ProSoundWeb Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: 1 2 [All]   Go Down

Author Topic: Voltage drop question  (Read 4407 times)

Jacob Shaw

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 87
Voltage drop question
« on: August 07, 2015, 06:19:01 pm »

I am having a little trouble with these online calculators.  Not sure if the aperage is both conductors combined or each conductor exc, so here is the senario.

Single phase run from main panel to another service panel.
Conductor will be 2-2-2-4 aluminum direct burial rated cable hung overhead
Run length is 420 feet.
Draw is 30a per leg (2 legs of course)
Single phase means that the original voltage is 120 per leg or 240 across both legs (duh but just in case someone asked)
What would the voltage drop be? Surely not 12.4 volts right?
Logged

Stephen Swaffer

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2319
Re: Voltage drop question
« Reply #1 on: August 07, 2015, 07:23:00 pm »

I get 8.03 volts dropped at 420 feet.   

#2 Al has a resistance of .319/1000 feet.

.319 X .840 (total distance travelled)=.26796 ohms

.26796 ohms X 30 amps=8.03 volts

How will this wire be supported if run overhead?
Logged
Steve Swaffer

Jacob Shaw

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 87
Re: Voltage drop question
« Reply #2 on: August 07, 2015, 08:35:05 pm »

They currently have a #4 service wire that is on prussics that are attached from trees, but I was thinking of replacing the prussics with kellums grips.
Logged

Jacob Shaw

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 87
Re: Voltage drop question
« Reply #3 on: August 07, 2015, 08:38:37 pm »

So you would have 111.7v at the other end?
Logged

TJ (Tom) Cornish

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 4284
  • St. Paul, MN
Re: Voltage drop question
« Reply #4 on: August 07, 2015, 10:06:33 pm »

So you would have 111.7v at the other end?
How much are you starting with?  Have you measured it?  Is your 30A per leg load continuous or is it an average draw with spikes higher than 30A like it would be with an audio load? 111v is pretty marginal.
Logged

Stephen Swaffer

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2319
Re: Voltage drop question
« Reply #5 on: August 07, 2015, 10:42:00 pm »

I agree that voltage is marginal-and the problem will be worse if you are trying to start any motors as ohms law always applies so at peak draw your voltage will be even lower.

URD (Underground direct burial) is not designed for overhead.  Overhead has a messenger (bare) wire that has a strand of steel in it for the strength needed.  The wire needs the mechanical strength to not only carry its weight, but also to withstand the stress caused by whipping around in 50+ MPH winds.  Of course, its your project so you can use what you want.  (One strand of steel doesn't seem like much -but I have seen it bend 2" rigid conduit quite handily)
Logged
Steve Swaffer

Jacob Shaw

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 87
Re: Voltage drop question
« Reply #6 on: August 08, 2015, 11:22:13 am »

The stuff is mobile home wire, and it is designed to go from A service pole to a home, but it is also approved for direct burial.  There may be some steel in it, but it is all insulated.  Service entrance wire is what is there now, and it has held up for years.  I would love for the voltage to be higher, but I simply cannot afford to donate a 4/0 wire.
Logged

TJ (Tom) Cornish

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 4284
  • St. Paul, MN
Re: Voltage drop question
« Reply #7 on: August 08, 2015, 07:09:31 pm »

The stuff is mobile home wire, and it is designed to go from A service pole to a home, but it is also approved for direct burial.  There may be some steel in it, but it is all insulated.  Service entrance wire is what is there now, and it has held up for years.  I would love for the voltage to be higher, but I simply cannot afford to donate a 4/0 wire.
Doing it poorly may be worse than nothing. How about one size up?
Logged

Jacob Shaw

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 87
Re: Voltage drop question
« Reply #8 on: August 09, 2015, 07:30:57 pm »

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.
Logged

Stephen Swaffer

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2319
Re: Voltage drop question
« Reply #9 on: August 09, 2015, 11:50:59 pm »

 I can buy 2-4 ASCR (0verhead steel reinforced) for less than 2-2-2-4 URD direct burial.

1/0 4 wire overhead would be 62 cents a foot more than the 2-2-2-4 URD. 

I suspect that they do make #1-but likely a special order that would not make sense.  There can be a big difference in suppliers costs and pricing.

While "doing better" is a good thing and something I might find acceptable, it won't be a defense if something bad happens.  That said, the State inspectors in my area don't/can't enforce voltage drop recommendations in the code. They view it as a quality issue not a safety issue.  OTOH, using URD overhead could lead to a broken wire leaving a live wire whipping in the wind or laying on the ground.

Also, consider the value of your time (even if it is donated.)  It might make the cost of doing it one time correctly seem more reasonable.  Of course, if the load is just incandescent lighting or electric heat-or a number of other loads, voltage drop is not as critical.

Just some things to consider.   
Logged
Steve Swaffer

TJ (Tom) Cornish

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 4284
  • St. Paul, MN
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.
Logged

Jacob Shaw

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 87
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?
Logged

TJ (Tom) Cornish

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 4284
  • St. Paul, MN
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.
Logged

Jacob Shaw

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 87
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?
Logged

TJ (Tom) Cornish

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 4284
  • St. Paul, MN
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.

Logged

Guy Holt

  • Full Member
  • ***
  • Offline Offline
  • Posts: 125
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
Logged

Jacob Shaw

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 87
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.
Logged

ProSoundWeb Community

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


Pages: 1 2 [All]   Go Up
 



Page created in 0.053 seconds with 22 queries.