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Author Topic: Power from multiple sources  (Read 10206 times)

George Dougherty

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Re: Power from multiple sources
« Reply #10 on: May 19, 2012, 12:47:03 pm »

Wondering if a trick like Tim Padrick's poor man's distro wouldn't be perfect in this case.  Different feeds tie in to a panel that has the ground bonded together behind the panel.  Instant tie between generator and service ground.

I've done this with a rackmount panel, plastic powercon connectors so they don't ground to the chassis, and a grounding terminal strip to tie up to 3 input circuits together.

Also worth noting, meter your voltage off the gennies.  I've killed a rackmount power conditioner on a gennie putting out some serious overvoltage.
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Tim Perry

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Re: Power from multiple sources
« Reply #11 on: May 19, 2012, 02:07:01 pm »

Tim keep it simple: run all audio (including backline) from the 20A main

Run lighting from the generators

Do not interconnect the two in any way.

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Frank DeWitt

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Re: Power from multiple sources
« Reply #12 on: May 19, 2012, 03:31:17 pm »

Back to the clamp on meter for finding what audio cable is carrying current that is causing a ground loop problem (Hum)

I am trying to figure out what meter to buy.  It looks like the meter must be capable of showing .1 amp.

Thinking.
100 foot is about the max cable I might run into.  Mic cable shield is about 1 ohm for 100 ft (Internet)  Hum is noticeable at as low as  .1 volt
.1 volt, 1 ohm = .1 amp.

Looking around the internet it seems that in order to resolve (see) .1 Amp on a digital clamp on meter, it must have a full scale setting of 40 amp or less.  Is that about right?

thanks

Frank
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Keith Humphrey

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Re: Power from multiple sources
« Reply #13 on: May 19, 2012, 04:24:53 pm »


I can drive a 6' ground rod easily with a proper post hammer.  I can put it right next to the generators and connect each chassis to ground with at minimum #8 THHN and clamps.

How, then, should I add the utility power to the ground scheme?  Can I use an Edison plug to wire a duplex to the ground rod (using the U-ground pin only)?


If you are adding a second ground rod you should bond it to the electrical service ground directly. I would not rely upon the installed wiring to bond the grounds as you do not know the capacity and schematic of the full circuit. By bonding directly you can be sure you are using wire of sufficient capacity and quality as to be safe. Remember you become liable if anything goes wrong in this scenario. 

My personal vote is with those that suggest using the 20A house electrical for PA and backline and leaving the generators for the lights. This option leaves liability with the promoter or property owner.     
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Mike Sokol

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Re: Power from multiple sources
« Reply #14 on: May 19, 2012, 04:48:48 pm »

Back to the clamp on meter for finding what audio cable is carrying current that is causing a ground loop problem (Hum)

I am trying to figure out what meter to buy.  It looks like the meter must be capable of showing .1 amp.

Thinking.
100 foot is about the max cable I might run into.  Mic cable shield is about 1 ohm for 100 ft (Internet)  Hum is noticeable at as low as  .1 volt
.1 volt, 1 ohm = .1 amp.

Looking around the internet it seems that in order to resolve (see) .1 Amp on a digital clamp on meter, it must have a full scale setting of 40 amp or less.  Is that about right?


Frank... Nearly any digital clamp-on ammeter with a 40 to 200 amp scale will resolve down to 0.1 amps (100 mA). If you need more sensitivity you can always wrap the cable in a circle a few times, which multiplies the current. But I've found that 100 mA is about the lower limit of audible hum from most amps. I've personally seen up to 5 volts difference on grounds, which makes for a whopping 5 amps of current on a XLR cable shield. And I used to wonder why some XLR cables felt hot to the touch. Now I know....

Get a clamp-on ammeter with a built-in voltmeter, and you've always got one of those handy too.

Update: Here's a video (yes, that's me) on how to use an ammeter on an extension cord: http://www.youtube.com/watch?v=z-KfZvbjyBY

I use a lot of Fluke gear, and a 322 is a solid investment for less than $100. http://www.amazon.com/Fluke-Corporation-322-Clamp-Meter/dp/B00012Z0ZM

But you can also get a pretty decent Extech clamp meter for $50 at Sears that will do the job: http://www.sears.com/extech-200a-clamp-meter-with-built-in-non/p-03414179000P?prdNo=4&blockNo=4&blockType=G4

You can use an AC current only meter (AC-DC is more expensive) and you don't need True RMS (measures distorted waveforms). Basically any standard digital clamp meter that resolves down to 0.1 amperes should do the trick.
« Last Edit: May 19, 2012, 08:10:15 pm by Mike Sokol »
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Mike Sokol
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Mike Sokol

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Re: Power from multiple sources
« Reply #15 on: May 19, 2012, 04:54:14 pm »

If you are adding a second ground rod you should bond it to the electrical service ground directly. I would not rely upon the installed wiring to bond the grounds as you do not know the capacity and schematic of the full circuit. By bonding directly you can be sure you are using wire of sufficient capacity and quality as to be safe. Remember you become liable if anything goes wrong in this scenario. 

My personal vote is with those that suggest using the 20A house electrical for PA and backline and leaving the generators for the lights. This option leaves liability with the promoter or property owner.   

Code does allow for a secondary ground rod, but it MUST be bonded to the primary service ground rod as well. The ONLY time a separated ground rod can be used is with an isolation power transformer, essentially creating a new primary service panel.

Note that a ground rod's impedance to the earth can be as much as 25 ohms and still considered safe. The ground rod's job is NOT to sink current from a short circuit to ground, but rather to establish a ground plane mostly for lightning protection. Separate ground rods without bonds to each other can (and do) have many volts difference between them. The earth is actually a pretty poor ground. 
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Mike Sokol
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Timothy J. Trace

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Re: Power from multiple sources
« Reply #16 on: May 20, 2012, 12:35:35 am »

Tim, please report back after the gig and take pictures of the power setup if you can. Hope everything works out for you....

First, thanks to everyone for contributing to this topic.

The problem was solved for me upon arrival.  The Parks and Rec department gave me two jobsite generators, an 8kW and a 5kW.  The 8kW was putting out 90VAC and the 5kW 135VAC.  I didn't feel like messing with them any further, so I put up one stack a side and wired the PA and backline to the house power.  Done.

Ten minutes before showtime, we spun up the 8kW on a 100' 12/3 cord and used it to power the band's cooling fans and my LED stage lighting.

Amazingly, I had enough oomph in my rig to get the job done, even on such limited power.  My Furmans reported 105VAC for most of the night (down from 120VAC at idle), but the GFI and the upstream breaker never tripped.  I talked to a few punters who would've liked the show to be much louder (as would I), but at the end of the night, everyone was happy and we all got paid :)

Thanks again to everyone for bringing their knowledge to this topic.

Tim ==





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Mike Sokol

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Re: Power from multiple sources
« Reply #17 on: May 20, 2012, 05:29:04 pm »

I talked to a few punters who would've liked the show to be much louder (as would I), but at the end of the night, everyone was happy and we all got paid :)

Very good that it all worked out.

Sometimes less is more.... I remember as a kid playing is a big rock band where I just HAD to bring in every speaker, amp, and mixer, plus my full keyboard stack with a B3 and Leslie, along with a 50KW lighting system with flash pots and strobes. Yes we dragged this all in even when playing on a tiny stage in a tiny room for a crowd of 100.

Nowadays I would be happy playing through a Fishman SA220 powered column and an acoustic guitar. One trip to carry it all to the stage, 10 minutes to set it up, and DONE...
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Mike Sokol
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Sam Feine

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Re: Power from multiple sources
« Reply #18 on: May 20, 2012, 06:03:03 pm »

Is that Front of house, are you running an SAC rig on your computer?
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James A. Griffin

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Re: Power from multiple sources
« Reply #19 on: May 20, 2012, 06:17:05 pm »

Tim keep it simple: run all audio (including backline) from the 20A main

Run lighting from the generators

Do not interconnect the two in any way.

+1.   You're asking for trouble any other way.  Too many unknowns.
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Re: Power from multiple sources
« Reply #19 on: May 20, 2012, 06:17:05 pm »


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