I want to re-site my CU. I have a 100A company fuse and a 100A main switch before the CU (which itself contains a 100A main switch).
I want to run tails from the first switch about 6m to 7m to a different location. They will be separated and buried in plaster and I want a paid-up Part-P-inspector type to be happy with it.
What size tails do I need? DO they have to be double-insulated? Any gotchas, apart from the fact that they will be bloody difficult to handle?
Peter
Does the "100A main switch before the CU" have a fuse in it as well, i.e. is it a switch/fuse? If not I don't think you will be allowed meter tails as long as six or seven metres. You'll have to have some sort of protection (e.g. a switch/fuse) at the meter end. Length limits do vary between suppliers but are usually two or three metres.
Yes, they do have to be 'double-insulated' - as in two layers of plastic around them. All meter tails I have seen are like this. I guess if they were in conduit they wouldn't have to be double skinned.
It was taken from another, small CU - it is identical to the one in the CU that is to be moved - i.e. it has, unless I am much mistaken, overcurrent protection.
If I have that, does that mean I am allowed, withing reason, whatever length tails I like to the CU? I would hope so because after the main switch in a conventional CU you can run appropriately sized cables as far as you like.
The reason I wondered is that I can see an argument that, in theory at least, these are no longer meter tails because they are the switched side of the/a main incoming switch. IYSWIM.
Would 35 sq mm be sufficient because TLC have them at a reasonable price (or can anyone point me at a cheaper source?).
Cheers,
Peter
No it doesn't, if it's DIN rail switch from a CU, at least I've never seen one of these with overcurrent protection. It may be *rated* at
100A (or something less) but that's just the current it can switch, not any form of protection.As I said above that switch almost dertainly doesn't provide the required protection, you need a proper switch/fuse unit. The 'downstream' cables running from a CU are all protected by the MCBs (or maybe fuses) in the CU, that's what the CU is for, among other things.
If you do provide a switch/fuse to protect the cable then, yes, you can have the meter tails as long as you like within reason. There may be earth loop impedance issues but it's unlikely unless you have unreasonably long tails.
Whatever they are they have to have two layers of insulation if they are exposed, just the same as for all the T&E mains cable you have around the house.
My OSG says 35sq mm is OK unless in a thermally insulating wall or similar.
Last time I got involved with this, the then LEB wanted either SWA or MICC from the meter to CU if more than about 1 metre or so.
I had assumed, from the action of the switch - being harder to switch on then off, that it did. Oh bugger.
I am looking for something not too big. From Wylex's website I downloaded
and there on page 24 (their marking, 26 if you look at your PDF viewer's page number) is the 106 which looks a sensible size. However, if you look at TLC's site it looks *much* bigger. Is the 110M really that big? Why should it be that much bigger than the 106? Or is it an old/incorrect photo I wonder. Hmmmmmm.
Any alternatives?
Does under plaster count as in a "thermally insulating wall"
Thanks for your help.
Peter
This is what I was hoping I would find ...
Is this what is required?
The dimensions are given on the pages that you cite.
Makes even better sense if you bottom post !!!!!
Dave
CLICK do a double socket sized 80 amp fused switch for about 55 quid. The contacts are rated 100 amps and you could uprate the fuse if required. I fitted one today and it was very nice to work with.
Is there a CLICK website?
Adam
MEM also used to / still do one.
You need to watch several points:
1 you must have a circuit protective device (most usually a CU with MCBs/RCDs) within 1m to 3m (specified distance varies with area of uk you are in). In your case you are moving the CU so you will need to replace it with a 100A switch fuse. I would also recommend that you put in an isolator switch before the switch fuse if there isn't one in the meter box. This may look like overkill, but it enables you to totally isolate your cicuits for later replacement/maintenance or whatever. A 100A switch fuse is quite a size so you could investigate a 100A MCB, which is much smaller, instead - I'm not sure whether that would be permitted - anyone know?So if you move your CU 8m: 25mm cable drops 1.85mv per metre per amp - ie 1.48V per leg or 1.29% in total. (35mm would be 1.35mv...0.94%). This eats into your 4%, and reduces the maximum length of each circuit from your CU. Exactly how much varies with the maximum current that can flow in that circuit. eg using 25mm*2 tails, an originally 40m long 6A 1.0mm^2 lighting circuit could only be 35m long from the new position. If you used 35mm^2 cable the new length could be increased to 37m approx.
These rating numbers are all in the IEE regs tables 4Dxx/4Exx - or you could work it out knowing the resitivity of Cu. If you use SWA the voltage drop figures stay the same as they only depend on the copper conductor size (apart from a small temperature effect), but cable max amp ratings differ - see the pirelli web site for BS5467 SWA cable ratings.
When you've weighed it up, would suggest that you repost with a firm proposal. Hopefully someone will give it the once over for you.
There could be problems with discrimination using an MCB as main fuse.
Owain
Afraid I do not understand what you mean - I am unfamiliar with the term "discrimination" in an electrical context. Could you explain please?
Peter
I already have a 100A switch immediately after the meter and before the existing CU. Can I put in *just* a fuse in the live side after the switch? One like ?
Would along the wall but above ceiling height (I am in a top-floor flat) be sufficiently out of the way? What can I do for the bit vertically from the existing installation to ceiling height?
My understanding is that it may only be one cable size down so 25mm^2 earth with 35mm^2. If I am routing 35mm^2 cables an additional 25mm^2 one is not going to be much of a problem. My problem stems from not having enough space above the entrance doorway over which to route all the circuits, so I want to move the CU the other side of this choke-point.
Good idea - thanks for your thoughts.
Peter
If you have a short circuit on one of your circuits, you would expect the MCB for that circuit to trip. A short circuit is likely to be much more than 100A, but you would not expect your upstream
100A MCB to trip. Well, MCB's don't have that level of discrimination (or at least, you'd have to work it out carefully from manufacturer's data), and you might well find the 100A MCB does trip too. This is also in contravention of the wiring regs which requires as far as is possible that the effect of faults be limited to the faulty circuit, since it would cut the power to other circuits too.Normally, you would use some type of fuse upstream from an MCB, such as a BS1361 or BS88 cartridge fuse (but I don't think 100A versions of these will fit in CU DIN rail cases).
He did!
It's perfectly reasonable to put an overall comment at the top of the post (particularly one related to snipping, cross posting or something like that) before posting properly, as Peter did.
I know it's easy to jump on the "don't top post" button whenever you see new material at the top, but in this case that's not what happened at all.
[sig deliberately quoted]
Exactly!
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