cable calcs


Postby maveric » Wed Jan 21, 2009 5:15 pm

:? could anyone one tell me in easy to understand terms how to calculate the cable and breaker size , including cf's and volt drop, i have a copy of BS7671 IEE Wiring Regs but i am struggling to get my head around the long calculation to reach Zs, fault current and minimum csa of cpc .
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Postby singer » Thu Jan 22, 2009 12:41 pm

Having copy of BS 7671 is commendable but not knowing how to use it shows you have insufficient knowledge to be able to design, install, test, inspect & certify anything you are contemplating.

Best to call in someone who can!!!
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Postby kbrownie » Thu Jan 22, 2009 1:14 pm

Ill try help it's not that easy to explain unless you already have a little knowledge of the calcs and the fact you are asking this question I am assuming you do?
It will be handy to have both BS7671 requirements (regs) and On-Site Guide (OSG)
Cable sizes
Step 1; Find design current (ib) using power divided by voltage (p/u=ib)
Step 2; Find protective device rating (in) page 49 regs table 41-3. in (fuse/mcb rating) must be greater than or equal to ib (design current)
Step 3; Find tabulated current (it) using the calculation in (fuse/mcb rating) divided by (Ca x Ci x Cg x Cc) Page 125 OSG
Step 4; Select cable size (page274 regs table 4D1A onwards)

Voltage drop
Page 124 OSG and 274 regs,
((mv/A/m) x ib x L)/1000 = milli volts per ampere per meter x design current x length in meters of cable and divide that by 1000.
voltage drop must not exceed nominal voltage by 3% in lighting circuits and 5% in power circuits.

So my question now would be you have 12 lights on a circuit all rated at 100 watts in a domestic property no grouping of cables or thermal insulation and the temp is a constant 30 degrees (230v), calculate design, fuse rating, cable size, volt drop.
KB
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Postby kbrownie » Thu Jan 22, 2009 3:42 pm

Sorry forgot, clipped direct and 25 meter run.
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Postby maveric » Fri Jan 23, 2009 8:12 pm

[quote="singer"]Having copy of BS 7671 is commendable but not knowing how to use it shows you have insufficient knowledge to be able to design, install, test, inspect & certify anything you are contemplating.

Best to call in someone who can!!![/quote] At what point did i say i was going to attempt anything ? i asked for help with the calculation,
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Postby kbrownie » Sat Jan 24, 2009 8:57 am

Hi maveric,
I hope you can appreciate that, when working with electrics there are certain dangers that can effect both lives and property.
singers comments may not have sounded that helpful, sometimes it is hard to intemperate what some questions on this forum are leading to next. I for instance sometime don't answer post if I think that the question has been made by someone with little knowledge. Then again if I think someone is going to put themselves and others lives and property in danger. I feel I should make them aware of the dangers that are posible. Even though I have not have been helpful with that particular post.
I will say I have seen some of singers replies within this forum and he is very astute and knows his stuff.
We are here to help, even if at times it seems that we have not, the key is safety.
I assumed by your post that you wanted to learn about cable calcs. others may have thought differently. I've been there, and know that sometimes it's hard to get your head around them, and like you I was not using them to do an install, just to learn.
We are are not mean, unhelpful souls. We offer advise for free and safety is paramount.
I will be only to pleased to offer any help you may need on calculation if I can, will also recommend some not too expensive books that can give great guidance.
KB
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Postby sparx » Sun Jan 25, 2009 1:01 pm

Hi Maveric,
If I understand the situation this is part of your 'self training'?
On that basis I will try to answer any calcs. you are struggling with in a basic way, you can then build on them and repost further stages.
Lets start with a radial circuit from Dist. board to a single load, say a 6kW heater of some kind, using twin & earth surface clipped cable.
The rating of the MCB will be quite simply the smallest that will run the load without tripping.
So first convert kWatts to AMPS eg 6000w/230v=26.09A,
nearest available MCB is 32A, (not exactly close protection, but as good as we can get).
now by using table 4D5a we need a rating from column C to carry our load of 26 amps, it would seem from this a 2.5 mm2 might do.
Unfortunately 2 reasons this won't comply, first the MCB won't protect the cable until it's over loaded by at least 6 Amps (32-26A).
second will be volt drop problem which comes next in calcs.
So going up a size to 4mm2 rated at 37 A gives a safe 5A margin.
Using volt drop column at end of table gives 11mV per amp per meter of circuit so if we say 25 mtrs from dist. bd to heater then we get:
26A X 25M X 11mV divided by 1000 to get volts from millivolts gives a volt drop total of 7.15volts.
since max value V-D allowed for non lighting is from Appendix 12 page 358 is 5% of 230 = 11.5V circuit complies so far!
Will come back with Zs etc. later today as computor required by others!!
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Postby sparx » Sun Jan 25, 2009 2:41 pm

Hi back again,
Zs = Ze (measured between L-E at intake terms.) + (R1+R2).of circuit in question.
say Ze = 0.18 ohms
R1+R2 either measure by shorting Line To CPC at one end and read at other direct on ohms scale or take from tables in various guides.
EG. ECA guide to wiring regs gives 4mm2 = 4.61 ohms per 1000m divided by our 25m run= 0.115 ohms for copper at 20Deg.C. as our R1+R2 added to Ze(external) = 0.18+0.115 =0.181 ohms
which is our circuit's Zs(system).
Next look at tables hidden away around page 49 of regs for the max allowable Zs for our circuit.
take top table for type B mcb or RCBO we are allowed up to 1.44 ohms at 'normal working temp' since we can't test at that we allow a derating based on 80% giving 1.15 ohms which is still way above our reading so still good to go!
In fact type C or even D would be OK but would not choose them unless instead of heating we had say a motor or discharge lighting load requiring a high start up current.
Not yet taken into account cable routing which may be through a higher temperature area or be grouped with other cables or have part of its route through insulation materials, all of which would mean derating factors from more tables being applied!
Nothing to this "sparkin' lark", dunno what the fuss is about any ol' builder knows what cable/MCB to use !!!!
please post back if you still want to go on....
regards SPARX
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Postby sparx » Wed Jan 28, 2009 9:01 pm

still active topic
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Postby ksddw2 » Wed Jan 28, 2009 10:21 pm

Sparx, Saw your excellent posts about cable calcs (thanks), but you mentioned that the routing of cables affects the calcs, which raises a quick question.

I am tidying up the wiring in my garage, where all the cables were just about attached to the wall. I have put up two 40mm square plastic conduits on the wall, which run about 6mtrs along the garage . In the top conduit I placed the kitchen ring main, kitchen lighting, and an earth bondng from the kitchen. In the second I put the cooker (6mm) and fridge (4mm) radials.

Can you think of anything wrong with this?

Additional info:

Radials are about 14mtrs single outlet on each
Ring main covers approx 30sq mtrs
Lighting covers 10 individually transformed 12volt ELV spots, and 1 transformed 12volt centre light, on 3 circuits

Thanks,

DW
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Postby sparx » Thu Jan 29, 2009 5:32 pm

Hi ksddw2
seems good to me, main problems are when several heavy loaded circuits run tight together for a long run, all cables have resistance so give off some heat, if trapped in trunking/conduit/buried in thermal insulation etc that heat builds up and can reach cable insulations limit.
70degC for domestic type twin & earth!
An overhead line cable of only 16mm2 can carry several hundred amps as heat can dissipate easily
regards SPARX
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Postby ksddw2 » Fri Jan 30, 2009 11:51 pm

Thanks Sparx, sounds like I am OK as it is freezing out in the garage. Ever thought of a job in IT? You'd make a fortune!
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Postby SparkyPaul » Sat Jan 31, 2009 12:27 am

[quote="sparx"]
An overhead line cable of only 16mm2 can carry several hundred amps as heat can dissipate easily
[/quote]
Don't try it tho :lol:
It'll cost you a fortune heating the great outdoors and volt drop might be a tad on the high side
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Postby sparx » Sat Jan 31, 2009 9:49 pm

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
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Postby chrisrr » Thu Feb 12, 2009 10:55 am

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]
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