Tail size from meter to cu

[Baker1988]

Hi every one how are u all on this nice monday morning. I wanted to ask i was at a job today and noticed that the tails from the meter to the consumer unit was in 10mm twin and earth and they was using the earth from the twin and earth to be the main earth From the cu and i did not think it is right and i suggested that they change the tails for 16 mm armoured cable ( i know u suppost to put 22mm tails in now but i though seen as it is a small flat 16 would be ok) and 10 mm earth cable it is a flat with 1 lighting circuit 1 ring final circuit and 1 cooker circuit and it is about 14 meter run from the meter to the consumer unit is this right or am i totally wrong any advice would be good

 

[Baker1988]

Sorry i mean i know it is suppost to be 25 mm tails sorry

 

[Ross1]

Are you saying there is a 10mm T & E leaving the meter because if there is that must look a right mess.

What size is suppliers fuse? I expect it would be 60A therefore 16mm double insulated tails are needed?

I`m sure there is sumthin in regs about Tails not to be longer than 3m as well.

 

[Baker1988]

Yes it comes out of the meter in 10 mm twin and earth and goes to a isolation switch about a meter away and then goes from the isolation switch about 13 meters to the consumer unit the meter is down stairs at the left side of the building and the consumer unit is up stairs at the right sude of the building so it is a lon run and yes i have been told that before that it should not be more then 3 meters if it is u have to put an isolation fuse in the line

 

[Baker1988]

It is a 80 amp supplyers fuse well so my work mate has told me i did not check the size of the fuse my self an i know meter tails are suppose to be less then 3m if they are more they should be in armoured cable and have a isolation switch

 

[tony mc]

There are a few things you need to look at here and what Ross has mentioned.

What is the earthing arragement for the premises?

 

[Ross1]

surley the meter cant have any DNO seals on it?

 

[Baker1988]

sorry but what do u mean by earthing arangement? as far as i can tell the main earth in the consumer unit is the earth from the twin and earth and that goes to the isolatin unit and then to the main earth block on the incoming supply. and no it hasnt got seal on it by the looks of it the cable has been in for a long time it is the old red and black colours

 

[malcolmsanford]

Is that isolation switch a switch or a switch fuse, if it's a switch fuse what size is that fuse?

From the meter to that swich fuse/isolator what size are those tails?

Unless it is a TNC-S system why do you lads think that 10mm T + E is not sufficient for a flat that as just 3 circuits?

 

[Ross1]

tony mc means TT or TNC-S etc

 

[Baker1988]

yes it is a switch with a switch with a fuse and the fuse is 60 amp from the meter to the isolator is the same twin and earth thats what i mean it definatly not right i think some one has come along in the past and just put 10mm t&e cos thats what they had in the van so they dont have to buy tails and im not saying the earhth is not good enough i wasnt sure thats why i am asking and also it is a tt system

 

[Ross1]

Is that isolation switch a switch or a switch fuse, if it's a switch fuse what size is that fuse?

From the meter to that swich fuse/isolator what size are those tails?

Unless it is a TNC-S system why do you lads think that 10mm T + E is not sufficient for a flat that as just 3 circuits?

i cant understand how you can acheive double insulation at points of termination at meter with a T & E cable. Never seen it before and is bad practice in my opinion.

 

[Baker1988]

i agree with ross i think it is bad practice too so am i right in suggesting to my boss that he needs to change the twin and earth for 16mm armoured cable tails

 

[Ross1]

25mm tails to the switch fuse, then 16mm from there on. earth must acheive under 200ohms preferably under 100. Others will prob have more to say on matter Baker1988, but i gotta go. Chaw 4 now

 

[malcolmsanford]

The tails from the meter to that switch fuse I would agree should be double insulated and as it is just a meter in length then no problems.

10mm T +E though can take 60amp on reference method C, so unless there is a good reason not to, that can stay, or you could fit a 40/45 amp fuse if the CCC don't work out.

 

[Baker1988]

Thanks for all the answer but my boss has just got back to me and said that he is not going to change any thing that it has been that way for years so it is staying that way thanks for all the advice

 

[malcolmsanford]

Thanks for all the answer but my boss has just got back to me and said that he is not going to change any thing that it has been that way for years so it is staying that way thanks for all the advice

Think your boss and I would get along quite well

 

[Adam W]

Remember the t&e will be protected by the switch fuse so that will govern how much current the t&e will take.
Rather than 'tails' (they are the bits between the meter and the switch fuse) think of it as a submain - there is additional protection provided by the switch fuse so you're not just relying on the service fuse for that length.
As regards reference method, presumably you wouldn't have a problem installing t&e that way for a socket, so why a submain? Unless it is a TT earthing system it wouldn't necessarily need rcd protection either, due to the age of the installation.

 

[Paul.M]

^^^^^ as above, do it all the time for loft conversions.

 

[lloyd1980]

he could put in 25mm tails rather than 16mm if he wanted to, as long as the supply fuse protects the tails then its all good, if the distance from the meter to the CU is a really long run, up a riser for example then Id say armoured would be better, although prob more expensive cost wise, depends if they are willing to pay the additional costs, labour and material for the install, I agree tho the 10mm twin is undersized and is a pile of rubbish, if the cpc of the t+e has been used as the main earth then this needs ripping out quickly and a seperate 16mm earth running in its place.
Hope this helps you mate and good luck with it.

 

[malcolmsanford]

Lloyd apart from the fact that now we have established it is a TT system and they may have to change that switch fuse if the disconnection times can not be met, why are you advocating ripping out things and installing 16mm main earth conductors for a TT earth system?

 

[lloyd1980]

ah ok missed the TT bit
thanks

and on the size of he earth (16mm) if he installed it, why would this matter ??

on a TT, your resistance should be under 200 ohms , if he wanted to put in an earth cable that was 125mm that would be ok (apart from look strange )
As long as the earth isnt undersized.

Cant see why replacing the CPC from the cut out to the CU would be a bad thing ?
Yes ensure this meets the disconnection time, but why wouldnt a 16mm earth from the cut out to the CU mean the disconnection time wasnt met?

 

[malcolmsanford]

Llloyd you could use spent Dilithium Crystals from the Warp Drive of the SS Enterprise if you are so inclined, as long as by Adiabatic equation proves it will carry the fault current.

But what you can't do is advise someone that a T + E sub main on a TT installations need ripping out quick and a 16mm installed, as if that T+E is going to suddenly explode into a roaring inferno, there is no need for a 16mm main earthing conductor on a TT system, and so why advise someone that is the minimum requirements as it isn't

 

[lloyd1980]

My point was not that the 10mm would suddenly explode haha , my point was that the cpc of the 10mm twin and earth would be too small and would need to have its size increased,
for a disconnection time to be met you would need an acceptably low impedence path back to earth, this would increase the fault curent and trip the disconnect in a faster time.
The larger an earthing conductor the easier it is for the fault curent to pass through (low resistance) and then the fault current spike increases in direct relation to the impedance.
So the higher the impedance the lower the fault current and the slower the disconnect will go, breaker tripping or fuse blowing,
So the lower the impedance the higher the fault current and the faster the disconnect will go, breaker tripping or fuse blowing.
I know my stuff when it comes to this so you cant tell me that leaving the CPC of a peice of 10mm twin and earth as the main earth conductor (TT system or whatever flipping system ) will be more beneficial to a disconnection time than replacing it with a 16mm seperate earth.

Maybe they taught you to use spent dilythiam crystals from the warp drive in college

They just taught me to use copper cable, dont think any wholesaler I know stocks spent dilythiam crystals or warp drives?
Maybe its a new square D range ?

 

[malcolmsanford]

My point was not that the 10mm would suddenly explode haha , my point was that the cpc of the 10mm twin and earth would be too small So you know that, you have done the equation and it's too small .....fair enough and would need to have its size increased,
for a disconnection time...........We are talking earthing conductors here not R1 + R2, remember the disconnection times are met by both conductors not purely the size of your earth to be met you would need an acceptably low impedence path back to earth, this would increase the fault curent and trip the disconnect in a faster time.......you seemed to have grasped that
The larger an earthing conductor the easier it is for the fault curent to pass through (low resistance) and then the fault current spike increases in direct relation to the impedance.
So the higher the impedance the lower the fault current and the slower the disconnect will go, breaker tripping or fuse blowing,
So the lower the impedance the higher the fault current and the faster the disconnect will go, breaker tripping or fuse blowing.
I know my stuff when it comes to this ......Do you so you cant tell me that leaving the CPC of a peice of 10mm twin and earth as the main earth conductor (TT system or whatever flipping system ) will be more beneficial to a disconnection time than replacing it with a 16mm seperate earth..............Yes I can if I prove it by calculation

Maybe they taught you to use spent dilythiam crystals from the warp drive in college....................not sure what they taught you

They just taught me to use copper cable, dont think any wholesaler I know stocks spent dilythiam crystals or warp drives?
Maybe its a new square D range ?[/QUOTE

Well I'll have to bow to your obvious expertise on this

 

[Spudnik]

And as the OP has been instructed by his boss, we can end it there.