Thanks, but I'd found that. 2"x2" seems rather excessive for a couple of
25mm2 tails, and it's too deep to be plastered over. I'm surprised that it's so hard to find trunking that enables tails to be buried.
Thanks, that's interesting and about what I'd expect. That's why I was surprised that someone said it was a real world problem for a domestic installation. In case anyone wants to know (which I doubt): the magnetic field from a single conductor decays linearly with distance, that from a pair with equal and opposite currents decays as the separation divided by the square of distance, and that from a coil decays as the cube of distance. Turning these theoretical gems into something to predict actual heating effect is beyond my current intellectual abilities, although I did once have cause to use Maxwell's equations to solve a real-world problem ... a long time ago!
Surface mount is possible, but not preferred.
I think I've just come up with an alternative. If the tails take a slightly longer route they can go via the roof space and drop down from the ceiling with all the other cables. At a guess the run would be about
3.5m and it would be visible (up from the Henley block), in steel conduit (across the top of the joists), and in the accessible boxed-in area for the cable drop to the CU.
What about steel capping (which you would need to earth)?:
If your meter rails exceed 3 metres you well need to put in oa switch-fuse...... at the beginning of the meter tail run.....
We use steel conduit for tails behind the plaster. Is there any reason you cannot just enter into the rear of the CU from the garage side?
That would not be compliant.
Yes, I could do it with a sub-main but the only reasons to limit the length can be impedance and risk of physical damage. The extra impedance of 0.5m of 25mm2 copper is not going to have an appreciable effect on the time it takes the service fuse to rupture if there's a fault, and I can mitigate the damage risk by having the whole run protected.
AIUI DNOs used to spec a recommended max tail length but not all now do, and I don't believe the regs spec a max length. I accept that 3m is often quoted as a maximum (sometimes 2m or 6m too); where did you find a
3m limit documented?
Unfortunately I need to put the CU on a wall that's at right angles to the garage wall.
The tails are about 13mm diameter and the capping is only 6mm high, plus it's very easy to drill through (also, I hate trying to fix the stuff!)
Agreed, it offers relatively little mechanical protection, but in reality no worse than many of the so called earth shield cables.
It just generally seems to be the norm.
eg
Sorry for the delay in replying.
As I said we use non touching 25mm galv conduit in such cases (L&N in different conduits) OR go deeper than 50mm.
Thanks for the link, but they say "If the length is more than 3 metres, you should ... as specified in the current IEE Wiring Regulations" ... and AFAICS 3m *isn't* specified in the regs.
No need to apologise, I'm very grateful for all and any info from people who know what they're talking about.
It was a NICEIC inspector that passed this as acceptable on a first fix inspection on a new build.
The non touching bit, I suppose, is there as it "stops" these pesky eddy currents.
I might have some photos of that installation.
FWIW I read that[1] to mean the *protective device* is to be as specified in the regs. not that the regs. set the 3m limit.
AIUI the 3m comes from guidance (on the ESQC?) rather than the wiring regs. IIRC you can challenge their decision. If still not satisfied you can refer their refusal to connect you to the Secretary of State who'll then appoint an independent expert to decide the point. But you may not get a connection in the meantime ;)
[1] in full for ease of reference "If the length is more than 3 metres, you should install an additional protective device at the nearest point to the supply inside the customer?s premises, as specified in the current IEE Wiring Regulations"
There are no current IEE Wiring Regulations*, so no requirement to do as specified :-)
Owain
They're now IET Regulations.
As an FIET I should have spotted that ;-)
Going deeper than 50mm isn't an option here on internal walls - OK to do from one side but then less than 50mm from the other! I tend to use mini-trunking, especially as the walls are hollow 'brittle biscuit' (whatever that's called).
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