bringing armoured cable into building

Hi,

I need to replace the power feed to my garage. The garage is separate to the house - it's about 30 feet away across my back garden.

The existing power feed is a 2.5mm 3 core armoured cable spurred off one of the ring mains. The cable emerges from the wall at the side of the house (no box or anything, it just pokes into a hole in the wall) and goes into the side of the garage wall in the same way. The hole is drilled straight into the wall (ie perpendicular) so the existing cable has to stick out a bit and then bend back to meet the wall before it runs down into the ground. This looks rubbish.

I've got a new 6mm 3 core armoured cable that's getting its own feed back to the CU on its own 32A RCBO.

Now the question is, is it still acceptable practice to just drill a hole in the house wall and have the cable coming out of it, or is it better to put an IP rated box on the wall with a terminal block inside and then have a cable coming through the wall into the back of the box, sealed with a blind grommet and some sealant?

If I could just drill a hole in the wall (which I'd do diagonally upwards* at about 45 degrees to make it easier to keep the cable bend radius gentle without having to have a big loop poking out) it will make joining it onto the power feed inside much easier, as I can then do this inside and won't need to worry about IP ratings, I presume.

[*] by "diagonally upwards", that's with respect from drilling from outside in, so the cable will be going upwards as it enters the wall from outside.

I would always go for taking the cable directly inside if possible, and would always drill the hole at an angle.

My peferred method is through the wall with the SWA terminate in a metal KO box butted up to CU and feed inner cores through directly into CU.

Unless its a metal clad CU where you terminate SWA - which is rare in domestic premises.

Has the garage got it's own CU if so I'd use MCB to feed garage and fit RCD at garage CU.

No need for RCD protection of SWA feed to garage.

What's the main earthing in house? Ze value?

DESL said:

No need for RCD protection of SWA feed to garage.

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Unless its TT of course

That's good to know, thanks. Taking cable straight in is going to be easier to do of course. I will drill upwards at 45 degrees so that the cable more naturally sits flatter to the wall. It obviously has advantages when it rains, too.

I don't know the Ze value but the supply is TN-C-S so should be no worries with that.

There is a metal CU in the garage with a couple of circuits in it (the former owner used it for his amateur radio) and there's also a separate feed from this into my wife's potting shed for a radio, light and beer fridge

I've already got a 32A RCBO especially for the garage feed. The armoured cable will be entering the house at the back, though the main CU is in the front so there's an approx 14m cable run from the main CU to where the armoured cable exits the house at the back to then go on to the garage. For ease, I was hoping it would be OK to use regular 6mm T+E as the main cable run through the house (it'll need to weave a bit and go through existing joist holes) and then join this onto the armoured cable using a 45A connector sitting in a metal box. I can site this well inside the house so no moisture concerns. I've got the gland kit for the cable already and this fits perfectly into a 20mm knockout hole and gives me a good way to earth the armouring.

Does that sound reasonable?

What's maximum run of feed to garage?

Have you calculated volt drop?

10mm T&E would be better for the 14m run to the SWA as csa of cpc will match that of SWA. If you're exporting earth then you a low Z as possible.

I assume SWA/T&E joint will accessible and not under floorboards.

DESL said:

What's maximum run of feed to garage?

Have you calculated volt drop?

Click to expand...

Allowing for cable path, from the point of connecting to the T+E it will be about 15 metres, so let's say 30m total, some of which is T+E and the rest is armoured.

At 32A (the rating on the breaker) my calculations give a voltage drop of 7.1v allowing for the cables running at 70C. Aside from a 1.2 metre run from the CU, there's no cable grouping along the run and at no point does the cable come close to any insulation.

Also, in practice the load will just be a few lights and a radio with the occasional trickle charge for the car battery and an electric garage door.

DESL said:

10mm T&E would be better for the 14m run to the SWA as csa of cpc will match that of SWA. If you're exporting earth then you a low Z as possible.

Click to expand...

Damn, I've already got the 6mm! Anyway, even with the 6mm T+E, allowing for 14m T+E and 16M armoured, I've calculated the earth loop from the CU to be 0.29v with cables at 70C. If I'm allowed to assume a max Ze of 0.35 ohms (because I've got TN-C-S), then that should give a max total earth loop impedance of 0.645 ohms, giving me a 346 amp potential fault current, if I assume the supply of 223v (allowing for the 7 volt drop I mentioned above).

This should be plenty to trip the breaker within 0.4s, especially as I've already rounded up the cable distances a bit (in reality the total distance is probably going to be more like 26 metres). I'm going to replace the CU in the garage and will probably keep max breaker size down to 20A in there so the only reason the breaker in the main CU should trip is if there's a fault in the supply cable to the garage or in the garage's CU at the end of it.

DESL said:

I assume SWA/T&E joint will accessible and not under floorboards.

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I've never understood why such things have to be that accessible, seeing as builders etc are easily capable of lifting the floor if necessary. Why is it? Is it more a case of how likely they are to find it if it's not in a cupboard or something?

very_pointy_ears said:

DESL said:

I assume SWA/T&E joint will accessible and not under floorboards.

Click to expand...

I've never understood why such things have to be that accessible, seeing as builders etc are easily capable of lifting the floor if necessary. Why is it? Is it more a case of how likely they are to find it if it's not in a cupboard or something?

Click to expand...

What happens if the floor is tiled or covered with laminate flooring at some point in the future?

Hmm I see your point. So what do you do if there really isn't anywhere other than a ceiling or floor void in which to put the connector box?

There is always another way.

It is not always the easiest or cheapest way, but there WILL be away.


In this instance, the easiest way is to not have a joint at all.

Not even a puff?