Number 8 wire - ohms resistance

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raytaylor

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#56725 21-Jan-2010 22:45

Hey everyone.
I am needing to run a low voltage along a fence wire, and was wondering since I am a city boy,

Does no.8 wire refer to its AWG size?
And does anyone have any resistance measurments or calculations to work out how many ohms of resistance it is per metre? - I think i worded that right.

This is one of my weird posts that i make every so often.
Thanks

Ray Taylor

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SepticSceptic

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#292119 22-Jan-2010 10:00

I'm pretty sure no 8 refers to the American AWG sizes - a look at Wikipedia shows all the details, and no 8 looks about right @ 3.2mm and a grand total of 2 ohms per km.
http://en.wikipedia.org/wiki/American_wire_gauge#Table_of_AWG_wire_sizes

raytaylor

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#292271 22-Jan-2010 16:56

Awesome, thanks for that.

Do you know what its made of? Iron steel? I know its not copper so i am wondering if the resistance would change between different brands of the wire.

Ray Taylor

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SepticSceptic

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#292274 22-Jan-2010 17:14

Quite true - the resistance will change a fair bit if it's not copper - the resistance per km was based on copper. It's made of iron, though I don't know the composition. Pure iron is quite soft - swordsmen of old were known to straighten bent iron swords with their teeth ! This was before they perfected iron blends...

Unless you are running km's of no 8, I don't think the resistance would have too much effect. It would also depend on the expected current draw of the device at the other end. If you are trying to re-charge a car battery, then yes, the resistance would have an effect.

Just wire it up, pump in your 12v, connect your device and measure the voltage at the device end. Most consumer devices will tolerate a minor voltage fluctuation.

For example, most plug packs, although stamped 12v, will pump out anything up to 14-16v with no load or a light load

LennonNZ

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#292277 22-Jan-2010 17:23

Looking at some electric fence webpages

8 AWG electric fence wire is about 22 Ohms/mile (~ 2 Ohms/mile if it was made of copper)
About 35 Ohms/km

raytaylor

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#292283 22-Jan-2010 17:53

Its some battery operated sensor lights that change colour when the temperature drops below 2 degrees so the helicopters that fly above the orchards on frosty mornings know where to fly and are more efficient.

Rather than fly around in circles for an hour pushing air around, not knowing if they are high or low enough and not being very efficient, we are trying out a system where they play a game.
The sensors every 100m along the fence change the colour of the led lights when the temperature drops below a certain point.

The pilot plays a game where he can fly in the red areas of the vineyard and warm them up and just needs to keep the lights green.

Works out to be about 12v @ 2 amps that we are trying to get to the end of a 1km fence. The amp requirement drops at each sensor along but i want to try and design it for 2 amps at the end.

Thanks guys for your help, its really much appreciated.

Ray Taylor

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raytaylor

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#292284 22-Jan-2010 17:58

LennonNZ: Looking at some electric fence webpages

8 AWG electric fence wire is about 22 Ohms/mile (~ 2 Ohms/mile if it was made of copper)
About 35 Ohms/km

Was that ohms/mile rating to the end and back (loop length) or just to the end of the mile?
Wouldnt it be 13.8 ohms per km because 1 km is .63 of a mile?

Just wanna clear those up to make sure i understand correctly

Ray Taylor

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LennonNZ

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#292287 22-Jan-2010 18:17

oh yea.. I multiplied by the wrong number..

22 Ohms/mile = ~13 Ohms/km :-)

when you buy electric fence wire it is usually quoted in Ohms/mile :-) (So Farmers know how far their electric fence unit can go)

http://www.gallagher.com.au/permanent-fence-components.aspx?mktcatid=649

for example.. there are many types of wire, made of different stuff so better just look at the specs of the particular wire you are going to use :-)

raytaylor

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#292298 22-Jan-2010 19:03

darn. I was just going to use whatever wire the fence was made of.

Ray Taylor

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LennonNZ

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#292301 22-Jan-2010 19:11

If the wire is just stapled to the fence and does it have insulators on it to the fence (like with an electric fence has) you'll find the wire will behave alot different as well.

raytaylor

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#292314 22-Jan-2010 20:20

Its raining today and ment to be tomorrow so the posts will be nice and wet, and uninsulated from the fencewire. I am going to go out and find some fences, short 2 of the horizontal fence wires with a copper link wire, walk 50m away and test the resistance with a multimeter and see what I get as a sample. Might do it 6 times and work out an average.

I have just been shown how to use the multimeter and the equasions on how to work out the loss.

Yay :-)

Ray Taylor

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