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Author Topic: Voltage drop question  (Read 4415 times)

TJ (Tom) Cornish

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Re: Voltage drop question
« Reply #10 on: August 10, 2015, 09:15:08 am »

What do you mean by poorly?  I don't understand how replacing what is there with the same type of wire but bigger could possibly be worse than doing nothing.  Could you elaborate?  They don't seem to make device entry or triplex wire in #1, so I would have to go up to 1/0.
You didn't mention in your OP that there was an existing service you were replacing, and still haven't said what the application is or how you derived your 30A load number.  Since this is a pro audio forum, it's a reasonable assumption that it is an entertainment application, where at least some portion of the load has a peak component higher than your 30A presumably average number.

Doing a job inadequately for the application - even if it's less bad than the previous presumably inadequate method is throwing good money away.  Either do nothing and rent a generator when necessary, or do it well enough so the power is adequate for the application.  If I came in to provide in this location and measured less than 110v - something likely in your situation, as the 111v number assumes 120v at the beginning of the 420' run and doesn't consider the voltage drop over wiring from the transformer through the first service, I'd be unhappy, and would either start downsizing the show to fit the available power, or be asking for a generator to be brought in.
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Jacob Shaw

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Re: Voltage drop question
« Reply #11 on: August 12, 2015, 11:58:02 am »

Does it need to be 4 wire to go from one service panel to another?  Or can I get by with triplex?  The wire that is there now only has 3 conductors.  What determines wether or not the ground AND the neutral need to go back to the main panel?
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TJ (Tom) Cornish

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Re: Voltage drop question
« Reply #12 on: August 12, 2015, 12:07:46 pm »

Does it need to be 4 wire to go from one service panel to another?  Or can I get by with triplex?  The wire that is there now only has 3 conductors.  What determines wether or not the ground AND the neutral need to go back to the main panel?
If this is fed from another service panel, it is considered a sub-panel and must have a neutral conductor and a separate ground conductor, and these must not be bonded in your sub panel.  In short, yes, you need 4 wires.  The only way that a separate neutral would not be required is if you had no 120v loads and were just powering 208v/240v equipment.
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Jacob Shaw

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Re: Voltage drop question
« Reply #13 on: August 13, 2015, 11:30:10 am »

You didn't mention in your OP that there was an existing service you were replacing, and still haven't said what the application is or how you derived your 30A load number.  Since this is a pro audio forum, it's a reasonable assumption that it is an entertainment application, where at least some portion of the load has a peak component higher than your 30A presumably average number.

Doing a job inadequately for the application - even if it's less bad than the previous presumably inadequate method is throwing good money away.  Either do nothing and rent a generator when necessary, or do it well enough so the power is adequate for the application.  If I came in to provide in this location and measured less than 110v - something likely in your situation, as the 111v number assumes 120v at the beginning of the 420' run and doesn't consider the voltage drop over wiring from the transformer through the first service, I'd be unhappy, and would either start downsizing the show to fit the available power, or be asking for a generator to be brought in.
  The op was a math problem.  Someone answered it in the first reply, then in the second reply I talked about the existing wire.  It is pro audio, but I don't expect to peak above 30a per leg (60a) as it is for about 150 people.  What wire would you use in this situation?
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TJ (Tom) Cornish

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Re: Voltage drop question
« Reply #14 on: August 13, 2015, 11:45:31 am »

  The op was a math problem.  Someone answered it in the first reply, then in the second reply I talked about the existing wire.  It is pro audio, but I don't expect to peak above 30a per leg (60a) as it is for about 150 people.  What wire would you use in this situation?
What wire is already there?  60A for 150 people sounds like a lot.  What gear are you using?  How are you arriving at 60A?  Adding up the theoretical output of your amps?  Using 1/8 power spec?

If you haven't already, the first thing I would do is measure your gear to figure out what it actually draws.  You may be pleasantly surprised that it is less than you think.  I could do a heck of a show for 150 people on 60A.

If you actually need all that power and if this were a regular event, I would pony up the extra $200 to get the larger wire Steve mentioned in his post to ensure that your gear will actually work and not cut out all the time.  If this was a once a year thing, I would probably rent a generator and bill the client.  If none of the above are practical, I would downsize the expectations of the event to fit the existing power - i.e. cut down on lighting, move to LED fixtures, or turn it down.  Turning down 3dB will halve your audio power consumption.

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Guy Holt

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Re: Voltage drop question
« Reply #15 on: August 13, 2015, 07:41:28 pm »

If none of the above are practical, I would downsize the expectations of the event to fit the existing power - i.e. cut down on lighting, move to LED fixtures...

While using LEDs rather than incandescent lighting will reduce your power consumption,  if you don’t take into account the generally poor power factor of theatrical LED fixtures you may find breakers tripping and portable generators running erratically. I am discovering that a lot of LED AC power supplies are not power factor corrected (pfc) making them much less efficient than tungsten lights that have unity power. With power factors as low as .45, LEDs can draw twice the current than a tungsten light of the same wattage and draw considerable harmonic currents. The harmonic currents non-pfc LED lights draw will cause additional voltage drop because of a phenomena, known as “skin effect”, in which the higher frequency harmonic currents travel through only a narrow band near the surface of the conductor. Where more current is traveling through less copper, resistance increases and voltage drops. If you don’t take into account the extra current they will draw and the harmonic currents they will generate, you may find equipment failing, breakers tripping, and portable generators running erratically.


(The Chauvet Slim Par Pro RGBA has a pf of .61 and Total Harmonic Distortion of 81%)

The manufacturers of LED Light fixtures generally do not give power factor specifications for their products. One would think that the less expensive LED lights would not be pfc, while the more expensive ones would, but that proved not to be the case in recent testing I did of fixtures. Over half of the fixtures that I tested at random (from the inventories of Boston area rental and lighting sales companies) were not pfc. With power factors ranging from .45 to .63, these fixtures generated considerable harmonic distortion (THD ranged from 75-85%.) To see which LED lights are power factor corrected or not, use this link - http://www.screenlightandgrip.com/html/emailnewsletter_generators.html#anchorHigh Output AC LEDs - to see some of the results of my tests.

Guy Holt, Gaffer
ScreenLight & Grip
Lighting Rental & Sales in Boston
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Jacob Shaw

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Re: Voltage drop question
« Reply #16 on: August 13, 2015, 11:33:34 pm »

Awesome link.  I use all Martin lights, but not a lot of fixtures.  I use 1/4 factor for my amplifier draw because it is dance music.  It is an annual event, but it is my own event, so I would be billing myself for a generator( not an attractive offer).  I could buy 1/0 wire for the price of one weekend of generator rental, or otherwise I will take a loan out on a generator.
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ProSoundWeb Community

Re: Voltage drop question
« Reply #16 on: August 13, 2015, 11:33:34 pm »


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