sizing of swa underground supply to garage

How should I decide the size of wire I need to price up to run a supply from my house to a 60A consumer unit in the garage 75 metres away? It's not a D-I-Y job but I'd like to estimate the likely cost of materials.

AJH

Reply to
andrew
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The info you need is here:

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?title=Taking_electricity_outsideDo you need the full 60A capacity in the garage? If so the problem that will bite you is voltage drop at that length and current.

Although 10mm^2 can take the current, even 16mm^2 SWA with a drop of 2.8 mV/A/m that is 12.6V (i.e too much).

35mm^2 might be a realistic minimum with a drop of 1.25 mV/A/m or 5.6V at that length and current. Given 35mm would be way in excess of your current carrying requirement, you can probably ignore the reactive power factor effects one would normally include in calcs for cables over 16mm^2.
Reply to
John Rumm

Here's a calculator

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decent prices for SWA to. . .

Reply to
nicknoxx

Indeed, TLC are usually pretty good. Although again they don't list larger SWA sizes on the web site. They may be able to special order them though.

Reply to
John Rumm

Thanks both of you and yes the TLC calculator only seems to go to 16mm2 swa. It looks like the cable alone will be over 500quid so I won't be rushing forward. My understanding was that if the consumer unit is rated at 60A the cable must be similarly rated even though the biggest single load is likely to be a 20A 1ph welder. 35mm2 seems to come as 4 core which is a bit wasteful for this not to mention expensive.

I'll have to measure the diameter of the incoming main to the yard, I'm told it is aluminium but the voltage drop from the transformer across the road some 200m from the house must be significant, EDF pay for this loss though.

AJH

Reply to
andrew

No the cable does not need to be sized according to the CU, but to the load & distance etc.

For a 20A 1ph welder I would redo the calculations for a 20A load & 5% voltage drop (power on the TLC calculator). I suspect 10mm SWA would be ok at about =A3150-175 if you shop around - but It is still going to cost a fair bit, but then 75m of H07RNF 10mm or 16mm would cost plenty too.

The supplier can alter transformer tappings to boost voltage so your voltage is within spec.

Reply to
js.b1

Nope, the rating of the consumer unit needs to match or exceed the load you plan to draw through it. If you only need it to supply 30A total, there is no harm in it being rated for more.

It sounds as if a 32A feed would be much more reasonable in terms of cost. The other option to consider is running more than one submain to the garage - say sticking the lighting on one (where you have least tolerance for voltage drop) and a larger one for power - and then you can use larger 5% drop allowance.

So for example if you allowed 6A for lighting, and 32A for sockets etc, that gives you 6.9V to play with on the lighting feed. Which would mean a 4.0mm SWA. The power could then have a voltage drop budget of 11.5V so a 10mm^2 SWA would squeak in with a volt to spare (in fact it would not be that bad, since you would be operating at well below maximum conductor temperature). Still not massively better on price but under £400

In fact, in this case a single 16mm^2 submain might be cheapest. Allow

32 for the main power circuit and a bit extra for lights. £333 for a 100m reel at TLC, and you could probably flog the offcut on ebay! ;-)

They may well be using 50mm^2 cable or similar.

Reply to
John Rumm

In message , John Rumm writes

snip...

I am 30 years out of date on this but Newey and Eyre used to offer a cutting service for SWA. You could try several suppliers. Huge discounts usually available on manufacturers list price.

Who's bothered about supply drop on an intermittent welding application anyway? Just whack up the tranny setting:-)

regards

>
Reply to
Tim Lamb

materials.

total,

11.5V so

under £400

yard, I'm told

EDF will not normally allow two supply cables into one building. When I uprated my barn supply from the previous single phase 2.5mm SWA to three phase 160 amps per phase they insisted on seeing both ends of the 2.5mm 'permanently isolated' before they would connect. I wanted to let it feed a small consumer unit with just a lighing circuit for emergancies.

AWEM

Reply to
Andrew Mawson

It could be worth shopping aroung for the cable.

Just comparing what I have paid recently with TLC prices

10mm 3 core SWA. TLC £3.63 + VAT per metre. I paid £2.52 + VAT per metre.

1.5mm 5 core SWA. TLC £50 + VAT for 50m (that is 4 core). I paid £43 inc VAT for 50m of 5 core.

A 16mm submain should be good for for a 45A supply on that distance.

Cheers

Reply to
ARWadsworth

TLC will do cut lengths as well, although there are price breaks at 50 and 100m.

well that's one option. Depends a bit on what else you plan to do in the garage.

Reply to
John Rumm

Yup, its probably worth phoning a few places since you want a reasonable length of it.

It would meet the 16th edition spec for voltage drop certainly.

Reply to
John Rumm

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