Temp electrical service at amphitheater

[taneglaus]

Hi all. I’ve searched the Control Booth info db and have found some valuable info but not quite what I'm looking for. I hope you can help.

I'm designing the lighting and sound for ‘the Sound of Music’ being performed in an out door venue this summer.

The site of the make shift amphitheater is in the back of a resort hotel. There is a grassy hillside for the audience to sit and a flat gravel area at the base of the hill where the stage will be built. The area does not have any electrical power.

As I'm designing the lighting and sound I figure I'm going to need at least 120 amps @ 120v. 100 amps for lights and 20 for sound.

There is a gazebo to one side of the seating area about 80 feet away which has both 240v and 120v outlets. There is also a large condo building about 100 feet behind the stage area.

The resort maintenance manager told me that he has 2 construction breakout boxes (?) with 125 foot 240v cables. From what I understand, these construction boxes are designed when contractors are building a house and power has been brought to the property line. You plug the 240v cable to the electrical service and the other plugs in to the breakout box. The box then has several 20Amp 120v outlets and a 30A 240v outlet. I don’t know about earth ground…but, I'm using 2 arch trusses on the hillside and using guy-wires with steel stakes in the ground to hold the arches up, so I figure I can bolt a wire on a stake and use it for local earth ground…I don’t know if this would matter or not.

So the manager says he can plug one cable into the gazebo 240v service and another to the condo building and give me 6- 20A 120v circuits.

Has anyone out there worked on a similar electrical setup?

We are using 6 DMX 4ch. dimmer packs. I'm wondering if the electrical setup might confuse the DMX signal?

Any advice is appreciated.

 

[derekleffew]

...Has anyone out there worked on a similar electrical setup?
We are using 6 DMX 4ch. dimmer packs. I'm wondering if the electrical setup might confuse the DMX signal?

Done similar many times. The electrical setup will have no effect on the DMX signal.

You're skirting on a taboo subject here, taneglaus (see FAQ/Content Policies/Safety), but as long as a http://www.controlbooth.com/forums/glossary/8744-qualified-person-per-nec.html (no reason that can't be the resort maintenance manager) does all the electrical set-up and connections, and you just plug into to NEMA 5-15/20 outlets, you shouldn't have a problem.

...so I figure I can bolt a wire on a stake and use it for local earth ground...

DO NOT go taking matters into your own hands to create your own earth ground. Leave that up to the http://www.controlbooth.com/forums/glossary/8744-qualified-person-per-nec.html.

 

[avkid]

Your dimmers don't care where their power comes from as long as the voltage and frequency are correct.

 

[Footer]

You're dealing with a spider box. Odds are it will be a Hubbell box. This is a pretty common thing when dealing with large amounts of portable power around a building during construction.

http://www.grainger.com/Grainger/items/4D572

 

[gafftaper]

Only 20 amps for sound? That's a pretty small rig. You sure?

 

[taneglaus]

I appreciate your input. I appreciate the controlbooth.com’s policy on not endorsing electrical setups due to potential of electrical shock and/or property damage. I wasn’t asking for a ‘stamp of approval’ although I now realize I wasn’t quite clear on what my ? was…

Often times I find that when troubled with something, if I pose the problem to someone else, during the process of openly describing the problem I often answer my own ?

After submitting the post I realized that I'm nervous about this electrical setup because I've never worked with this type of ‘spider box’…the image is correct. But as I thought about it, I wouldn’t have any problem with running a 100’ 12g extension cord and pulling 20 amps through it…this setup is about the same thing just much more power. In effect I'm using a high power extension cord with a power strip that has circuit breakers on each circuit.

I guess I was most concerned that when we got it all setup that there would be voltage fluctuations or not enough current capability or poor earth ground…or whatever. But I've worked with long extension cords before without problem so as long as the resorts electrical service has been installed correctly, and I believe is has, then I should be ok. And I've looked up a local electrical contractor who will stop buy to check it all out. So I'm feeling much better about this setup.

The sound is only reinforcement so I don’t have massive power amps, a 24 ch. mixer, 4 JBL powered speaker cabinets, and 4 stage monitors running off of a 250w/ch. power amp. So 20 amps for sound is close, I might need 10 more amps for the monitor amp…I hope. I've never had to budget my power so tightly like I am at this venue.

Thanks again for your feedback.

 

[n1ist]

If you really want to play it safe, you can use an optoisolated splitter on your DMX with a separate branch feeding the dimmers plugged into each spider box. I would also make sure all of the audio gear is on the same spider to avoid problems caused by different grounds.

 

[Dionysus]

so I figure I can bolt a wire on a stake and use it for local earth ground…I don’t know if this would matter or not.

When it comes to bonds and grounding it is necessary to deal with the authorities, especially when using more than one service as source.

I am glad to hear that you have already now decided to deal with an electrician. I'm not sure about the local authorities where you are doing your show but I know that the Ontario Electrical Safety Authority can be very, may I say militant on issues of that nature.

There are lots of things you can do, but you'll get in lots of trouble if you do it without a proper permit AND inspection, with the inspectors permission on that particular setup in advance.

 

[gafftaper]

Caution is always a good thing when dealing with electricty. However, if you get the assistance of a certified professional electrician, using the spider box will be safe and reliable. It's done all the time in the industry for applications just like your own.

 

[TimmyP1955]

But as I thought about it, I wouldn’t have any problem with running a 100’ 12g extension cord and pulling 20 amps through it…this setup is about the same thing just much more power. In effect I'm using a high power extension cord with a power strip that has circuit breakers on each circuit.

You'll get a lot of voltage drop. I once had to run my PA on a single 100' run of 10 gauge. I had a 5V drop with the system at idle. According to manufacturers specs on the gear, I was drawing less than 10 amps.

 

[LightStud]

I had a 5V drop with the system at idle.

That amount of drop is excessive and atypical. See Stealth 316 - Wire Resistance and Voltage Drop Calculator for an explanation and calculator.