cable calcs


Postby chrisrr » Fri Feb 13, 2009 7:22 am

If anyone's around that can shed some light here I'd be grateful?
(and another question: why is 4mm cable not available anywhere? Very few places stock it - Screwfix dont and neither do B&Q.)

[quote="chrisrr"]Sparx, your posts on this subject are superb and have really helped this newbie a lot. Quick question about working out mcb's - we need to start with the design current, so in a lighting circuit we know will demand (for example) 3 lots of 60 watts is easy enough, but what do we do to work out design current for a ring or radial circuit with nothing plugged in? Surely that could range from zero to 1000's of watts?

Reason is that I am putting a cu (from another cu in the house) in a garage that will have just a ring or radial and a lighting circuit. Can you steer me in the direction of my calcs for mcb/cables. (eg 4mm2 to garage, then 2.5 single core to radial/ring but what mcb at both ends etc?)
I'd be really grateful.

[quote="sparx"]Sparkypaul, as i'm sure you know I was illustrating a point, in fact in my current area of work 16mm hard drawn bare overhead line is doing what I said! ( I can't say more).
ksddw2,
Been there .done that. used to work on weapons control/ navigation 'putors for RAF, programmed in mc.code & Basic.
worked on NIMROD Hawk, Jaguar Tornado etc, lost interest when these horrible PC's came on scene.
Got C&G 222 'Digital Logic Techniques' distinction in 1976!
Was system designer using 'latest' ttl logic gates & fluidics pneumatic logic which most people have never heard of.
Just an old leckie now, leave the clever stuff to my kids, one of whom is in IT. I'm happy doing Insp/test & getting paid by HM gov. dept to do leckie design for server rooms etc.
BW, Sparx[/quote][/quote]
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Postby sparx » Mon Feb 16, 2009 11:04 pm

Hi,
4mm is not popular as not often used until recently, I use quite a bit for radials in kitchens etc but buy from wholesalers not DIY sheds.
As you say the loading on power circuits is unknown which is why the IET set out guides on circuit areas in 'standard circuits', which should ensure no overloading,
E.G. 20A radial on 2.5mm for area up to 50sq. mtrs.
32A radial on 4.0mm for area up to 75sq. mtrs
32A ring on 2.5mm for area up to 100sq. mtrs.
So for your garage power 20A radial would do in 2.5mm.
32A mcb on supply end and use a 'garage unit' sold for the purpose which have 6A plus 16 or 20A for power and main switch for isolation, If house consumer unit not on RCD already then use a 32A/30mA rcbo to give full protection for cable.
Don't forget installing a new circuit is 'part-p' even if garage attached to house.
regards SPARX
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Postby chrisrr » Tue Feb 17, 2009 8:46 pm

Sparx, you're a day at the beach. Thank you very much indeed for this. Brilliant.
Re 4mm, Screwfix don't stock it - period, neither does B&Q, Wickes, Homebase etc etc. I even phoned Screwfix just in case I couldn't see it on the website, but nope - don't sell it. I'll continue the search amongst the smaller electrical wholesalers when I next get an hour away from the coalface.
Meanwhile, again, many thanks for the pearls.
Cheers
Chris

[quote="sparx"]Hi,
4mm is not popular as not often used until recently, I use quite a bit for radials in kitchens etc but buy from wholesalers not DIY sheds.
As you say the loading on power circuits is unknown which is why the IET set out guides on circuit areas in 'standard circuits', which should ensure no overloading,
E.G. 20A radial on 2.5mm for area up to 50sq. mtrs.
32A radial on 4.0mm for area up to 75sq. mtrs
32A ring on 2.5mm for area up to 100sq. mtrs.
So for your garage power 20A radial would do in 2.5mm.
32A mcb on supply end and use a 'garage unit' sold for the purpose which have 6A plus 16 or 20A for power and main switch for isolation, If house consumer unit not on RCD already then use a 32A/30mA rcbo to give full protection for cable.
Don't forget installing a new circuit is 'part-p' even if garage attached to house.
regards SPARX[/quote]
chrisrr
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Postby sparx » Tue Feb 17, 2009 10:48 pm

Hi chrissrr,
just a point, all the suppliers you mention are DIY 'sheds' not electrical wholesalers.
ANY/ALL proper wholesalers will stock 4.00mm, such as CEF, Newey&Eyre, Rexel-Senates, and many more listed in phonebook under 'electrical wholesalers' .
I never use any of the 'sheds' as too limited stock as you found & too expensive too,
regards SPARX
sparx
Rank: Project Manager
Posts: 2166
Joined: Wed Mar 21, 2007 8:33 pm
Location: The fifth continent.

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