meter tails != 25mm^2 PME supply (TN-C-S)

[rlc]

Could someone clarify table 54G and confirm whether meter tails of 16mm^2 (OD~9.1mm) are acceptable. I'm assuming as they are OK as they are still specified in the table? Is there any reason to get these updated to 25mm^2 and if so what benefit is there?

In addition, the protective conductor is currently 6mm^2 (OD ~4.8mm) which as I see it's wrong according to Table 54G, minimum is it should be equal to the phase conductor csa to comply with 16th ed. Also, the tails go into the CU as well does the main earth i.e there is no MET.

Thanks

~rlc

 

[rlc]

so reading a little further......Table 54H tells me that the main equipotential should be >= 10mm^2.....so is it correct to assume that the earthing conductor should be >=16mm^2

 

[Lectrician]

Bonding min is 10mm.

I always install 25mm tails and 16mm earth as standard.

BUT

On periodics, you need to look at the loading and fuse size. Many installs will only have a 60 or 80amp fuse despite the popular belief that they are 100amp. If this is the case, 16mm still complies fully, and could then have a 10mm main earth.

I would still always fit 25/16/10 on new installs, rewires etc.

 

[rlc]

Cheers Lectrician, dwelling is candidate for increase in loading through adding an extension soon, I have two old wylex boards fitted, wire fuse, no RCD protection and sub-standard bondind - 16mm tails coming from meter into CU. SWA feed to garage ring + lighting circuit.

Suggesting best course of action would be to contact DNO, get them to:

(i) upsize the tails to 25mm from the meter
(ii) fit 100A DP isolator switch & Henley Block (for future expansion for annex CU)

Upgrade CU & carry out remedial updates to bonding throughout.

Would you agree?

~rlc

 

[Lectrician]

I dont agree with having isolators fitted. It is another link in the chain for failure and damage. Once a CU has been changed, why would you really need to ever isolate it again?

The tails from YOUR CU to the meter would be best replaced, but the tails from the meter to the service fuse is the DNO's property and not covered under BS7671, and as such they may be happy with that size (The service drop to the house is usually only 16mm). They may replace them, they may not. My DNO usually change them if you ask nicely.

Upgrade of bonding is a requirement when you are going to be doing major additions/alterations.

A CU change is deffinantly a good idea.

Incidently, this may not be appropriate, but worth a mention.....even if just to learn something! A 4 way wylex CU with the typical 30amp cooker circuit, 30amp ring main, 15amp immersion, and 5 amp lighting circuit can only ever be loaded to 80amp - 30+30+15+5. The main switch was also only ever rated at 80amp originally. Therefore, 16mm tails are fine, even if the service fuse is 100amp.

 

[rlc]

I dont agree with having isolators fitted. It is another link in the chain for failure and damage. Once a CU has been changed, why would you really need to ever isolate it again?.

Fair point, other advise appreciated.

cheers, ~rlc

 

[Spark123]

Where PME conditions apply you should consult your DNO about the size of main equipotential bonding conductors as they may require them to be larger than those in the IEE regs.