2off 2.5mm T&E cables in 20mm Plastic Conduit....

[boyyo]

Hello all,

First post so here goes..

Having just bought / routed / fixed 30M of 20mm plastic Conduit, and realising it was bl***y hard to get 2 x 2.5 cables it to run smoothly through ...in fact used some gel from work which we use for cable ident sleeving (electronic assembly)...

Can someone advise if I muststrip it out and fit 25mm...or can I put it down to experience and use the right size next time.

The conduit is in a Carport / shed that will only be using domestic power tools...no fridge / compressor etc...

I know what I want to hear but am willing if told 'I must' ...start again!!!

please anyone ....thanks

 

[BS3036]

Can't see why you should have to take it all out again, if you've managed to get it in now. Using conduit is not mandatory, you could have clipped direct.

Only thing is that for long runs of conduit you should derate the cable. I seem to remember 2.5mm in conduit as 20A, and two cables derated to 20/1.414 = 14A. So if you have made a ring final, it might be better to use a 20A MCB.

 

[JonoPashley]

You could have you know just spent the extra few quid and bought singles and not even had the problem of the cables sticking.

 

[boyyo]

Thanks for the replies,

I know now that I could have used 'singles'...been reading up since the event!...and guess what we have reels of unused stuff at work!

With regards to long runs...the furthest from the CU is 9m is that considered to be a long run?...and there are only 4 double metal clad skts in total..

 

[BS3036]

By long run, I meant more than 6 inches really. It's to do with over-heating the cable. You didn't say if it was a ring. To use a 32A MBC on a ring final the cable should be able to carry 20A on each leg, so you should derate it. If you don't think the current is a problem, then you won't think that using a 20A breaker is a problem.

 

[holmslaw]

..

 

[sparkyspike]

Or scented. It will make your work smell nice.

 

[securespark]

And your hands like you've just bathed a baby...

 

[boyyo]

Thanks for the feedback.

Yes it is a Ring and I will derate the breaker to 20A, which should not be a problem for me....

OK....a stupid question!!!...

because I have used 'inspection Ts and Bends....6 in all, as I couldnt bend the conduit manually, wht's to stop me leaving the covers off for any heat to disperse 'naturally'?.....Is it because localise heating is the problem and this could be some way from the open vent therrfore derating still occurs at this specific point?

I'm still going to derate...but wonderd.

 

[boyyo]

Ohhh and just came across another wee problem!!!

Have a 2 Gang metal clad switch with each side being part of different 2 way switch circuits.

So my reckoning is that I'll have two 4 core and teo 3 core cables going into the switch unit....thats too much for 20mm conduit....even I know that!!!

what do I do....having now purchased the switch etc??...

 

[ericmark]

2.5mm clipped direct is 27A so even with de-rating it is unlikely to reach the 21A for Reference Method 100# (above a plasterboard ceiling covered by thermal insulation not exceeding 100 mm in thickness) likely to be Reference Method B (enclosed in conduit on a wall or in trunking etc.) with rates 2.5mm at 23A.

Not got books to hand but I would expect that it can still be supplied from a 32A MCB?

 

[boyyo]

Actually I've just realised what a stupid question the '4 cables in one 20mm conduit 'was....

I can use two feeds into it with two cables in each...the back box has two 'ports' per side!!!

check the timeof my post it was flipping early...brain not warmed up etc...

 

[BS3036]

Eric: Did you allow for two identical cables carrying the same current, in the same conduit when you said that?
I don't have my books to hand either, but my recollection is that you divide the current rating by the square root of the number of cables. Willing to be proved wrong though!

 

[JohnD]

just an observation for DIY work:

if it is for a ring, it is more common to run the cables round the room, with outlets off it at intervals, so the two legs are not usually contiguous.

I use oval conduit for T&E sometimes, but only one cable in it, including in chases, I finfd it easier toi replaster over than capping. I would probably have used a separate conduit for each leg, rather than a bigger conduit.

Anyway, the cable may be difficult to pull through, but you probably won't ever want to pull it out again.

 

[ericmark]

As I have said can't find book at moment but if it is not allowed there are a heck of a lot of houses with two legs of 2.5mm in oval conduit buried in a wall which don't comply.

Considering without de-rating it is 46A it is unlikely that it will go below 32A after de-rating. It would need to be de-rated 0.7 or less not to comply and from memory it was over that figure.

PS Square root of 2 = 1.414214 which is over unity so does not make sense. If divide then = 32.52 so just within requirements.