Swa for garage

[squarenut9]

yes and no..

you can either use a bit of flexible conduit or something similar ( blue water pipe etc.. or you can bury it in the concrete.. ..

you'll need an inspection pit outside and an earth rod.. best your sparky gives you the details.. liasing with him now before the build starts is a good idea to see what he wants done..

Sorry I forgot to mention the earth pit. I dont do much domestic installation nowadays. Its all part of the wiring regulations. If you are doing the job yourself ensure that the garage end of the SWA is insulated (assuming you are PME) and nothing can come into contact with it. Dont even attach it to your wiring enclosure, or C/U if they are metalic. The enclosures should be of PVC form anyway if my memory serves me correct. I used to put heatshrink sleeve over the armour where it was cut off to expose the inner flex, some would use insulation tape. I think the earth cable between the garage c/u and earth pit is a minimum of 10mm but for mechanical reasons it is best to use 16mm earth cable. Any how the RCD test has to be carried out on the installation and you will need a loop impedance meter and an RCD tester so unless you are a qualified bod you won't have these items. Make sure your sparky gives you some form of test certificate because this work is notifiable and when you come to sell the house this peice of paper will be required. Not that anybody looking at it will understand it of course..lol

 

[RF Lighting]

It's easier just to export the earth surely?

There is no extraneous metal work in the garage, but the SWA is big enough anyway.

Not that it is needed, but 2.5mm² would be big enough from the rod to the MET if mechanically protected. 4.0mm² if not.

 

[pippo]

Jesus its 4am lol

From some research I think the house has a TN-S earth system as the earth comes from the sheathing on the service cable. Will this make a difference to whether an earth stake is required for the garage? Also, should I still buy 2 core swa or will the earth be required in the cable too ?

Pippo

 

[RF Lighting]

You do not need to install an earth stake. This is an old habit some people have, but it is not required.

You would be better installing a 3 core SWA. If you install 10.0mm² or 16.0mm² 2 core XLPE SWA, the CSA of the armouring is not big enough to comply with table 54.7 in BS7671.

If you install PVC SWA, it is on the limit and would be better to see 3 core installed.

 

[pippo]

ok thanks RF

 

[mfarrow]

If you install 10.0mm² or 16.0mm² 2 core XLPE SWA, the CSA of the armouring is not big enough to comply with table 54.7 in BS7671.

Isn't the armour coated steel, protected by a sheath?

 

[RF Lighting]

I'm not sure what you're getting at?

Steel has a higher resistance than copper, so despite having a larger CSA, it has a lower k value.

 

[ban-all-sheds]

you can either use a bit of flexible conduit or something similar ( blue water pipe etc.

Not really.

 

[mfarrow]

I'm not sure what you're getting at?

Sorry, I had the 16th in front of me so was looking at the wrong table.

543.1.4 relates to Table 54.7 as an alternative only to the calculation in 543.1.3.

Assuming we have nothing more than a 100A BS1361 cutout, the I²t value total at 240V is 57,300 A²s (Busman data). So even if we have a fault close to the supply origin, the minimum size of protective conductor we require shall be

S = sqrt(57300) / 46 = 5.2mm²

Therefore the armouring will be adequate as a cpc.

EDIT: ... I mean to satisfy that particular regulation. You still need to make sure you satisfy all the other regulations (which means at least 16mm² of armour [which you'll have], before calculation).

 

[RF Lighting]

What temperature will the CPC of the cable rise to under those fault conditions?

 

[mfarrow]

What temperature will the CPC of the cable rise to under those fault conditions?

In an insulated environment, about 8000 degrees Kelvin

OK so I've not quite grasped the whole let through energy thing, and I probably won't any time soon. Also unless we know this guys Ze value, length of tails etc. and the type of protective device he's planning to use, it would be pointless to work out anything else.

The nearest analogy I can make is with Twin & Earth. Compared with T&E the specific heat capacity per meter of the armouring on a 16mm² 2c cable is far greater than that of the 6mm² cpc in T&E. So if we can use the cpc in T&E safely without further calculation regarding temperature increase in domestic installations, then I would safely presume the armour would be OK as a cpc.

 

[RF Lighting]

But the CPC in a T&E cable is copper, so it has a lower resistance, and will not warm up as quickly as the STEEL armour.

We can not ASSuME that it will be ok.

There is a handy table in the wiki which shows when the armour is suitable and when it is not suitable for use as a CPC.