Size of cable to shed?

[B Wilde]

Hi, I’d appreciate any insight on the following matter.

I’m having 5 dual gang sockets installed in a relatively large tool shed. I’m not planning on any large loads, namely a some battery chargers, LED lighting and a powered table saw. I would estimate the maximum load to never exceed 20A. The cable length from the CU is about 40m.

An electrician has quoted some expensive 16mm2 SWA for the job but This seems like an awfully large cable for modest load, even with the distance, am I wrong? I had envisaged 6mm max would do the job but wondered if there might be a regulation requiring this due to the number of sockets or cable run?

I’d be very grateful for an expert view.

regards
Ben.

 

[sxturbo]

You probably could get away with 4mm, according to calculators, I'd personally advise 6-10mm, although as time goes on it is likely the loading will increase and therefore the sparky is overspeccing to cover eventualities. I imagine the spark has seen or done a lot of na I'll never have more than 20a loads and then seen them with 32amp plus later on.

https://www.clevelandcable.com/cable-calculator/

 

[RF Lighting]

6.0mm² is really pushing it bearing in mind that things like a table saw will have an inrush current much higher than its rated current when running and an undersized cable could cause the saw to not start, or the voltage to fall low enough that the lights go out for a second or two while the saw gets up to speed.

Also be aware that the stroboscopic effect produced by some LED lighting may cause the blade of the table saw to appear stationary when it is in fact spinning which must be avoided.

 

[Aragorn84]

i did 10mm to my workshop in the old house, and bought 16 for the new place (though it will be running an EV charger in the new place as well)

The cable cost difference is usually fairly small, especially when taken into context of the whole job, including potentially digging up the garden etc.

Have you actually priced the difference between 6, 10 and 16mm?

Perhaps agreeing a compromise to 10 would be acceptable?

 

[charliegolf]

I ran 40m of 4mm swa to my shed, via a spur on the downstairs ring without any problems at all. I have a table saw, router table and hand power tools; with 3 sockets and a strip light. I only have 2 hands, so only one of those things is in use at one time. I don't run a heater in there- that might make a difference, I suppose.

 

[oldbutnotdead]

All the above, especially the one about headline cost of cable looks high but fades into insignificance compared with the cost of digging. You will thank your electrician in a few years time when someone decides a hot tub in the back garden would be just the thing. Well worth shopping round for the cable- think I saved about £90 on a 35m run of 10mm last one I did & that was from a proper supplier (not chinesium).

 

[JohnW2]

The cable cost difference is usually fairly small, especially when taken into context of the whole job, including potentially digging up the garden etc. .... Have you actually priced the difference between 6, 10 and 16mm?

I'm inclined to agree. At TLC prices (excluding VAT)for 50m ...

2-core: 6mm² = £99.00, 10mm² = £132.00, 16mm² = £230.50
3-core: 6mm² = £128.00, 10mm² = £209.50, 16mm² = £319.50

... so, I agree that in terms of the big picture of the entire job, the marginal difference in prices of different size cables is probably not a major consideration (particularly if digging it all up and replacing it with larger cable might conceivably be necessary in the future if one uses 'small' cable).

Kind Regards, John

 

[oldbutnotdead]

If the budget is really tight, run a duct in your trench for power (as well as another duct for alarm, network etc), then if you do need to upgrade the cable its just a case of pulling a new one through & reterminating.