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Country Discussion Topics
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My Last DC Question - I Hope!!
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Sparky    Posted 09-19-2003 at 09:47:30       [Reply]  [No Email]
Worst case scenerio: Two conductors, #2 and #4. Would it be safer to use the #2 for the negative and the #4 for the positive in a 24 VDC circuit? The load is less than 20 amps so the cables are not near their capacities.


DL    Posted 09-19-2003 at 20:12:06       [Reply]  [No Email]
It all depends on the resistance of your wires. For example, if the total resistance of one wire is 0.4 Ohm ( I think it's a reasonable number), there will be 0.4X20 = 8 volts voltage drop on the wire. The power disipated on the wire is 20X8=160 Watts. So can your wire handle 160 watts of heat? Not mention that with only less than 16 volts left (the 2nd wire) for your machine. As a good reference look under the hood of your car. When starting a midsized car the battery crank out about 12 amps and the car battery is 12 volts, smaller than yours. Compare your wire gage with that connecting the battery in the car. Considering that it only take 1 minutes to start a car. The people who design automobiles would not put an extra large wire in the car without a good reason. Those people know what they are doing.


Larry 8N75381    Posted 09-19-2003 at 17:08:10       [Reply]  [No Email]
Simple answer = NO

As Red Dave and JimBob have essentially said in their detailed replies. There will be no difference in SAFETY between either choice - #2 neg and #4 pos versa #2 pos and #4 neg.

At less than 20 amps, you won't be able to measure the difference in the resistance/voltage drop between the #2 and #4 unless you have miles of wire. Book I have rates #2 at 95 to 219 amps and #4 at 70 to 163 amps depending upon wire insulation and type of installation (in conduit vs. open air).

Regards,
Larry


Red Dave    Posted 09-19-2003 at 09:52:05       [Reply]  [No Email]
It won't matter. The current will be equal in both the positive and negative conductors.
Unless you are running this a very long distance, #2 & #4 are way over sized for less than 20 amps.


Jimbob    Posted 09-19-2003 at 11:02:50       [Reply]  [No Email]
I would not find any advantage, I also do not know how long the feed is in length. I would use the #2 on the positive like an auto battery has the larger + post. Wire is resistance & for some reason, the lower resistance wire is put on the return side or positive side. DC flows from negative to positive, however I do not have different wire sizes when installing large 500 to 2500hp DC motors.

If any of you out there wire such large motors, we always combine the + and - wires in each conduit so a polarized field does not develop in each conduit.


Red Dave    Posted 09-19-2003 at 11:35:39       [Reply]  [No Email]
Battery posts are different sizes merely to differentiate between them. The current in one post is the same as the current in the other unless somebody changed the laws of physics last night while I was sleeping ;)
20 amps should not induce a strong enough magnetic field to make cancelling it out matter. Direction of current flow will also not matter unless he is designing a circuit that includes semi-conductors, as I understand his info, he only needs to observe polarity.
I assume that he is using such large diameter wire because he already has it and it won't cost anything, I would never design a circuit like that because of the cost of the conductors.
It sounds to me like what he's doing will work fine.


Jimbob    Posted 09-19-2003 at 11:44:35       [Reply]  [No Email]
I will repeat my post. It does not make any difference if one wire is larger than another. It also does not make a difference if the larger wire is in the positive or negative lead. However, like car batteries, a larger wire on the positive is more standard than not. It started back in the '40s with positve grounded cars. I also have not changed ohms law, simple power calculation formulas nor physics.


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