SWA cable rating

[Pandora]

I am putting in a new underground SWA XLPE 2-core cable to the garage/workshop at the bottom of the garden.

The cable length from the consumer unit to the worshop is 16m inside the house then 48m outside the house (total 64m).

The load in the workshop will be protected by a RCD there. I anticipate that the worst case simultaneous load will be a drill, soldering iron, 2kW heater, desktop p.c./monitor. and some lighting.

What should the conductor size of the SWA cable be (6 sq.mm? 10 sq.mm? more?) to avoid excessive voltage drop?

The calculator at

10 sq.mm. for a 20A load

What rating MCB should be chosen to protect this cable from the consumer unit?

[Christian McArdle]

I'd say 10mm is OK. Don't even think about 6mm. You can use T&E inside and switch to SWA with a gland when you get to the outside wall.

20A for 10mm. 32A for 16mm.

Christian.

[Alex]

Just to add, earth the armour of the cable at the house end, but make sure it is isolated at the garage end. Then connect the garage end to an earth rod, testing for impendance. You probably already know this, but probably better said too many times than not enough!

Alex

[PJDesign]

Should that not be 3 core? I am pretty sure that you should not use the Armour as an earth. Baz

[Alex]

No need for an earth, you do not want to be using the house earth 48m down the garden!!

Alex

[Jb]

Using the armour IS permissible, just make sure your fault calcs are accurate. Jb

[Mike Tomlinson]

In article , Alex writes

Why not? (and why not export the house earth and supplement it with an earth rod at the garage for equipotential bonding?)

Not disagreeing - just want to understand the issues.

[Christian McArdle]

It is difficult to achieve the required earth fault loop impedence. It is extremely difficult to achieve main equipotential bonding if that is required. It is less safe in the event of damage or failure of the cable between the buildings.

Christian.

[T i m]

Going with the OP and not disagreeing etc but wouldn't having both ends earthed (by stakes at both ends etc) actually lower the impedence (resistances in parallel?)

It is

So you might get earth current flowing say from the house end to the earth at the garage end (or something else)?

It is less safe in the event of damage or failure of the cable

Being earthed at both ends 'less safe' ? (this must be true as you know what you are talking about but I just wondered if you would explain please Christian?)

All the best ..

T i m

[Christian McArdle]

An earth rod won't reliably produce an impedence low enough for TN earthing. Such earthing would have to be professionally installed and regularly maintained and checked. You'd still need RCD protection for the whole installation as per a TT.

I'm not sure why. I would be tempted not to isolate the earth myself (but still going with full TT precautions). However, when I suggested it once, someone suggested that this would not be the done thing. I can't remember exactly why.

Christian.

[Neil Jones]

impedence

Is it possible to have a fault on the supply, whereby the house 'earth' is at a much higher potential than the local earth, though the rod? If so I imagine that could be dangerous, and to be avoided.

Neil

[Christian McArdle]

That's the only thing I could think of. However, it doesn't quite stand up for two reasons.

a) If that happens, your shed is the least of your worries

b) The provision of the local earth rod may actually produce enough fault current to trip a protective device, actually improving safety.

Christian.

[Neil Jones]

Probably so. I was wondering whether such a fault would ever happen, in reality.

Neil