Connecting twin and earth to SWA

1 Oct 2007
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United Kingdom
Hi all,

I've got power running to the garage but the previous owner achieved this by running 6mm T&E direct from the CU at the front of the house, under the floorboards of the first floor, out the back of the house and along some fence panels in conduit, to the garage. The conduit is screwed to the fence panels.

I want to make this safe. I don't want to do a whole new cable run of SWA from the CU as this will be a right pain so can I take the T&E into a waterproof junction box as soon as it comes outside (through the back of the box) and continue the run with SWA, from this junction box, down the house wall, clipped along the dwarf wall under the fence panels, and into the garage? And will 2.5mm SWA be okay for this purpose? The MCB for the garage, in the CU, is rated at 32A. The current run of 6mm T&E is about 20m, with about 8m running outside.

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You haven't said what the current load is in the garage. This is vital as 2.5mm may drop too many volts if the load is too big.
Please advise what your calculated demand is.

In general it would be better if you were to run 6mm SWA so the CSA is the same throughout.

The 32A MCB is OK for 6mm but too big for 2.5mm. If 2.5mm meets the VD requirements you would need to changethe MCB to a 20amp MCB (or 25A if you can find one for your consumer unit).
You say there is a 32A at the garage end. What is this protecting?

What fuse protection is there for the cable leaving the house?

What is the load on the cable?

Regardless of the 6 milli, the whole circuit can only carry the maximum permissible for the smaller cable.

You have to work out how much the total load is going to be on the garage cable, work out voltage drop on the circuit to make sure it is within limits and make sure the garage circuit cable is protected at the house supply end by a suitable overcurrent device and RCD.

That's before you worry about how to join the two cable types together!

Which I would do with a sturdy and roomy square PVC adaptable box. You take the cable into the back of this box, seal it well, and the SWA is glanded into the bottom with a CW gland. Use a Piranha locknut on the inside and assure good cpc continuity.
Sorry, the 32A MCB is in the CU in the house, not the garage. The CU was replaced a couple of years ago so all is RCD protected. There is an old cartridge type CU in the garage. Not sure what this is rated at without looking.

Load wise, the only two things in the garage is one strip light and one double socket, both run in 2.5mm T&E. That's all I need anyway. The sockets are to be used for typical power tools and lawn mower. It's not a workshop of any kind.

I'm more than happy to use 6mm SWA if that's what it needs.
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So use 6mm SWA, then you are giving yourself a good margin for future proofing. You may need to run a bigger load some time in the future, and you wont need to mess around with changing the MCB at the house end.

Connection is done as outlined by securespark. You'll need a weatherproof adaptable box at both ends of the SWA.

These are good for the job,
and you can use these
to provide the earth connection to the armour.

Note: the 308B box is described as 4mm² but will be fine for 2x6mm² which is what you will be terminating.
Many thanks for your help, all. Much appreciated.

To be honest I would probably just leave the existing cable as-is but attach it, within the existing conduit, to the dwarf wall you describe, ensuring the cable is not exposed at all along its route outside to the garage. There's nothing inherently wrong with T&E outside as long as it's adequately protected. Ensure you apply appropriate sealing though so the water doesn't sit or go in the conduit.

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