meter tails question

[Richard]

I wish to run my 240v domestic meter tails on a route of 7m through a cavity wall, can I run 25mm standard meter tails or do the regulations require other measures over that distance?

[Bob Mannix]

You will almost certainly find the electricity supplier will only allow about 3m of tails. To get over this problem, you need to install a sub-main. The meter tails run to an overload protection device (100mA for 25mm^2 submain) and the submain to the CU. The submain is then protected by *your* overload device not *theirs* (which is all they care about).

Depending on the supplier, you may be able to install the submain protection device in the meter box but I'm not sure on that point (mine is in a box on the dining room wall and I have 7m of submain).

[Andy Wade]

No, the maximum length for tails is 2 or 3 m (depending on who your supply distributor is). The distributors will not allow their cut-out (fuse) to provide protection to any consumer's wiring longer than that. What you need to do is install a suitably rated switch-fuse (usually 100 A - something like a Wylex 110) within 2 m of the meter.

After that you can do whatever you like, provided that you comply with BS

7671. Your distribution circuit (between aforesaid switch-fuse and consumer unit) can use 'normal' tails cable (6181Y) or SWA cable or singles (6491X) in conduit or trunking. The size may need to be larger than 25 mm^2 if you are running anywhere near thermal insulation, or for voltage drop reasons on a long run. The earth (CPC) should be 16 mm^2 (for phase & neutral up to and including 35 mm^2) unless you show by calculation that a smaller size is OK.

The main bonding conductors to water, gas, etc. should go to the main earth terminal near to, or inside, the switch-fuse, and not to the consumer unit's earth bar. (Attention should be drawn here to regulation 413-02-13 regarding the impedance of the CPC of the distribution circuit and/or the need for additional bonding, but in your case with a short run of 7 m and a

16 mm^2 CPC, this regulation will clearly be satisfied without any further concern.)

Avoid running any cables in a cavity wall unless there really is no alternative.

If you look on Google Groups you will find lots of previous threads on this subject.

HTH

[Colin Wilson]

Another thing to note with regards to earths - the supply company do not work to the 16th edition, but to the supply regs, and they are *not* obliged to provide an earth.

If they haven`t provided an earth, they might be able to provide one depending on whether the type of network in the area is suitable, or you may have to sink earth rods etc.