Safety Concerns & Total Amperage Question

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  #1  
Old 06-20-07, 09:28 AM
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Safety Concerns & Total Amperage Question

Thank you in advance for any assistance you can provide. All comments and suggestions are welcome and very much appreciated.
I have a couple of questions concerning electrical service for a recently aquired portable/mobile trailer to be utilized as an entertainment stage. It currently has a 200 amp electrical service panel wired for 240v with a 200 amp main breaker and 5) 20 amp & 5) 30 amp non-GFCI breakers. One leg of the service is fed by a 12/4 type SO wire all connected together. The other leg is wired with 12/2 type SO wire all connected together. The Ground wire is a 10/3 with bare copper wire type NM-B again all connected together. All feed wires are 40' in length. Power could be provided via portable generator and/or existing service panel.
First and foremost: Is this a safe electrical setup?
What would be the maximum amperage this setup is capable of pulling on each leg?
Should the metal service panel be connected to the frame of the trailer, to a non-conductive material connected to the trailer or on a separate pole not mounted to the trailer at all? (floors are plate steel covered by indoor/outdoor carpet. Plate is welded to steel supports that connect to the steel trailer frame)
Would the stage service panel feed wire ground be sufficient to protect a person on the stage should a surge be encountered or should additional grounding be provided?

Thanks again for all support,
Mike
 
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  #2  
Old 06-20-07, 09:35 AM
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I think you already know the answer. This is NOT safe.

This is NOT safe. The power feed needs to be wired to be a proper feed. Using multiple sections of NM cable is very wrong. NM-B cable is not allowed to get wet or damaged. This could easily happen. You need a proper cable assembly.
 
  #3  
Old 06-20-07, 09:42 AM
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> First and foremost: Is this a safe electrical setup?

Not even close. The feeder cords are totally inappropriate. This unit should have a four-wire feed made of an appropriately sized cord assembly. What is the spec'd load? What are these 20A and 30A breakers for and how are they wired?

> What would be the maximum amperage this setup is capable of
> pulling on each leg?

Potentially 200A, which would melt your jerry-rigged cords and catch fire.

> Power could be provided via portable generator and/or existing service panel.

Only if the generator or existing service panel has an appropriately sized four-wire receptacle.

> Should the metal service panel be connected to the frame of the trailer

Yes, but should be isolated from the feeder neutral (which based on your original description I assume it's not)

> Would the stage service panel feed wire ground be sufficient to protect
> a person on the stage should a surge be encountered or should additional
> grounding be provided?

I don't have my code book handy, but I believe performance spaces have additional requirements above basic bonding. Since this seems to be intended for outdoor use, there should be GFCI protection probably on every circuit and appropriate use of outdoor rated panels, boxes, receptacle covers, etc.
 
  #4  
Old 06-20-07, 10:21 AM
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Thanks for the advice.

The 20 and 30 amp breakers are for a sound system PA and stage lighting. They are currently wired to 10) grounded (3 wire) dual 110v outlets, 1 per breaker. I did suspect that this was not safe when I aquired it hence my reasons for searching out those with more electical knowledge than myself. I do know electicity basics but by no means know code. This equipment was provided with the mobile stage trailer and has not been used since aquired. I would like to set it up correctly and know it is safe.
I would like to be able to provide two isolated feeds so that the lighting (noise) does not interfere with the sound system. Based on my crude calculations the sound system (3 amps, 1) 3000watt 110v, 1) 2400watt 110v, 1)600 watt 110v) and other light amp draw gear such as cd players, 9v adapters, etc... could require up to 80 amps at full peak power and one for a lighting system (18) 500 watt PAR 64's, 8) 150 watt PAR 38's and a few led type laser lights) that could require as much as 50 amps peak. I also need the setup to be reliable (not trip) under the specified loads. I would also like to have approximately 40 feet between the generators or electical service and the stage. That being said: Could you provide info on specific wire requirements? Do they make GFCI main panels or would I only need to outfit with GFCI individual breakers?

Thanks again for all advise and assistance,

Mike
 
  #5  
Old 06-20-07, 12:57 PM
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I'm glad you're interested in doing this safely. I used to work at a community theater, and some of the traveling acts were extremely unsafe electrically. I recall one where the sound guy had a set of hacked up jumper cables he used to clamp onto the feeders of one of the building's main panels. I unfortunately at this time didn't have the training or authority to stop him, but looking back I realize how terribly unsafe that situation was.

> The 20 and 30 amp breakers are for a sound system PA and stage lighting.
> They are currently wired to 10) grounded (3 wire) dual 110v outlets, 1 per
> breaker.

This is fine for the 20A circuits (provided they are installed with #12 wire); however the 30A circuits should only be feeding 30A receptacles and equipment which has been designed for 30A feeds. It's my understanding that many do-it-yourself DJs modify their amplifiers which are supposed to run on 20A and feed them with 30A circuits to get more power. This is not a safe practice, and I would recommend you check your 30A circuits to see if you have this sort of situation.

> I would like to be able to provide two isolated feeds so that the lighting
> (noise) does not interfere with the sound system.

In a theater this is usually accomplished by installing the sound and lighting systems on separate transformers which is feasible in a commercial building, but really isn't possible with a portable stage unless you're certain that the venue can provide isolated power feeds at the correct voltage and amperage. Beware that many concert venues provide power in a three-phase configuration and you need a single-phase configuration. You could also use separate generators.

If you want separate feeds, you'll need separate subpanels and separate feeder cords for each.

> sound load calculation

80A @ 120V = 40A @ 120/240V

It sounds like a subpanel with a 50A main breaker for the sound equipment would be acceptable. This would require a minimum #6/4 cord. You would need a generator in the 12-15 kW range for just this feed; it's also important to make sure the generator has (or can have) the correct receptacle.

> lighting load calculation

50A @ 120V = 25A @ 120/240V

I think a 30A subpanel would be a little too close -- probably a subpanel with a 40A main breaker would be better. This would require a minimum #8/4 cord. You could also make this one a clone of the sound panel and use a 50A main breaker with #6/4 cord. For this feed, you would need to be looking at generators in the 10-12kW range.

> Do they make GFCI main panels or would I only need to outfit with
> GFCI individual breakers?

There are some GFCI panels that are designed for use on construction sites; although there are disadvantages to using a GFCI panel. False trips are more likely and when you have a trip you lose the entire light/sound system. I would advise GFCI receptacles for the 20A circuits as they are much cheaper than GFCI breakers and if you have a trip you only lose one bank of lights or sound amp. I think some more investigation of the 30A circuits is required before making a judgment on those.

> Regarding cords and subpanels.

In this type of situation, one would usually wire the subpanel feeder to a sort of backwards receptacle called a "flanged inlet" on the outside of the vehicle which accepts the female end of your cord. The male end of course goes to the generator or site power. The advantage of using the flanged inlet is that the vehicle and the cord are separate so that various cord lengths can be used and the cord can be removed and stowed for safer transport.

One of these to feed each of the sound and light panels would be great:
http://www.gen-tran.com/eshop/10Expand.asp?ProductCode=63651

Gen-tran sells matching cords for the inlets in 25', 50', 75' and 100' lengths as well as adapters for locking and non-locking receptacles:
http://www.gen-tran.com/eshop/10Browse.asp?Category=50A
 
  #6  
Old 06-20-07, 02:28 PM
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Excellent & Informative Info. Thanks Very Much ibpooks

Thanks for the informative post. Your are indeed correct on the outlets. I went and verified this and all of the 110v outlets are rated at 125v 20amp including the one's wired to the 30 amp breakers. I was assuming this rating was for each individual plug on a 2 gang plug but based on your earlier reply I am thinking this is incorrect. So am I now correct in thinking the 20 amp rating is shared for each total outlet? I've saved the links you provided and like the idea of a weatherproof plug in type connection wired to a sub panel. It would definitely make setup a lot safer and easier. The jumper cable thing sounds pretty darn scary although I can think of a few people who would probably use it anyway. Fortunately I am NOT one of them. I've been looking at the new Generac 15kw or 17kw. http://www.generac.com/Products/Portable/Portable.aspx?src=commercial
I had considered using the existing 200 amp service panel and the Generac transfer switch that comes with the 17kw as a means of wiring to either generator or utility service via a 4 wire 50 amp plug and assorted cabled adapters for different hookups. I know some of the venues I have been to use plugs similar to a 240v welding plug or dryer plug. I'm also not sure if I can find GFCI breakers for the existing panel. It is a Challenger SB20CT. I have not been able to locate any info on it from the internet. Should I only use a sub-panel w/o a main breaker in this application or would the breakers on the generator and/or utility service still work if I used a main panel with the 200 amp breaker and GFCI individual breakers? I am not 100% on my amperage requirements just yet since I am still in the process of aquiring all of the data sheets for the amplifiers and such. The 80 amps was more of an educated guess based on equipment I use in a relatives studio. I came up with this based on Ohms law using the 120v and the wattage outputs of the various amps. I not sure this is correct since much of the gain could take place internally on the amps. I'll have a more realistic figure in a few days. I'm hoping it is closer to 50 amps. If you do not mind me asking, how did you come up with the load requirements in your earlier post? Is it a simple formula such as Ohms law? Thanks again for all of your help.
 
  #7  
Old 06-21-07, 01:27 PM
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> I was assuming this rating was for each individual plug on a 2 gang plug
> but based on your earlier reply I am thinking this is incorrect.

20A receptacles (no matter how many are on the circuit) should have a 20A breaker and #12 wiring. The solution in your case is to either replace the 30A breakers with 20A breakers; or, contact the amplifier manufacturers and determine if those units are rated for 30A circuits. If so, replace the 20A receptacles with 30A receptacles and get the appropriate 30A cords from the manufacturers.

> I had considered using the existing 200 amp service panel and the
> Generac transfer switch

That would be fine.

> I'm also not sure if I can find GFCI breakers for the existing panel.

You don't need GFCI breakers. Replace the standard 20A receptacles with GFCI receptacles instead. GFCI receptacles provide the same level of protection and are only about $12 whereas GFCI breakers are usually around $40. Only use the breakers if you want the central control of reseting all trips at the breaker panel.

> Should I only use a sub-panel w/o a main breaker in this application or
> would the breakers on the generator and/or utility service still work if I
> used a main panel with the 200 amp breaker

The subpanels need a main breaker if there are more than 6 circuits in the panel. As long as you have an upstream OCPD, this breaker can be sized to match the panel, so a 200A panel can have a 200A main breaker. This breaker is used as an emergency disconnect to shut down the whole panel in a hurry if needed.

The 50A breaker at the generator, inlet or venue panelboard will provide the actual OCPD (over-current protection device). This breaker is the one that limits the flow of current so your feeder cord won't melt. I strongly recommend that you use the 50A breakered inlet I linked yesterday, because then you know the cord breaker is the correct size. Otherwise, you have to trust the venue "electrician" to have the proper 50A breakers in place on your feeders.

> I came up with this based on Ohms law using the 120v and the
> wattage outputs of the various amps.

Your estimate is probably off then. Manufacturers overstate the power of their amplifiers in the marketing numbers and those numbers do not include the energy that is wasted to heat. If you actually have the amplifiers, there will be electrical requirements stamped on the unit somewhere that has the accurate amperage.

> how did you come up with the load requirements in your earlier post?

Add up all of the input wattage from the nameplates on the amplifiers (not the output power); for light bulbs, you can use the actual bulb value. If any of these loads are continuous (intended to be used for more than 3 hours without a cooling period), then you should multiply their wattage by 1.25 before adding. Divide this number by 240V; and you have your load in amperes. W / V = A.
 
  #8  
Old 06-21-07, 01:47 PM
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80A does sound quite high for sound equipment for the scale of what you're looking at. When computing the power needs, you need to look at the INPUT power required for the amplifiers not the output power. Looking at a random power amp, the output wattage is 3500w, but the nameplate rating for the input is 11.5A @ 120v (11.5A @ 120v = 1380w INPUT).
Other than the amps, the rest of the equipment is minor (mixers, CD players, etc).
 
  #9  
Old 06-21-07, 07:30 PM
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Thumbs up Thanks again for the Excellent advice.

Ben, I really appreciate your advice. Rest assurred I will take it. I truly beleive I can now move forward with this project knowing that safety is of the utmost concern. The impact of having an individual harmed by doing this wrong would cost much more in many ways than the cost of doing this right the first time. Thank you for your time, knowledge and understanding. It does seem that I came to the right place for advice not to mention that I have learned tremendously from this encounter.

Zorfdt, I also appreciate your input. This is definitely where I made my miscalculation. Back to the drawing board as they say. The good part is my number should be quite a bit lower. That's a good thing for me.

Sincerely,
Mike
 
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