Power feed to a shed

[wazzag]

Hi all,
I plan to run power to a large shed at the bottom of my garden. I have a CU in a concrete outhouse/office next to the house from which I plan to run SWA to the shed. The shed is about 25 metres from the cu. in the shed I want to power lights, some workbench tools such as a grinder, small drill etc. maybe a small fridge in the future.
What size SWA do I need?
I assume I need an rcd on the feeding CU, what rating does this need to be?
Should I go for a small CU in the shed and what should it contain?

Many thanks!

 

[ericmark]

Length of piece of string comes into mind. OK I know cables length but although in general 6mm² is normally ample to cater for lights where maximum volt drop is 3% and power which could rise to 32A even a 10 mm² sq cable may not be enough.
6 mm² volt drop per amp per meter is 7.3 mV
10 mm² = 4.4
16 mm² = 2.8
Quick calculation I get 37 amp max with 6 mm² at 25 meters but you must clearly include all cable within the shed and house as well as the SWA run.

 

[riveralt]

Length of piece of string comes into mind. OK I know cables length but although in general 6mm sq is normally ample to cater for lights where maximum volt drop is 3% and power which could rise to 32A even a 10 mm sq cable may not be enough.
6 mm volt drop per amp per meter is 7.3 mV
10 mm = 4.4
16 mm = 2.8
Quick calculation I get 37 amp max with 6 mm sq at 25 meters but you must clearly include all cable within the shed and house as well as the SWA run.

Psst Eric, I found the ² under the special characters, below the smillies on the reply page.
BTW I only found it after being prodded by BAS, still got the bruise to prove it.

 

[wazzag]

I reckon the shed wont ever consume more than 20v at any one time. Will 6mm be ok at 25 m?
What rcd setup will I need?

 

[ericmark]

Sure you mean 20A and I would if my shed use 6mm² SWA but I would use lighting not too dependent of volt drop. Likely a HF florescent more expensive than standard but as well as larger voltage variation also better with rotating machines with stroboscopic effect.

As to RCD position this has been debated a lot. Clearly if working in shed handy to have RCD in the shed but where freezer in the shed easier to reset from house without having to walk out to shed.

The loop impedance is important for a passive RCD to work but not for an active RCD. So unless you can measure the loop impedance then passive in house and active in shed is way to go which means if there is a power cut the shed power will not auto restore when an active is used.

If you use a TT system then double pole RCD is required if using the TN supply the single pole is OK.

As to TT with earth rod or TN much depends on other buildings not necessary owned by you. Where there are houses beyond your shed then TN likely OK. But where fields beyond the like will need TT.

Problem with TT for DIY is you need to test earth rod so likely importing TN will be a better option assuming house is already TN.

I would of course test with low ohm meter, Insulation meter, RCD meter, and loop meter but likely you will not which means you need to be more careful and adopt methods I would not to allow for lack of testing.

I should say don't DIY but I know you will so trying to suggest method with least risk.

 

[ban-all-sheds]

What size SWA do I need?

• 
  
  
  
• //www.diynot.com/wiki/electrics:books
  
  
• //www.diynot.com/wiki/electrics:mcbtocable
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm (all subsections)
  
  
• http://www.tlc-direct.co.uk/Book/4.3.11.htm
  
  
• 
  
  http://web.archive.org/web/20070327014113/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
  
  
• http://www.tlc-direct.co.uk/Book/5.3.1.htm
  
  
• //www.diynot.com/wiki/electrics:mcb2
  
  
• http://www.tlc-direct.co.uk/Book/7.13.3.htm
  
  
• http://www.tlc-direct.co.uk/Book/8.1.1.htm
  
  
• //www.diynot.com/wiki/electrics:swa-gland
  
  
• http://electrical.theiet.org/wiring-matters/16/elect-inst-outdoors.cfm?type=pdf
  
  
• //www.diynot.com/wiki/electrics:part-p
  

 

[wazzag]

Thanks Eric. I think I'll go for 6mm with a small cu in the shed. MCB in the house and two passive (lights and fridge in future) and rcd for tools.
I'll also look into the lights you suggest.

 

[wazzag]

Thanks ban-all-sheds, ill get reading

 

[Taylortwocities]

This will be a new circuit from teh consumer unit.

So you'll need to set up a Building regulations application with your LABC, and pay thier fee. BEFORE YOU START WORK.
They will then inspect the job at various stages, and will expect you to test the circuits in accordance with BS7671. You'll do this with all that calibrated test equipment that you have at home.

Alternatively, you could have a registered electrician in to do the work. it will probably be cheaper.

 

[wazzag]

Thanks everyone, sounds like the best route is to get an electrician to do it.
Out of personal interest now that I've read a bit, can this job use 2 core swa (the armour being used as earth) or must it be 3 core?

 

[wazzag]

Thanks everyone, sounds like the best route is to get an electrician to do it.
Out of personal interest now that I've read a bit, can this job use 2 core swa (the armour being used as earth) or must it be 3 core?

 

[Taylortwocities]

That's why you need a competent electrician,

There are a number of factors which may determine if the armour can, or cannot be used to export the earth from the source building to the outbuilding.

 

[JohnW2]

There are a number of factors which may determine if the armour can, or cannot be used to export the earth from the source building to the outbuilding.

There are, indeed - but that doesn't directly impact on the question asked as to whether 2- or 3-core SWA would be used - particularly if the decision were that the earth could be exported (I accept that it would be a bit daft using 3-core if the earth were not being exported).

Kind Regards, John

 

[ericmark]

It does not have to be 3 core. But where two core is used and the armour is used for earth then the cable needs to be tested to ensure the plastic coating is not damaged as even a small hole can cause the steel to corrode and so lose the earth. So it is common to use belt and braces so if the SWA does fail the earth is not lost. On long runs it also helps keep the ELI down.

6mm² 3 core is a very common cable size. Where I worked we always had a role as it can be used for so many jobs. Because of it being so common we also found it tended to be cheaper than other sizes as so much more is made. And easier to source when you want short lengths.

 

[wazzag]

Excellent, thanks ericmark!