Cables under subfloor

[michaelsf90]

Happy new year guys. I'm gonna be in the process of slowly rewiring my house over the next 12 months. It's an old house with wooden floorboards on the ground floor and a subfloor. The consumer unit is at the bottom of the stairs in the cavity. To get upstairs I plan to run cables underneath the floor to under the stairs. Go up there in a trunking and then up onto the landing floor. The amount I need to rewire that will go under the floor is 1 x 10mm t+e, 1 x 1.5mm t+e and 7 x 2.5mm t ÷e. Shower circuit, upstairs lighting circuit, boiler supply circuit, upstairs ring. A utility ring main and outside power supply ring main will follow the same route but won't go upstairs. To save on clipping and not that I'd have room to clip them all. Would bunching them together and all round banding suffice? It's a metal consumer unit as well so would bunching them create a problem with eddy currents as they go through knockouts but obviously won't all fit through the same one

 

[ImpededLoop]

Bunching the cables will require their current carrying capacity to be derated as they won't be able to dissipate heat efficiently. This will need to be calculated.

Provided that all three conductors from each cable enter the consumer unit through the same hole, there'll be no problem with eddy currents.

 

[timhoward]

Maybe consider 4mm T+E radial circuits for the utility room and outside circuit, meaning two less cables.

If having de-rated a cable using table 4C1 according to how many are grouped together, if the expected load current is less than 30% of the de-rated CCC it can be excluded from the cable count when considering the remainder.
Lighting circuits are likely to be excludable these days with LED lighting drawing very little current.
The upstairs ring and the boiler circuit may well be lightly loaded too.

I think the trunking is going to be the biggest issue if you religiously follow Appendix 4. If "under the stairs" is a cupboard then clipping direct slightly spaced and boxing in could be an option.

Hope that's some food for thought.

 

[michaelsf90]

Under the stairs is a small cupboard. Not one you can walk in. I'd put trunking right in the corner to the side of the door as you go in. Can I not put trunking there? I'm wondering how I'm gonna clip so many cables along the same run of joist. On the first floor holes are drilled and cables kept together so I figured the all round banding would work. Wouldn't be tied together but where rhe banding is theyd be bunched I guess. Do you all just clip direct to the joists under subfloors? I was going to use 4mm for the outside power but 4mm twin is only 2.5 earth and it feeds a small consumer unit in the shed and does a pump for the pond at the end of my garden. It's a 2.5 ring inside to a Wiska box then it goes to 4mm swa to the shed

 

[michaelsf90]

My plan was to put a large pvc trunking so plenty of spacing factor. Like a 4 x4 or something. I know with twin eddy currents isn't a problem but the shower circuit will go through one knockout. The outside power and utility room through another. Upstairs light goes through the same as downstairs lights. Upstairs rings on it owns. If they were bunched occasionally through a joist would that not cause eddy currents?

 

[timhoward]

Under the stairs is a small cupboard. Not one you can walk in. I'd put trunking right in the corner to the side of the door as you go in. Can I not put trunking there?

I didn't mean you can't put trunking there. I was more thinking of the cables bunched together in trunking where derating is harder to justify avoiding.

My plan was to put a large pvc trunking so plenty of spacing factor. Like a 4 x4 or something.

Makes sense and that should be fine.

I know with twin eddy currents isn't a problem but the shower circuit will go through one knockout. The outside power and utility room through another. Upstairs light goes through the same as downstairs lights. Upstairs rings on it owns. If they were bunched occasionally through a joist would that not cause eddy currents?

You don't need to worry about eddy currents. Think of something else to worry about!

 

[michaelsf90]

Tim I worry about everything haha and want the job to be right. Domestic install is a different ballgame. Especially when trying to make as least mess as possible! I need to check the derating factor in the regs but in such a large trunking I can't see their being an issue. In the trunking it would only be the shower, boiler, lighting and upstairs ring circuit. I just need to figure out where to fix all those cables under the floor