Cable size for future extension.

[Deluks]

Future proofing while the floor is up, by running SWA in UG ducting:

What would be a 'rule of thumb' sized cable to run 6m from existing CU to new CU in a potential kitchen extension on the back of the house?

 

[ban-all-sheds]

 

[holmslaw]

..

 

[Taylortwocities]

You size ducting/trunking etc according to the size of the cables.

Appendix 5 of the On-site guide tells all.

In summary, you should onlu populate trunking, ducting etc up to a max of 45% of the available cross sectional area. Also you'll need to apply grouping factors when doing cable calculations.

Not quite as easy as popping the cables in some 1½" waste pipe, is it?

 

[Deluks]

Just noticed your heading, 'insulation' makes a big difference, its usually better to include the heading wording in your post, to avoid any confusion.

I don't think the insulation is relevant here tbh. I'll elaborate, most of this is unlikely to be of much relevance to the original question, but will hopefully clear up why I asked it in the first place.

Background: I have some swa which is going to be run to the garage at some point in the not so near future. I'm going to run this under an internal concrete floor which is currently up. Will be going in 150mm utility ducting under the hardcore, and hopefully under the foundation at the back of the house.
One end will be coming up through the floor under the stairs beneath the fuse box, other end will be coiled up in a corner of the patio until I am ready to dig trench to the garage. (long way off yet)

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The house has never been extended from the original footprint, although many other houses in the street have done, and if funds/time became available it could easily happen and I'd like to prep for that eventuality, even if it means a little more work (hardly any) and expense (hardly any)

Future proofing:
If an extension was built, I'm thinking it would make the wiring up of the leccy easier if there was already a spare cable ready to connect up. This would mean the extension would have it's own direct feed from the CU, and would mean no disruption to existing circuits in the house. The cable can simply sit UG in the ducting, and dug out when the extension is being built, ready to connect.

What sized cable could be used to feed this extension? Max of about 15sqm (99% chance it will end up as a kitchen)
I would only need about 6 metres to get me from current CU to the proposed extension area.

 

[holmslaw]

..

 

[PrenticeBoyofDerry]

The circuits in a kitchen could well exceed 100Amps, depending on number of circuits and the output required for your cooker/oven.
If installing a SWA cable you'd be looking at 16mm2 to carry that sort of current.

 

[Deluks]

This is a hypothetical extension, but as kitchen's need more power than almost any other type of extension, then we'll call it a kitchen.
Like I said, future proofing.

Lay 6m of 16mm3c in the ducting. One end taped to the wall by the main house CU and the other end sitting UG in a duct. Come building of any extension, simply dig down, cut into the pipe to retrieve the end of the cable and bring it up into the extension ready for the spark to connect up each end.

Am I going about this the wrong or right way? (ie: how would you do it?)

 

[timtheenchanter]

IMO, the better option would be lay the large ducting, then put in a draw cord or two, you never now what the power demands might be in 5/10 years time.

this option would allow the relevant sized cable to be installed when the time comes.

 

[Deluks]

IMO, the better option would be lay the large ducting, then put in a draw cord or two, you never now what the power demands might be in 5/10 years time.

this option would allow the relevant sized cable to be installed when the time comes.

I had considered that, but the ducting will stop up against a sleeper wall, and it's a further 1.5m to the understairs fuse box. The 10mm cable I'm putting in for the garage will be passing through a hole about the size of half a brick. I foresee problems trying to pull another cable alongside that in the future.

 

[Chri5]

Why use SWA for what will end up (post build) as a internal sub main ?

16mm sq TE is easier to handle, easier to terminate (no banjo / swa armour to earth) and since the route will either be in floor void or sub floor buried in duct I don't see the need for the mech protection that swa affords.

 

[plugwash]

One good reason to use SWA is that it lets you place any RCD protection at the far end. This means you can split up the RCD protection between different circuits on the submain.

If you use T&E and any part of the run is concealed less than 50mm deep you have to protect the submain with a 39ma RCD.

 

[ban-all-sheds]

Only in walls.

 

[Chri5]

39ma RCD.

Typo ? 0.03 amp / 30 mA !

The sub main will require RCBO's or RCD based CU no matter what, even though the A-B cable route between the CU's shouldn't warrant the 'main' cable needing an rcd per BAS remark.

 

[Deluks]

Why use SWA for what will end up (post build) as a internal sub main ?

16mm sq TE is easier to handle, easier to terminate (no banjo / swa armour to earth) and since the route will either be in floor void or sub floor buried in duct I don't see the need for the mech protection that swa affords.

So would it be acceptable to use t&e about 600mm underground in a duct? Running under a foundation and through a sleeper wall?

It is also possible that the footprint of the ext could leave the duct running alongside the foundations on the exterior.

What about when the cable is removed from the duct to feed the new extension. where is the protection between duct and the surface. How can you tee off a duct pipe such as this? (150mm ribbed)