protect the suppliers equipment .

 

[telectrix]

The way I look at 100amp cutout is a fused circuit so cabling has to be suited to that circuit 25mm, as 16mm from memory is only rated at 92amps.

Having said that the fuse is there to protect the suppliers equipment .

interesting. so if the fuse is, as you say, to protect the DNO's cable etc., then the tails are not, themselved protected. only downstream in the CU, and indirectly by the actual max. load that may be relevant at any one time. therefore, using 25mm tails just because the DNO fuse is 100A is not justifiable. tin hat on by a guy who regularly fits 16mm tails.

you say 92A for 16mm, but according to table F4(i) in osg ( green 17th ed), no figures are given for 16mm single cables in free air (ref. method F). 25mm CCC has a 45% highr rating method F as opposed to methoc C, which woulf extrapolate for 16mm as 114A.

 

[7029 dave]

Just looked up the technical data of some cable manufactures, 16mm tails rated at 87A , just saying.

 

[Dartlec]

interesting. so if the fuse is, as you say, to protect the DNO's cable etc., then the tails are not, themselved protected. only downstream in the CU, and indirectly by the actual max. load that may be relevant at any one time. therefore, using 25mm tails just because the DNO fuse is 100A is not justifiable. tin hat on by a guy who regularly fits 16mm tails.

you say 92A for 16mm, but according to table F4(i) in osg ( green 17th ed), no figures are given for 16mm single cables in free air (ref. method F). 25mm CCC has a 45% highr rating method F as opposed to methoc C, which woulf extrapolate for 16mm as 114A.

From a quick look most places seem to use the same table in their literature, even the cable makers - though the one in the OSG seems to suggest it's more for Twin and Earth, not singles (with the 2 cables reference), Though if they are run cable tied together that probably has no effect anyway.

Cable tails do often need to be clipped to secure them so the actual rating is likely somewhere between Free Air and Clipped Direct

Prysmian give 87 for 16mm and 114 for 25mm (clipped direct) but don't give a "Free Air" rating.

You could probably up that a bit in many locations because of ambient temperature being lower than 30C.

And I imagine a majority of houses with gas heating and without multiple electric showers never come close to that load for any length of time in any case.

So it's likely an issue of theoretical 'risk' rather than a practical one. I've seen youtube videos of 63A RCDs that have clearly been melted/damaged by overload where they are protecting multiple high load circuits, but not seen any equivalent damage to the tails.

Personally now that Toolstation have the Doncaster 19 strand easi fit tails I tend to get them automatically because they are so much easier to wrangle into place. It also means one less thing I have to worry about when deciding what is needed.

 

[buzzlightyear]

Prysmian give 87 for 16mm and 114 for 25mm (clipped direct) but

Page 356 BYB flat and touching.

 

[Dartlec]

Page 356 BYB flat and touching.

Is there a good reason why they don't give ratings for 16mm for Reference Method F in table 4D1A in BBB or F4(i) in OSG? Is 16mm single core not approved for Ref Method F installation or something?

Logic based on the 25mm figures would suggest that spacing 16mm tails would increase the capacity and might end up over 100A.

But then what load will a 100A fuse take almost indefinitely without tripping? Not keen enough to look it up, but probably something like 160A? Which is beyond 25mm tails too.

So it's all being done with some 'common sense' factors - just whether that is specifically stated or done by the installer I guess.

Chances of 16mm tails being an issue are almost zero I'd say - but if something did happen people would expect the installer to justify their use with figures, regulations or engineering knowledge.

 

[telectrix]

Just looked up the technical data of some cable manufactures, 16mm tails rated at 87A , just saying.

that's clipped direct, method C. most CU tails are method F , free air.

 

[Dustydazzler]

the physical lenghth of most meter tails is abit 1m-1.5m

to melt a 16mm csa cable of that length I reckon you would have to run 300-400a through it

 

[Ian1981]

Yes I agree however it goes against the regs by using 16mm tails on 80A/100A fuse

No it doesn’t

 

[LastManOnline]

if I had my way 16mm tails we be the norm for all domestic property.

25mm is total overkill imo...

Agreed on overkill comment. On the positive side there is no question that UK consumers are absolutely futureproofed with a 25mm supply. You do wonder at times though about cable current ratings in general when a 6mm is deemed adequate for a 40 amp mcb (all other factors permitting) but a 25 mm is required for an 80 amp fuse.

 

[Ian1981]

Agreed on overkill comment. On the positive side there is no question that UK consumers are absolutely futureproofed with a 25mm supply. You do wonder at times though about cable current ratings in general when a 6mm is deemed adequate for a 40 amp mcb (all other factors permitting) but a 25 mm is required for an 80 amp fuse.

Nothing wrong with 16mm tails clipped direct on a 80 amp fuse.
Nothing wrong with 16mm tails on a 100amp fuse if they’ve been there for years without suffering thermal damage and the loading of the installation does not exceed the ccc and they have adequate fault protection.

 

[LastManOnline]

Nothing wrong with 16mm tails in free air or clipped direct on a 80 amp fuse.

That's good to know. I was under the impression that 25mm was your standard DNO supply in UK. Am I correct in assuming that the 25mm is required for a 100 amp supply?

 

[Ian1981]

That's good to know. I was under the impression that 25mm was your standard DNO supply in UK. Am I correct in assuming that the 25mm is required for a 100 amp supply?

New supplies like new build housing up my way , the dno stipulate 25mm tails and a 16mm earthing conductor with a 100 amp main fuse.