T & E in steel 20mm conduit

[Mark G]

I have just about completed re-wiring my entire house using 20mm steel conduit and the rings are all in 2 x 2.5mm !

Will this pass inspection ?
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moderator

Mark G, please note 17 the post you replied to was nearly a year old

 

[andy]

use singles. id be surprised if you get 2 2.5 T+E in 20 mm conduit

 

[ban-all-sheds]

I'd be surprised if you didn't have to de-rate the cable as well.

If so, and it's on a 32A breaker, then it will not pass inspection.

 

[gjh]

i've done all my rings in 2.5mm cable as well. Not in steel conduit though, just buried in the plaster where nessicary [ie from the floor up to the socket and back again, and in the corner of the room to get the cables between floors] otherwise they are under the floorboards, would this fail too?

 

[mapj1]

No, 2.5m is not normally a problem. The only problem is that in conduit, with more than a 50% or so filling factor, the air cannot convect well to get the heat out to the masonary around it. Even so, the derating is not normally so severe as to be a problem - rememembr you are allowed 20A cable on a 32A ring -just.

 

[ban-all-sheds]

Stuffed, then. (anyway - isn't it 21.3A minimum?)

2.5mm² is rated at 23A in conduit.

But we know from his other questions that our supplicant has put two cable in his conduit, so there's a grouping factor of 0.8, making the rating of the cable 18.4A.

MarkG - looks like you have done something which will not pass inspection, does not comply with the wiring regulations and could be dangerous....

 

[breezer]

Stuffed, then. (anyway - isn't it 21.3A minimum?)

2.5mm² is rated at 23A in conduit.

But we know from his other questions that our supplicant has put two cable in his conduit, so there's a grouping factor of 0.8, making the rating of the cable 18.4A.

MarkG - looks like you have done something which will not pass inspection, does not comply with the wiring regulations and could be dangerous....

That's diy for you

 

[ban-all-sheds]

Not necessarily - I wouldn't do it.

 

[Mark G]

Well, I am as they say speechless.

I have rippped out all my old rubber which was in the existing conduit and replaced it with new and now I find that it's not allowed ?

Are you absolutely certain that this won't pass inspection ?

Would 25mm steel have been ok ?

I suppose 2 x 20 mm would rectify the situation ?

Help !

 

[mapj1]

I think you need to be clear that we have not misunderstood what was there before - was the conduit always undersized, or was the original circuit wired in singles, which would have given better air flow. Its only the sections where the conduit is too full that would give me any concern - and only if the ring main conductors have been reduced below a 20A ratig by proximity.
Exactly the same trouble applies to cables or wires feeding a ring main sharing capping or oval conduit, but the derating factors are dependant on the filling factor..
http://www.iee.org/Publish/WireRegs/2001_Amd_1.pdf
shows the 20A rating I am on about, needed for ring mains.
Then use the grouping factors here http://www.tlc-direct.co.uk/Figures/Tab4.4.htm and the basic rating here.. http://www.tlc-direct.co.uk/Book/4.3.9.htm
from your description, I think you are looking at 80% of 23A for a normal 2.5mm T and E with 2 current carrying cores, unless they are spaced. That is a little light of the mark. However, small changes in the description might mean it is OK after all.
(the 'spaced ' case allows similarly for air flow.)
Hope that helps you decide your safety margin is negative or positive.
Oh, and its not totally unknown for 'proper' electricians to fail to properly allow for the derating when two T and E wires share the same capping to feed a ring socket.

 

[Mark G]

The original steel conduit was the imperial equivalent of 20mm and yes there was only one conduit carrying both cables to each socket.

 

[mapj1]

And the originals were 'cables' with a double insulation, not single insulated wires?
If so, well then it has always been a bit undersized, I don't think you have made it any worse than it was before, though it may not pass the letter of the current regs, as it sounds as if the tubing is overful, and the derating effects should have been applied, but on the other hand I don't think it will suddenly catch fire either, unless you alter your loading habits suddenly.

If you ever extend the system however, rather than just replace existing cables with their metric equivalent, as you already have, then you would be wise to bear these effects in mind, and size the cables or run double conduit accordingly.

 

[ban-all-sheds]

I have rippped out all my old rubber which was in the existing conduit and replaced it with new and now I find that it's not allowed ?

It's not that it's not allowed, but you didn't design it properly, and now it won't carry the current that you expect.

Are you absolutely certain that this won't pass inspection ?

Pretty sure it shouldn't, if the inspection is done properly, unless you reduce the rating of the MCB on the circuit. 25A would be my view, but it would be good to get that confirmed.

Would 25mm steel have been ok ?

I don't think it would have helped - the cable would still be in conduit and would still be grouped....

I suppose 2 x 20 mm would rectify the situation ?

Yes.

As mapj1 says, you may be no worse off than before, but it doesn't conform to the rules. You mention an "inspection" - what is this, who is doing it and why?

Also, I'm still a bit confused about what the combination of these two seem to mean:

The original steel conduit was the imperial equivalent of 20mm and yes there was only one conduit carrying both cables to each socket.

Is it possible to route the return cable from the farthest socket back through the previous sockets so that the same conduit can be used for both tails of the run ?

Why are you having to double back from the "farthest" socket?

What did you originally have? Was it

or

or