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Author Topic: Voltage drop question  (Read 1496 times)

Stephen Swaffer

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Re: Voltage drop question
« Reply #40 on: April 17, 2017, 11:08:35 pm »

So, 2 smaller wires of the same cross section will have a greater surface area to dissipate the heat that is generated.
 
The parallel DC resistance of the 2 wires should equal the resistance of the larger wire-so for a given voltage/current the amount of heat generated should be the same-but the greater surface area would allow the 2 smaller wires to run cooler.

Just a basic, practical example of physics in action.
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John Roberts {JR}

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Re: Voltage drop question
« Reply #41 on: April 18, 2017, 09:56:28 am »

So, 2 smaller wires of the same cross section will have a greater surface area to dissipate the heat that is generated.
 
The parallel DC resistance of the 2 wires should equal the resistance of the larger wire-so for a given voltage/current the amount of heat generated should be the same-but the greater surface area would allow the 2 smaller wires to run cooler.

Just a basic, practical example of physics in action.
Physics..."It's the law"... Since resistance is linear with cross sectional area, the two smaller wires adding up to the same cross sectional area as one larger wire will have 1.4x the surface area.  Cross sectional area increases with the square of the radius, while surface area increases linearly with diameter.

JR

PS: For a wild card think about skin effect where current doesn't flow in the middle of the wires (crowded out by magnetic field), tiny wires have more skin all else equal.
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Jonathan Johnson

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Re: Voltage drop question
« Reply #42 on: April 18, 2017, 11:30:38 pm »


PS: For a wild card think about skin effect where current doesn't flow in the middle of the wires (crowded out by magnetic field), tiny wires have more skin all else equal.

An issue at higher frequencies, not so much at 60Hz. And no issue at all with DC.

At extremely high AC voltages (even 60Hz) you also have a corona effect where the magnetic (electric?) field ionizes the air surrounding the conductor, so some current is carried by the air.
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Stop confusing the issue with facts and logic!

John Roberts {JR}

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Re: Voltage drop question
« Reply #43 on: April 19, 2017, 08:58:17 am »

An issue at higher frequencies, not so much at 60Hz. And no issue at all with DC.
True, it's an AC phenomenon so not a consideration at DC. For 60Hz the effective skin depth where conduction occurs is around 0.33" deep, so moot for wires less than 0.66" across... IIRC I think some big dog AC wires use a steel center with (lower resistance) aluminum conductors wrapped around the steel core for good strength and good "effective" conduction in the outer 0.33" at 60Hz.
Quote
At extremely high AC voltages (even 60Hz) you also have a corona effect where the magnetic (electric?) field ionizes the air surrounding the conductor, so some current is carried by the air.
I just realized I am guilty of my signature rant... doing an information dump instead of answering the OP's question... ::)

JR
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Kevin Graf

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Re: Voltage drop question
« Reply #44 on: April 19, 2017, 03:17:59 pm »

For any situation that we are likely to be in, loop inductance will overwhelm skin effect.
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Mark Cadwallader

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Re: Voltage drop question
« Reply #45 on: April 19, 2017, 04:58:21 pm »


 I just realized I am guilty of my signature rant... doing an information dump instead of answering the OP's question... ::)

JR

I appreciate the additional education I am getting on the general topic. My original question was promptly answered (thank, guys), and the rest is all good.
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Scott Holtzman

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Re: Voltage drop question
« Reply #46 on: April 20, 2017, 03:15:18 am »

I am sure y'all know the higher the frequency the more skin effect is relevant.  7/8" and larger RF transmission cable uses hollow center conductors to lower weight/cost.  Also seen copper low density foam for center conductor.  It works even at VHF frequencies.



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Kevin Graf

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Re: Voltage drop question
« Reply #47 on: April 20, 2017, 08:13:58 am »

And even at HF frequencies. That's 3 to 30 MHz.
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