Conduit regs/best practice

[EvilDonut]

Could do with a quick sanity check on part of my rewire design.

To run the upstairs ring / upstairs lighting I would like to re-use an existing boxed-off compartment that serves (copper) pipes to the boiler. There is enough room in this compartment to slide down a length of rectangular 'maxi' trunking (40mm x 25mm), which will comfortably take 2x2.5mm and 1x1.5mm cables.

The conduit/trunking will originate in the crawlspace under the house, and terminate just under the upstairs floorboards, at which point the lighting cable continues up into the loft via a 20mm plastic conduit.

Questions:
1) Does this generally sound ok, or is it a bit 'odd'?
2) I've read that all conduit must be ingress protected at both ends - is that correct?
3) The 'join' from the rectangular maxi trunking to the 20mm conduit might be a bit messy. Any more elegent way to do this? (I believe I'll also need to build in some cable supports at this join, as the total vertical run will be about 6 metres)
4) Whilst the heating pipes do not appear to 'touch' the trunking, they do run fairly close to it and I cannot inspect the full length of the compartment, because it is enclosed (plasterboarded over, kitchen cabinets, etc). For this reason I also cannot install clips/supports at the standard spacing Is this ok?
5) Do I have to de-rate the cables at all? My design for the lighting circuit is for about 15xLED lights, totalling about 102W in a 1.5mm cable. As the design doesn't load the lighting cable by more than 30% of its rating, the grouping can effectively be ignored, and no de-rating need be applied to either circuit. Is logic this correct?
6) I have had to notch some joists (rather than drill - too near the ends). I've not used steel conduit here, instead just cable clips and steel safe-plates over the notches, with enough slack to allow the cables to droop a bit. Is this ok? I don't need steel conduit here for mechanical protection? (Seems a bit overkill!)

I should add that I've actually already run the cables

Just to justify the design choice a bit... The Consumer Unit is under the stairs, and the stairs have a winder at the top which makes it not possible to go directly up from the consumer unit, or use the stairs at all for routing. The only alternative would be to go down into the crawlspace under the ground floor, across to an internal wall, then run up the wall in a chased out channel in the safe zone, covering with some galvanised capping. That seemed a lot messier and more difficult.

 

[winston1]

Another one using 1.5 mm cable for lighting when only 1.0 mm is required. Why?

 

[EvilDonut]

Ultimately...lack of experience If I could exchange it now I would.

I'd let myself get all worried by regs (that actually aren't proper regulations) such as allowing 100W per outlet and 3% volt drop.

I'd also planned to clip to joists under insulation, mandating a whopping 0.5 derating factor. I now know a better solution is to loosely lay it on top of the insulation, then an air gap, then my loft boards (on stilts) which if I understand correctly doesn't require derating.

Didn't see any harm moving up to 1.5mm to give myself more headroom to compensate for these factors.

 

[JohnW2]

Ultimately...lack of experience If I could exchange it now I would.

Fair enough, but don't feel bad. Even though it is virtually never necessary, for some reason (habit?) many people, including many electricians, use 1.5mm² - and we have the electrician from across the Irish sea telling us (if I recall correctly) that only 1.5mm² is used there, and that he believes that 1.0mm² cable is going to be 'banned'

Kind Regards, John

 

[EFLImpudence]

I'd let myself get all worried by regs (that actually aren't proper regulations) such as allowing 100W per outlet and 3% volt drop.

Just to say.

With a 6A MCB, which you probably have, the limit of 100W lamps would be exceeded long before you would need larger than 1mm² (16A) cable.

As for volt drop - you don't count the total amperage of the loads for calculating.

For example, the total amperage only applies as far as the first lamp position, then total amperage minus that of the first lamp as far as the second, and so on until only the amperage of the last lamp from the penultimate to the last lamp.
So, if the lamps are roughly equally spaced, then you could calculate with half the total amperage for the whole circuit.

However, if your type of lamps are not fussy about the voltage, then it's all academic and unnecessary.

 

[industryspark]

1, Little bit odd but sounds do-able, in affect your concealing the trunking within the box compartment, right? Is this circuit RCD protected? Be careful not to fall fowl of the concealed cable requirments if its not.

2, Best practice dictates it ought to be but im guessing your NOT using twin and earth cables within the trunking but rather singles? If so you shouldnt leave the trunking open ended and you need to find a solution that allows your conduit to be joined to the trunking with the singles inside. If your using twin and earth it wouldnt matter so much.

3, Can you get some end stops for the maxi trunking brand your using? If so you could conduit onto that.

4, If cables and trunking aren't touching the pipes then it'l be ok but you really ought to keep an inch or so minimum clearance. Again, you should stick to the recommended support intervals but a sensible judgement might allow you to stretch things a little.

5, Forget about de-rating

6,Which joists and where are they?

 

[EvilDonut]

1) Yep, 17th Ed. split load CU. All circuits will be on RCD side.
2) All TnE, no singles anywhere in the new installation
3) OK. I've since bought a selection of end stops/glands/terminators to experiment with.
...
6) Joists between ground/first floor. Re-used existing notches and holes where possible, made a couple of new holes (between 25%-40%), and where I needed to route near ends of joists resting on internal wall I notched between 7%-25%.

Thanks for the advice!