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Forum Index : Electronics : 12 volt cable sizing

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Marcus20VT
Regular Member

Joined: 02/09/2008
Location: Wales
Posts: 49
Posted: 12:17pm 10 Jul 2011
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Hi all,

I have a small pv and turbine system that I am thinking of charging a set of 12v deep cycle batteries to power a 12volt timer to control a 12V immersion heater element.

Problem is, my system is out in my shed, and I obviously want to have the immersion element for the hot water inside my house.
Therefore I would need to run cable from my shed to my house - about 10M or so.

I have looked at the regs book for current carrying capacity and volt drop for cable for a 600W element.
Erring on the side of caution, I'm getting that I would need 10mm twin and earth cable - surely that's not right is it?????
 
RossW
Guru

Joined: 25/02/2006
Location: Australia
Posts: 495
Posted: 09:53pm 10 Jul 2011
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  Marcus20VT said   Erring on the side of caution, I'm getting that I would need 10mm twin and earth cable - surely that's not right is it?????


10 sq mm cable is 1.678000 ohms per Km. You have 20m. (10m out and 10m back).
600W @ 12V is 50A
50A through 0.03356 ohms is 1.678 volts drop, wasting 84 watts.

I'd be using heavier wire, or using a suitable inverter and 240V element.
 
norcold

Guru

Joined: 06/02/2011
Location: Australia
Posts: 670
Posted: 08:42pm 12 Jul 2011
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reuk.co.uk have a lot of info on wire sizes also a handy online calculator to show line losses. I find this calculator very handy, allows keeping the expense of long runs of copper cable down without going to an expensive overkill. Basically it is an online calculator that does what RossW has, you just input the voltage, current draw and cable length and the loss you are prepared to accept in percentage.
Very handy as is a lot of info on that site.
We come from the land downunder.
Vic
 
Fred Bloggs
Newbie

Joined: 21/09/2010
Location: United Kingdom
Posts: 2
Posted: 03:02pm 20 Jul 2011
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Hi

I've been a silent member of this forum for a while so here goes with first post.

'Just noticed you want some method of checking out cable sizes, I did a excel spread sheet a couple of years ago that works out volt drops etc for any size cable upto 90mm2 single core at any voltage. I would post it on this forum but have just noticed that you cannot add an attachment, or am I being stupid

Best Regards

Fred
 
Gizmo

Admin Group

Joined: 05/06/2004
Location: Australia
Posts: 5036
Posted: 05:09am 21 Jul 2011
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Hi Fred

You can attach ZIP files. Zip it up and use the file upload button to upload it.

Welcome to the forum too, I see you've been a member for some time.

Glenn
The best time to plant a tree was twenty years ago, the second best time is right now.
JAQ
 
Fred Bloggs
Newbie

Joined: 21/09/2010
Location: United Kingdom
Posts: 2
Posted: 12:32pm 21 Jul 2011
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Glenn

Thanks for that, I didn't realise what the symbol was for!

So here goes
2011-07-21_223153_cable_calculator.zip

Best Regards

Fred

Ps the spreadsheet was done in office 2003!
 
Gizmo

Admin Group

Joined: 05/06/2004
Location: Australia
Posts: 5036
Posted: 01:56pm 21 Jul 2011
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Thanks for that Fred

Opens in Open Office just fine too

You sure can appreciate why a higher battery voltage is better. Smaller wire for the same power.

Glenn

The best time to plant a tree was twenty years ago, the second best time is right now.
JAQ
 
Bub73

Senior Member

Joined: 10/12/2009
Location: United States
Posts: 116
Posted: 03:06pm 21 Jul 2011
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Thanks Fred;

Works in Open Office under linux just fine for me also.
I've been working on a spread sheet for blade length , using Hugh's formula and will share it when finished

Bob
 
Warpspeed
Guru

Joined: 09/08/2007
Location: Australia
Posts: 4406
Posted: 04:34am 28 Jul 2011
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An easy way to work this out for yourself, you only need to remember one number.
A one metre length of rectangular copper bar 1mm x 1mm has a resistance of .0172 ohms (at 20C)

So 30 metre stretch of 10 mm^2 (is actually 60m of wire).
60 m of 1mm ^2 wire would be .0172 x 60 = 1.032 ohms.
If 10 mm ^2 wire is used instead, it would be one tenth the resistance or 0.1032 ohms.

If you can remember the .0172 number, it is then dead easy to work out the resistance of any length of any gauge of copper wire.

600 watt hearting element is 50A at 12V or 0.24 ohms at full power.
Total resistance of wire plus element 0.3432 ohms.
Total current at 12v input 34.97 Amps

Voltage drop across wiring 3.61 volts
Voltage at heating element 8.39 volts

Edited by Warpspeed 2011-07-29
Cheers,  Tony.
 
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