fuse box question

[nospam]

Hello,

I have the electric meter in a box outside. The tails then come inside under the stairs and across the room to the fuse box in the corner. I don't like having this long stretch of permanently live cable. I'm upgrading the fuse box and thought that where the tails enter the house I could fit a 100A incomer in something like this:

then I could always isolate the section of tails between the incomer and the fuse box. Does this sound like a good idea?

Can you recommend any other good enclosure for an incomer? I can only find consumer units with a number of spaces in and I really only need a 2 way box for the incomer. The Wylex does not come with any cut outs or grommets, etc. what would I need to buy to use with this?

Thanks.

[Bob Mannix]

What you have is normally not allowed as meter tails are usually limited to

3m or some such. What you should have is exactly what you propose, which is called a sub-main. The suppliers do not like protecting long runs of heavy cable inside a property with the supply fuse and require you do do, and be responsible for, your own protection. The box you refer to is IP65 rated for use outside (so has no cutouts). You can get small enclosures for DIN rail stuff but you may have to go to an electrical supplier.

Disclaimer: "Doing any of this work yourself without proper qualification or inspection will undoubtedly be in contravention of part P etc., and I am obliged to suggest you don't".

[Dave Plowman (News)]

And rightly so; it's usual to use SWA for this job.

[nospam]

I think the tails are about 3m. The house was built 30+ years ago so perhaps the regulations were different when this was fitted. I'm not sure how the tails are fixed as they *seem* to be inside some 32mm pipe that is hidden beneath the plaster!

[Andy Wade]

The proposal was for "an incomer," which is rather vague. If the circuit length between the mater and the consumer unit exceeds the maximum allowed for meter tails (3 m, or less in some areas) a switch-fuse will be needed.

For 60 or 80 A supplies the MEM 800KMF

popular with electricians for this job. Alternatives are the Wylex

108 or 108M units or the 110M for 100 amp supplies which is called a sub-main.

Actually it's a distribution circuit. It only becomes a submain if it goes to a separate building (e.g. to a detached outbuilding).

Remember that the main equipotential bonding must be connected to the main earth terminal at the origin of the installation. It is permissible for the bonding to be made at the consumer unit but then the earth conductor of the distribution circuit acts as both circuit protective conductor and main bonding conductor and must be sized for the more onerous of those two requirements. For 25mm^2 tails that means that a 16 mm^2 earth will be needed, if copper, or 36.1 mm^2 for steel armour (so the armour of 2-core 25 mm^2 SWA (= 60mm^2) is OK). However

16 mm^2 T&E could not be used without running a separate 16 mm^2 protective conductor.

Building notice required, and that should trigger an inspection.

[nospam]

Hello,

I'm phoning around this afternoon to see if I can find something. If not, what's the best way to proceed? Drill a couple of holes in the one mentioned and fit grommets?

Thanks.

[nospam]

I was thinking a small enclosure with an 80A incomer would be cheaper than these. Why is the switchgear so much more expensive?

[hzatph]

At the risk of being shot down by those who know far more than I do, is there not enough room in the meter box to mount the protection that you need? It would avoid another device in the hoiuse although I guess under the stairs is not a problem if a traditional cupboard type space rather than open plan living.

[Bob Eager]

It's a switch fuse, not a switch (to put it simply).

Andy was concerned that the length of the meter tails was such that you really ought to have protection for those tails in their own right. All that protects them at the moment is the supply fuse. If you fit a basic incomer then there is no fuse included.

If the tails are less than 3m, you could use an incomer, but it would be better to use a switch fuse.

Go to

and look at:

PL02545

then fit a standard incomer. That's what I did.

[nospam]

Yes, I suppose you could fit the switch in the meter box but under the stairs is preferable because, as you say, it is a cupboard space. I think this for two reasons:

1. if the switch is outside then it makes it easier for undesirables to switch off your electric!
2. you've got to be careful about switching it off when its raining!

[nospam]

Hello again,

I agree with Andy that there should be some protection; that's why I want to add "something" between the meter and consumer unit.

The tails are roughly 3m; perhaps slightly over but no more than 4m. Like I said they were added when the rules were different in the 70's. Since moving in I have been concerned that this a run of cable is permanently live. I think it unlikely that I would ever drill through it, but I'd like to be able to isolate it in case something terrible happens.

So as the consumer unit is about to be upgraded, now seems to be the right time to do something about those tails. I am puzzled though about protecting long tails with a separate fuse. If there's a 100A fuse at the meter, what good would another fuse at the switch be? Or is it that the second fuse is slightly lower rated at 80A, which stops the tails from melting?

[Bob Eager]

by 'protection', I (and I suspect Andy) mean some kind of fuse or something that will break the circuit should damage or fault occur to those long tails. All you have at present is the supply fuse.

Isolation doesn't protect; after all, it might be an accidental damage that you couldn't foresee.

Yes, essentially. I doubt the supply fuse would go at 100A, anyway. And it's a damn sight more inconvenient.

[nospam]

I can see that a second, slightly lower fuse (80A vs. 100A at the meter) will provide some protection but even so, 80A is a lot of current before the fuse blows! It would help if there was a short circuit but I'm not sure how much use it would be in other disasters if less than 80A were being drawn; after all, it only takes mA to kill someone. OTOH any protection is better than nothing. Thanks to the link, I think I'll get the MEM unit recommended unless anyone here knows anything better? Thanks again.

[Andy Wade]

It's about the wiring regulations and demarcation of design responsibility, rather than technical considerations alone. The DNO's fuse is there to protect their equipment, not to protect parts of your installation. The wiring regs (BS 7671) require you to provide overcurrent protection as near as practicable to the origin of your installation. It is accepted by the supply industry that you can rely on the DNO's fuse to perform this role if and only if the tails are short, i.e.

[Andy Wade]

That's not allowed, unless it's an isolator provided by the supplier.

[nospam]

Don't worry you convinced me to put in a switch fuse in your first post. It's exactly because I share your concern about the long tails that I wanted to have the layout changed and why I asked here for ideas. I want this to be a very safe installation. Thanks again.