Tw & E Meter Tails

[Midwest]

I have done a little research on my question, but most of the previous threads are 2010 or older, and this issue somewhat different to current ones (I think!). I'm new to the 17th and still learning.

I went to a potential job today, to have a look at a job for a (friend) plumber. It's a large 5 bedroom house, and the work is on converting an existing bedroom to a bathroom, therefore all work notifiable. However on inspection of the CU I noticed a potential problem. The house had an integral garage, that has been converted into a living room. The then CU had been moved to a cupboard of the living room. I saw that the CU had been fed from a 100amp fuse/isolator (where the old CU was) with a 16mm 6242y cable (meter tails). The main protective bonding conductors are 6mm 6491x. The meter tails are in excess of 3m. The CU is 17th edtion 12 way split load with a 100amp isolator. I have not calculated the max load, but lets be generous and say 80amp. The cable is proably clipped direct in the joists of the room above.


I know I will be told to do my cals, and I will (when I have time), but I'm thinking the cable is underated, 87amp (if clipped direct, not sure about insulation). From my OSG, for TN the earthing conducter should be 10mm for a 16mm line conductor (16mm 6242y is 6mm cpc).

My thoughts are that I could derate the consumers isolator to 80amp to allow for discrimination, but potentially the cable will be at less than 50mm in the floor void (requiring RCD protection), and the cpc is too small to act as the main earthing conductor. I could just walk away, but my (friend) plumber bought in the previous electrician. So before I state my case...am I right in what I'm saying?

 

[Octopus]

I would say that you are changing a circuit or two and as such existing tails aren't relevent. Bonding is.

Take your approach and we would have to price ourselves out of loads of work.

Tin hat on

 

[jaydee81]

did you check size of fuse in isolator if it is 100 amp would change to 80 amp 6mm earthing conductor is fine if new upgraded bonding conductors are connected to origin

 

[Octopus]

did you check size of fuse in isolator if it is 100 amp would change to 80 amp 6mm earthing conductor is fine if new upgraded bonding conductors are connected to origin

Change the DNO fuse - really?

 

[jaydee81]

of course not i read it as meaning fused switch op even mentions down rating isolator to 80 amp

 

[bigbaddave]

did you check size of fuse in isolator if it is 100 amp would change to 80 amp 6mm earthing conductor is fine if new upgraded bonding conductors are connected to origin

I hope you where drunk

 

[Deleted member 26818]

Midwest, use of a 30mA RCD is not an acceptable method of providing additional protection for cables above ceilings or below floors.
By all meand, provide RCD protection, if that is your desire, but you will still need to provide an acceptable method of additional protection in order to comply with the Regs.

You haven't informed us of the earthing system.
owever, the minimum CSA for an earthing conductor in TT and TN-S systems is 2.5mm², and for bonding conductors 6mm².
For TN-C-S, the minimum CSA for both earthing and bonding conductors is 10mm².

 

[Engineer54]

Really, someone actually extended tails with T&E to feed a 12 way CU!! I dare say it was your mate the plumber that did that little job!! lol!!

 

[Midwest]

Really, someone actually extended tails with T&E to feed a 12 way CU!! I dare say it was your mate the plumber that did that little job!! lol!!

No unfortunatly not, that would make the situation simple. It was an electrician he 'introduced' to the client, therefore if I come in and say the meter tails/earthing conductor are too small and need replacing before I rewire new bathroom etc, it won't look good on my friend plumber.

I haven't been able to verify supply, as access to the cutout was unavailable (locked side gate), however could not see signs of TT and house is newish, so suspect TN-C-S. Therefore I can't see that the tw+e 6mm cpc is large enough (542.1.3.1, Table 54.8) i.e. main earth should be min 10mm & main pbc 10mm. CCC of flat sheathed 16mm (+6mm cpc) ref method 100 table 4D5 is 57amp or ref method c table 4D1A is 87 amp. The cable is run in the floor joists of what was an integral garage. As the property is only a few years old, I suspect the floor void would have insulation in it.

As regards RCD, the current tails not being mechanically protected need some fault protection. I have read/seen somewhere in this forum/elsewhere, that such tails (i.e. over 3m) should be protected in conduit/trunking or swa?

Would you guys really be happy to add circuits to the current set up, and submit EIC? I would really like you advice, as I suspect this will be a common problem to face. I'm due to go back and see the client Saturday. I think I might be telling my friend to get his other electrician back.

Sorry table for main earth wrong, but should be least half of line coducter (can't find table and late for work).

 

[telectrix]

the existing lack of RCD on the tails is not your problem. it's pre-existing . as long as the earthing and bonding are adequate, then you're OK. i would put a note on the cert. though.