2.5mm Twin + earth in 20mm Conduit

[peterduckett]

I have just been looking at some 21 way flush mount consumer units, assuming this was to be mounted at so call roof heigh i.e about 6ft from floor,
It would be safe to assume that 90% of all cables would go into the floorspace above, but the unit only supplies 12 20mm ko and a 32mm ko


Now i would guess the 32mm is for 10mm cable for shower and 6mm possibly for cooker it leave 12 20mm ko for lightning and mains and 17 spare ways this would mean some cables would have to double up

now guessing that the run would be less than 100mm to the floor above would it be possible to run two 2.5mm twin and earth down one conduit for a ring main? if so would their be much if any de-rating needed to be applied

 

[Chri5]

Would it not be easier to mount the CU on 20mm stilts and then use the cable entry behind the CU. Then box in any area between the cu top and the ceiling height ?

I'd also consider the height of the CU, imagine a family of 5ft elders (eventually) bought the house. The CU shouldn't need a set of steps to access when the lights trip

If you want to go with conduit drops and use the knockouts 2 x 2.5mm will easily get in a 20mm.

If you have any doubts about cable capacity them cut some bits of 1.0 - 1.5 - 2.5 - 4.0mm TE and play with them in a bit of tube. Derating for group factors is minimal over small distances.

 

[peterduckett]

Would it not be easier to mount the CU on 20mm stilts and then use the cable entry behind the CU. Then box in any area between the cu top and the ceiling height ?

Not really as i was looking at flush mount consumer units the idea being to make as much room in a small space as the space may turn into a downstairs WC though the cut out and meter would have to be moved as i dont fancy P*ssing on that

Derating for group factors is minimal over small distances.

At what point do you thin de rating has a significant factor?

 

[Chri5]

At what point do you thin de rating has a significant factor?

When the cable design load and required device load are close.

Take 1.5mm 6a lighting, the cable can happily take more than 2 x the fuse rating (clipped direct about 20a) so minor amounts of grouped cables, cables run in insulation are very unlikely to derate below the 6a fuse load.

A shower run in 10mm needing to take a 45a load is close to it's limits before any derating, so derating is a more important consideration and thus calculation must be done

 

[peterduckett]

I can see where your going with that though that only really stands for radial circuits how about ring?

A single 2.5mm twin and earth is rated at 24 amps but is generally used a 32 MCB even though the total ampage would be close to 48,
How do you de-rate a ring?

 

[BS3036]

How do you de-rate a ring?

The cable must be capable of 20A after derating. If I recall correctly, this is not quite met for 2 x 2.5mm in a conduit, but I would need to look it up.

 

[RF Lighting]

Why not drill the holes out and use bigger conduit?