Shed wiring

[kmb27560]

Hello,
I'm just about to wire up my garden shed and was after some advice on the cable size in relation to the distance from the house.
So far I have installed 16 x 6' HF fluorescent tubes for the lights, and a ring main that will use a variety of domestic tools, probably nothing greater than 3KW at any one time.
I plan to bury the SWA cable from the house to the shed, which is a distance of 75 metres.
Can anyone help with advice on the ideal cable size to use please?
Cheers,
Kevin.

 

[PrenticeBoyofDerry]

How big's the shed? 16 number 6ft fluorescent tubes seems excessive!
I assume they are rated at 70 Watt?
If we design the installation, so you have a 32A socket circuit offering about 7.4kW of power and a 6A lighting circuit, offer 1.38kW of power, then with voltage drop you would need 10mm SWA

 

[Sherbertlion]

16mm2 3c, 32A supply.

 

[kmb27560]

It is a big shed. The floor is 900 SqFt.

 

[PrenticeBoyofDerry]

It is a big shed. The floor is 900 SqFt.

FFS! mine has enough room for my bikes(2) and me to hide (just)
That is a big shed you may also need to consider volt drop of cable lengths within the shed as well!

 

[kmb27560]

Haha yeah, I can get totally lost in there sometimes!
I figured that I'd be looking at 10mm.
Thanks for the advice.

 

[PrenticeBoyofDerry]

If the total load of the shed exceeds 9.2kW you will then need to consider 16mm SWA, so depending whether you consider one radial or RFC circuit enough? Would start me thinking of a higher rated distribution cable.

 

[JohnW2]

If we design the installation, so you have a 32A socket circuit offering about 7.4kW of power and a 6A lighting circuit, offer 1.38kW of power, then with voltage drop you would need 10mm SWA

Is there something wrong with my arithmetic? If one designed for that load (38A), then I make the VD with 10mm² cable 12.54V, far too much for lighting. Indeed, even 16mm² would seemingly still be a bit over the maximum VD for lighting, at 7.98V - so one would actually need 25mm².

However, if one designed on the basis of the OP's stated max power load of 3kW (say 13A) and added on 6A for lighting, 10mm² would just do, with a VD which I make 6.27V (maximum permissible is, of course, 6.9V).

However, as I said, has my arithmetic gone wrong?

Kind Regards, John

 

[kmb27560]

I don't think that I'd ever be using more than 4kw at any one time really. Just 1kw for the lights, and the rest will be domestic tools, most of which are no more than 1kw.
The heating is taken care of by a log burner, rather than an expensive juicy electric heater.

 

[JohnW2]

I don't think that I'd ever be using more than 4kw at any one time really. Just 1kw for the lights, and the rest will be domestic tools, most of which are no more than 1kw.

Fair enough - ion which case, as I said, 10mm² cable would just do, by the skin of it's teeth. However, I personally would never go that close to the line (with something that involved an awful lot of digging!), and therefore would build in at least some 'future-proofing' by using at least 16mm².

Kind Regards, John

 

[kmb27560]

My 75 metre length was down to making several detours to keep the cable next to the perimeter. But I could cut that in half if I went the direct route.
Would that help with the size of the cable?

 

[Sherbertlion]

16mm2 allows for a 2% VD on 75m allowing room for the final circuits.

If you cut the length of run in half you will be fine with a 10mm2 SWA for a 32A circuit.

If you are exporting PME down there then be aware of any bonding you may need to do hence using a 3c cable.

 

[kmb27560]

Thanks very much for all the advice on this one. It's the first time I've asked a question on this site and the help has been really useful.
Cheers,
Kevin.

 

[Taylortwocities]

The length of the cable run is measured from the mains intake (ie the meter)

Not sure if that makes a difference in your calcs?

 

[PrenticeBoyofDerry]

If we design the installation, so you have a 32A socket circuit offering about 7.4kW of power and a 6A lighting circuit, offer 1.38kW of power, then with voltage drop you would need 10mm SWA

Is there something wrong with my arithmetic? If one designed for that load (38A), then I make the VD with 10mm² cable 12.54V, far too much for lighting. Indeed, even 16mm² would seemingly still be a bit over the maximum VD for lighting, at 7.98V - so one would actually need 25mm².

Surely the origin of the installation would be after the distribution cable at the shed CU?