2.5mm or 6mm SWA to feed garden room/workshop

12 Oct 2018
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United Kingdom
I'm nearing the end of my garden room build and planning on laying the SWA this weekend. Have got an electrician lined up to do the checks who had suggested 6mm SWA.

The distance from the house CU to garden room CU is about 15m so not a huge distance and dropping some numbers into a calculator here suggest that 2.5mm would be Ok.

The plan was to hook the SWA up to a 32A MCB on the household CU and run a 32A ringmain and 6A lighting circuit in the garden room. No really heavy machinery, probably total 6kw max which is roughly 26A.

I'm trying to do this fairly cost efficiently so don't want the 'belt and braces' of a 6mm SWA if I don't need it. Do you think I'd be ok with 2.5mm?
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I doubt theres much difference in cost, in which case go for the bigger

In 2 core theres about 70p/m difference in price.
If you want a 32A circuit in the workshop (ring not necessary) then you cannot use 2.5mm² swa.
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Now you have mentioned 1.5mm.
That ain’t gonna hack it.

Your electrician has determined the cable size to be 6mm
Your electrician has to certify that he DESIGNED the installation.

That’s what you install.
Screwfix do 4mm so I think this makes sense. 6mm double the cost of 1.5.

The plan was to hook the SWA up to a 32A MCB on the household CU

Its not quite so simple. The cable must have a higher rating than the circuit protection.

I understand you want to spend the minimum, but futureproofing with a bigger cable than your immediate needs is only an extra £20 -its hardly worth thinking about.

As TTC says your electrician needs to design the whole circuit with consideration to earthing, circuit protection etc etc so best just let him decide.
Has your electrician actually designed the circuit, or has he just said to put 6mm in because he couldn't be bothered to calculate the cable size required so just went overkill? On the other hand & as already said, is it really worth the hassle over saving 20 quid (25m of 6mm vs 4mm SWA)?? If you think it is, then speak to your electrician as 4mm should be ample.
Do you think I'd be ok with 2.5mm?

What type of supply earthing is in the house, and are there any extraneous conductive parts in the garden room?

Is there an RCD in the house consumer unit, and if there is are you intending to use it for the new circuit or have the RCD in the garden room instead?

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