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#1
04-02-2009, 03:27 PM
 benchaz Junior Member Join Date: Feb 2009 Posts: 14
AWG current ratings

Hey guys, I have a pretty noob question,

I know there are different sized wires rated in AWG, and there are current ratings for each diameter. http://www.powerstream.com/Wire_Size.htm

But how come whenever I see charts like that, the maximum current column never defines at what voltage. I don't get it. I tried searching everywhere.

So does it mean, say 20AWG wire, can transmit 1.5 amps, both at 200volts DC and 0.1 volts DC?

well ultimately, the question is, can 24AWG wire transmit 3amps @12 volt DC for power?

Thanks
#2
04-02-2009, 04:05 PM
 SgtWookie Expert Member Join Date: Jul 2007 Location: In the vast midwest of the USA; CST Posts: 22,030

It depends upon how long the wire is, and how much voltage drop you can tolerate.
AWG 24 wire has a resistance of 25.67 Ohms per 1,000 feet, or 25.67mOhms/foot.

Let's say you wanted to connect a 3A load to a battery that was 25 feet away using AWG 24 wire. You'll need two wires, one for the +12v, and one for the return, for a total of 50 feet of wire. 25.67mOhms x 50 feet = 1.2835 Ohms.

Let's figure out what the voltage drop will be across the wire.
Ohm's Law says E = IR (Voltage = Current x Resistance), so:
E = 3A x 1.2835 Ohms = 3.8505 Volts lost in (dropped across) the wiring.
You would then have 12v-3.8505v = 8.1495v across your load.

With a 3A load on AWG 24 wire, for every 13 inches of length you will lose 1/10 volt.
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#3
04-02-2009, 04:12 PM
 beenthere Senior Member Join Date: Apr 2004 Location: Missouri, USA (GMT -6) Posts: 15,815 Blog Entries: 10

The insulation rating is the only thing that is concerned about applied voltage. Recall that most voltages are fairly low - most insulation is rated at 600 volts or less.

The wire has some resistance which causes it to get hot (dissipate power) when passing current. The voltage will be only a bit different between one end of the wire and the other, so it really doesn't play a part in figuring current capacity of the wire. One way to think about that is that if you push enough current to develop a significant voltage drop in the wire, it will get hot enough to burn open.

20 ga can handle 1.47 amps at those voltages.

24 ga can only handle 577 ma at any voltage.

Remember, as the wire gauge numbers get larger, the cross section of the wire gets smaller, and it can carry less current.

Working off this chart - http://www.interfacebus.com/Copper_Wire_AWG_SIze.html.
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#4
04-03-2009, 04:01 PM
 benchaz Junior Member Join Date: Feb 2009 Posts: 14

oh, so the current rating has nothing to do with how much voltage is being pushed through.

It's just hard to imagine a small 24awg wire (if insulated very well), can take maximum 0.5amps @ 1kV and maximum of 0.5amps @ 0.1volts.
#5
04-03-2009, 04:48 PM
 SgtWookie Expert Member Join Date: Jul 2007 Location: In the vast midwest of the USA; CST Posts: 22,030

It's the current across the resistance of the wire that causes a voltage drop.

If you put 1 Ampere of current through a wire that has 1 Ohm resistance, you will get 1 Volt dropped across the resistance of the wire.
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If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input.
#6
04-03-2009, 07:11 PM
 beenthere Senior Member Join Date: Apr 2004 Location: Missouri, USA (GMT -6) Posts: 15,815 Blog Entries: 10

A bit of amplification - the power lost in the wire can be calculated by taking the square of the current and multiplying by the resistance of the length of wire.

These so-called I squared R losses cause inefficiency in every aspect of electronics.
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