shed electrics


Postby treelow » Sat Aug 23, 2008 8:50 pm

I have a set of three garden sheds all in a row all of which have a light in each, I want to put a 13 amp twin socket in the end shed but I have been told that I must run a new armoured cable with a separate rcd from the fuse box. The sheds at the moment are fed by 1.5mm twin and earth cable, which goes to a 16A mcb on my fuse box, over a distance of not more than 20 mts. Surely just placing a rcd fused spur just after the fuse box would serfice?
treelow
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Postby ericmark » Sun Aug 24, 2008 5:08 pm

For 20 meters at 16 amp volt drop alone requires 2.5mm² which would still give a volt drop of 5.76 volts you are allowed 6.9 volts where it feeds lighting. Twin and earth cable should not be in sun light as the UV rays degrade it. And anything feeding something in a garden has required RCD protection for years. Black (Not as affected by UV) aerial cable may be able to feed the shed but there is not enough information to know if it could be used in your case. Issue 16 Autumn 2005 Electrical installations outdoors: a supply to a detached outbuilding see http://www2.theiet.org/Publish/WireRegs ... /index.cfm does a full report on how to wire a shed remember since this was published there has been a completely new set of wiring regulations published but I don’t think it really affects what is said to any great extent and of course in a garden is special location so Part P will be required see the projects section.
Also you may want to use an active rather than passive 30ma RCD.
Eric
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Postby treelow » Sun Aug 24, 2008 8:23 pm

Thanks for the reply.
I am a bit confused though with the wireing calculation. According to a 17th edition wiring calculator, for a 20m run, clipped to wall, drawing 4KW. it states a 1.5mm cable is suitable! Surely if I use an RCD spur fused at 13A, this would comply?
treelow
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Joined: Sat Aug 23, 2008 8:38 pm


Postby ericmark » Mon Aug 25, 2008 6:41 am

4Kw = 17.39Amp
Cable depends on reference method but yes method C clipped direct does give 20Amp that’s if it does that all the way. Voltdrop 29 mV/A/m giving 10.08 volt drop 3% of 230 max allowed for lighting = 6.9 volts since I don’t know Ze I can’t calculate Zs but assuming at origin 0.35 then with 1mm² earth and 1.5mm² live conductor that’s an R1 + R2 or 1.02Ω giving Zs of 1.37Ω max should be 2.3Ω on 20 amp MCB so does pass. But you are close and using a spur you must include volt drop to spur and find Zs of spur and add 1.02Ω at which point you are most likely over the 2.3Ω. If a registered electrician under Part P was to do the job he would need to measure Zs and enter the results on minor works cert as well as Zs you also have the prospective fault current which must be enough to open the MCB within required time so for a B20 MCB needs to be 5 x 20 = 100amp not including the wiring to spur you calculate at 108amp so likely will fail. Plus how you can draw 20 amp from spur as normal 13amp fuse which will not give you your 17.39amp to start with? I think you need to re-calculate what you are doing.
Eric
ericmark


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