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Author Topic: What kind of power distro?  (Read 913 times)

Jared Bratsberg

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What kind of power distro?
« on: September 06, 2017, 09:49:47 pm »

Hello everyone, Jared here - and this is my first post ever!

I'll be providing the PA and running sound for an outdoor/open air event at the end of October.  I'm very comfortable with distroing power to my indoor rig, but this will be my first hack at powering an outdoor setup with this many speakers at once:

Four (4) JBL SRX835P
Four (4) JBL SRX828SP
Two (2) JBL PRX715

Mixer, laptop, IEMS, 2 smaller guitar cabs, maybe some mini LED PARS from China if they'll fit in my trailer ;)

I'll be speaking with the city about the generator they're providing; I'm guessing they'll drop a spider box under the stage, and then I'll build the power distro from there.

Assuming that'll be the case, I plan run four (4) 10ga extension cords, one to the subs, one to each side of the stage for the mains, and then one more for powering up the mixer, laptop, guitar cabs etc which are all on a 50' stinger running across the back of the stage.

I've been learning this stuff on forums and as I go, so be gentle :)  Is there a better or best way to distro power from a large generator and spider box?  I'll be calculating my total power draw hopefully this weekend so I don't overload any one circuit.

Cheers!

Jared
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Nathan Riddle

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Re: What kind of power distro?
« Reply #1 on: September 06, 2017, 09:51:24 pm »

Before anyone can answer, is "B" your full last name?

You need to use your full real name in order to participate.

Thanks!
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I'm just a guy trying to do the next right thing.

Jared Bratsberg

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Re: What kind of power distro?
« Reply #2 on: September 07, 2017, 09:04:50 am »

Done; thanks.
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Mike Sokol

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Re: What kind of power distro?
« Reply #3 on: September 07, 2017, 09:19:19 am »

Done; thanks.

Jared, welcome to the forum.

Jared Bratsberg

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Re: What kind of power distro?
« Reply #4 on: September 07, 2017, 09:32:30 am »

Thanks Mike!  I've been lurking in the background for a while; decided it was time to become a member :)
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TJ (Tom) Cornish

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Re: What kind of power distro?
« Reply #5 on: September 07, 2017, 09:39:37 am »

Hello everyone, Jared here - and this is my first post ever!

I'll be providing the PA and running sound for an outdoor/open air event at the end of October.  I'm very comfortable with distroing power to my indoor rig, but this will be my first hack at powering an outdoor setup with this many speakers at once:

Four (4) JBL SRX835P
Four (4) JBL SRX828SP
Two (2) JBL PRX715

Mixer, laptop, IEMS, 2 smaller guitar cabs, maybe some mini LED PARS from China if they'll fit in my trailer ;)

I'll be speaking with the city about the generator they're providing; I'm guessing they'll drop a spider box under the stage, and then I'll build the power distro from there.

Assuming that'll be the case, I plan run four (4) 10ga extension cords, one to the subs, one to each side of the stage for the mains, and then one more for powering up the mixer, laptop, guitar cabs etc which are all on a 50' stinger running across the back of the stage.

I've been learning this stuff on forums and as I go, so be gentle :)  Is there a better or best way to distro power from a large generator and spider box?  I'll be calculating my total power draw hopefully this weekend so I don't overload any one circuit.

Cheers!

Jared
Jared, take a look at these articles I wrote.  They should help you get started. http://tjcornish.com/articles/power-and-electricity-artic/
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Jared Bratsberg

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Re: What kind of power distro?
« Reply #6 on: September 07, 2017, 10:29:11 am »

Great read!  I especially like tip on how to find outlets on different circuits with a volt meter - I'll definitely be using this!

So one of your articles mentions a mult-outlet "service" device, and it shows a 50A feed with a breakout box of numerous 15/20A circuits... this is what I expect the City of North Las Vegas will provide from their diesel generator (I'll be confirming this soon).  I plan to run extension cords from this service box to my equipment.

My main point of confusion, is I'm not confident on the power needs of my equipment.  Here's an example: I'm shooting from memory here, but I believe the input power requirement for each powered SRX828SP (from data plate on the amplifier) is 250W, or 2A at 120Hz.  So four (4) subs would need 8A; so if I'm correct, a single power cable rated for 15A, running from a 20A circuit would supply all four (4) subs, is that right?
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TJ (Tom) Cornish

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Re: What kind of power distro?
« Reply #7 on: September 07, 2017, 01:26:13 pm »

My main point of confusion, is I'm not confident on the power needs of my equipment.  Here's an example: I'm shooting from memory here, but I believe the input power requirement for each powered SRX828SP (from data plate on the amplifier) is 250W, or 2A at 120Hz.  So four (4) subs would need 8A; so if I'm correct, a single power cable rated for 15A, running from a 20A circuit would supply all four (4) subs, is that right?
This article covers that: http://tjcornish.com/articles/power-and-electricity-artic/understanding-power-consump.html

2 amps (spec sheet says 2.2A) sounds a little light so I would plan one circuit dedicated to your subs.
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Tim Weaver

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Re: What kind of power distro?
« Reply #8 on: September 07, 2017, 01:45:41 pm »

If the city provides a spider box, then "there's yer distro". Just run extension cords to it and balance your loads on different circuits.

As always, CYA and meter the power before plugging anything in, and hopefully do it before the electrician leaves the site.
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Stephen Swaffer

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Re: What kind of power distro?
« Reply #9 on: September 07, 2017, 02:28:10 pm »

Not necessarily.  There are a number of threads on this-but music presents very dynamic loads to the power system.  "Ampacity" is a long term rating primarily concerned with heating of the cable.  Voltage drop on the cable is usually more critical-especially as the distance go up.

Voltage drop is calculated with Ohms law, and 14 AWG has a resistance of roughly 3 ohms per 1000 feet.  An 8 amp steady load on a 50 ft 14 AWG cord will have a voltage drop of:

.3 ohms X 8 amps =2.4 volts or 2% which is OK-but if a a hit triples that load , now your voltage drop is 7.2 volts or 6 % which is poor design-but probably still be workable.

In this case, you are probably OK-unless  your supply voltage is low.  If you are actually only getting 112 VAC and drop to 105 vAC, you might have an amp drop out.  Also, voltage drops are additive for every cable involved-including the run from the genny.

It pays to do the math and check.  You'll be surprised at how much a problem voltage drop can cause. I overlooked it on a recent project because I am only pulling 6 amps fpr LED's-but with the length of run I would up upsizing to 10 AWG (30 amp) wire.

On this gig, running <=50' extension cords it may not be a big deal-but something to keep in mind.
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Steve Swaffer

Ray Aberle

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Re: What kind of power distro?
« Reply #10 on: September 07, 2017, 02:28:56 pm »

To be candid, if I was in your situation, I'd run each pair of speakers (sub and top) to one circuit on the spider box (you have 6 circuits available). Then one circuit does stage power/backline, and one does FOH and your two wedges. That's going to balance everything really nicely over the available circuits on the spider box.

(I typically see spider box outlets wired X then Y then X then etc -- so the first Edison you find is on the X leg of the incoming service, and the very next one to your right is on Y, and then X, etc. So, you can use that pattern to make sure you're splitting everything between the two legs of the 50A service from the generator. Metering hot to hot should read 220v, thus allowing you to verify that pattern.)

If it's a 25kW generator, upstream breaker should be 60A, so you'll be fine.

As noted, make sure everything meters properly before the electrician leaves! Some newer generators have a "low engine speed" switch- if you're reading 60V or something scary like that, where you're expecting 120V, that's usually the issue.

-Ray
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Jared Bratsberg

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Re: What kind of power distro?
« Reply #11 on: September 07, 2017, 06:11:00 pm »

That's really good insight Ray, exactly the kind of thing I was looking for!
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Mike Sokol

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Re: What kind of power distro?
« Reply #12 on: September 08, 2017, 09:02:36 am »

And  be aware than many/most whisper generators of any size can be changed from single-phase 120/240 volts to 3-phase 120/208 volts. Plus there's generally a 480/277 volt setting you want to avoid. If I'm connecting to the genny with tails, I'll measure at my tail camloks FIRST before connecting to anything else. I've had a few electricians leave the genny on 480 volts and tell me it was 120/208, so only believe your own meter.

You didn't ask but my favorite field meter is a Fluke 117. Cost is around $150, but buy it once and it will likely last for your entire pro-sound career. https://www.amazon.com/Fluke-117-Electricians-True-Multimeter/dp/B000O3LUEI
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