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Author Topic: Voltage drop question  (Read 2115 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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Steve Swaffer

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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Guy Holt

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Re: Voltage drop question
« Reply #48 on: May 03, 2017, 05:40:39 pm »

You might actually notice some current from triplen harmonics generated by dimmers/switch mode power supplies and such but should still be pretty low.

Where that was once true, it is no longer the case. I am discovering that a lot of LED fixtures use switch mode power supplies that are not power factor corrected (pfc.) In recent testing I did of the fixtures in the inventories of Boston area rental and lighting sales companies, over half were not power factor corrected. With power factors ranging from .45 to .63, these fixtures generated considerable harmonic distortion (THD ranged from 75-85%.) If you don’t take into account the extra current they will draw and the harmonic currents they will generate, you may find unexpected voltage drop, breakers tripping, portable generators running erratically, and the neutrals in three phase systems getting hot. Even those that were pfc generated harmonic currents when dimmed. For instance, the pfc of the new Litepanel Astra 1x1 dropped from .99 to .54 when dimmed 50% (THD increased to 83.2%.)

For those interested in the results of my survey of fixtures, I have an article coming out in the spring issue of Protocol on the harmonic profile of a number of common LED fixtures and how it effects portable generators. If you are not familiar with Protocol, it is the quarterly publication of PLASA – an international organization working to raise standards, improve skills and strengthen the events, entertainment and installation industries. If you can’t find the print edition of the magazine, there are links to it and other articles in my “Production Power on a Budget” series in Protocol at http://www.screenlightandgrip.com/html/hd_plug-n-play_pkg.html.

Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com
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