What size SWA?

[flubber33]

Hi folks. I have a friendly electrician mate who has a bit of a reputation for over engineering so I want to confirm the advice before I spend a small fortune.

I want to add a circuit to an existing (new) domestic CU which has a 100A supply. The CU currently powers a granny annex so the load is small. The new circuit will be for a garage 100m away. I need to run SWA underground across the garden.

From the new garage, I also need to then run a circuit to a greenhouse and games room, again underground across the garden which is a further 70m away.

The garage will likely only power basic diy tools, power washer, lighting circuit, electric car charger (requires own 20A circuit), nothing major, ie no arc welding! Games room may have a heater, power and lighting only so again, small ish load.

I asked for a specification that would suit and leave room to grow in the future.

My mate has specified a 32A supply from the annex CU to a 4way garage CU using 25mm SWA followed by a 16A supply from the garage to the games room again using 25mm SWA.

What I'd really like is a second opinion on this one as he originally specified 16mm SWA then changed his mind, so I'm felling a little unconvinced.

Can anyone help please? I've tried various calculators which seem to indicate 10mm or 16mm SWA would be fine, but I'm not totally clear on the calculator inputs so I don't want to get it wrong.

Can anyone categorically state what I need please?

Thanks very much

 

[PrenticeBoyofDerry]

what size heater?

 

[HERTS P&D]

How big is your garden?

Andy

 

[flameport]

10mm is a total bust.

16mm might do as an absolute minimum provided you ignore volt drop requirements for lighting, the terminal load is small and you have made various unrealistic allowances for what items will be in use at the same time.

25mm is probably the minimum size to comply with BS7671.

35mm or larger is likely to be the realistic option to both comply and provide a realistic capacity for the various loads.

There are also the practical and financial considerations for such a length of cable - 170m of 2 core 35mm will weigh around 250kg and cost well over £1000. Digging a trench of that length will not be cheap either.

 

[ericmark]

At a 170 meters with 16A load and 16mm² cable you will just get away with volt drop for lighting.

So the point is what is the design current. I would say your friend is not far out but always a bit of guess work as no one knows the design current.

 

[cam1989]

Where is the main incomer in relation to the granny annex?

I'm thinking that the annex will already be fed from the main house CU, and if so you need to include the volt drop on that part of the run.

Or am I reading it wrong and you're taking the feed from the main house CU?

 

[flubber33]

Thanks Folks.

Andy - 5 acres!

Looks like 25mm is appropriate then. The Annex has own independent supply, not fed from main house CU. I'm going to hire a chain trencher for the day which will cut me a 600mm deep trench the whole way for maybe £200 for the day - not doing all that by hand!

Might look at 35mm to future proof if incremental cost not too big.

Thanks for your help.

 

[bernardgreen]

Don't forget to also lay in a duct for ELV services ( phone, intercom, alarms, etc etc )

Unless you know 100% for sure there are no pipes, cables etc on the planned route a cable locator might be sensible.

 

[RF Lighting]

I work it out as requiring 35mm² for the first leg. You should be able to drop to 25mm² for the second leg.

 

[flubber33]

I work it out as requiring 35mm² for the first leg. You should be able to drop to 25mm² for the second leg.

Hi RF Lighting - interestingly, my friend initially worked out the cable size for each leg separately but a friend of his then said you can't break the legs out like that, you have to work out the total distance voltage drop to calculate the cable size for the whole route.


Thanks bernardgreen - I am going to lay 3 ducts, one for the SWA, one for ELV and one spare. The 'garden' was a paddick in the countryside until I received planning permission to use as a garden last year - 99.9% certain there are no services underneath, but I'll hire a cable finder just to be sure.

 

[Lectrician]

Hi RF Lighting - interestingly, my friend initially worked out the cable size for each leg separately but a friend of his then said you can't break the legs out like that, you have to work out the total distance voltage drop to calculate the cable size for the whole route.

That's nonsense. You always break down each sub-main run.

If what his friend says was correct, the socket and lighting circuits in the sheds would need to be done in 35mm too! Lol.

 

[bernardgreen]

If what his friend says was correct, the socket and lighting circuits in the sheds would need to be done in 35mm too! Lol.

Remind me, what CSA is the cable in the street as that would be the CSA needed for all the wiring in the house