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Author Topic: single phase vs 3 phase rac pac power load for amps  (Read 2639 times)

Matt Greiner

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single phase vs 3 phase rac pac power load for amps
« on: January 02, 2018, 12:44:09 am »

Before I go out and purchase some Motion Labs Rac Pacs, I want to make sure I'm looking at the correct ones.

I currently have 3 amp racks that I am looking at adding some Rac Pacs to.  The 2 racks I use for my mains are identical, the monitor rack is different.  Almost all of the events I have been doing don't have 3 phase power available.  I use a 14-50 to connect my current distro.  However, the few events that have do have 3 phase available, I rent a motion labs distro and stringer boxes for those.


My amps are all Peavey CS series for the mains and a mix of Peavey CS series and QSC Mx1500a for monitors.  I also don't think I'm actually driving them to the full 1/8 power ratings.  Maybe the sub amps, but not any of the others.
Using the 1/8 power rating from the manufacturers, I have calculated my main racks could draw 36.625 amps if all driven to 1/8 power ratings.  If I add an additional power amp to run the system as a 4 way, it would increase the draw to 46.5 amps.  These are all at 120v. 

For the monitor rack, same ratings, I could draw 19.75 amps from 2 of the power amps, and less than 42 amps from the other 4 power amps, which totals a potential 61.75 amps.  I find the ratings a little high for monitors, as I don't think I would ever be running all 6 monitor amps at the same time, at full tilt.  Normal is using the 2 amps (4 monitor mixes) that draw 19.75 amps or the bands use IEM's.  The other 4 amps are for the bi-amp monitor rig, and I don't bring those out unless it's a big show.  But I would like to plan in case I would have to use all 6 monitor amps.

Looking at the available products, I feel like the 1104-2-DD-050-0012 would be the correct choice for my racks. (L14-30 In/Thru w/4 20 amp circuits to duplex)

Also, would I able to tie these single phase rac pacs to a 3 phase distro for those times when I need to rent one because of venue power tie ins?  Attached is a photo of the distro I rent if needed.

I'm ok with spending the money on the right gear, just not the wrong gear.

All thoughts and suggestions are appreciated!
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Tim McCulloch

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Re: single phase vs 3 phase rac pac power load for amps
« Reply #1 on: January 02, 2018, 02:09:56 am »

Hi Matt-

The quick answer to your actual questions - will the 1104-2-DD-050-001 be the right choice for your amp racks, the quick answer is "yes".

Will these interface with the rental distro you pictured?  Yes.  Are you buying a single phase version for your inventory?  You have to get power into those Rac PacsŪ somehow...
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"Practicing an art, no matter how well or badly, is a way to make your soul grow, for heaven's sake. Sing in the shower. Dance to the radio. Tell stories. Write a poem to a friend, even a lousy poem. Do it as well as you possible can. You will get an enormous reward. You will have created something."  - Kurt Vonnegut

Matt Greiner

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Re: single phase vs 3 phase rac pac power load for amps
« Reply #2 on: January 02, 2018, 09:34:14 am »

Thank you Tim.  I currently have a single phase distro that I've had for over 5 years.  I would just swap out most of the 20 amp circuits for some 240v 30 amp twist locks to accommodate the rack pacs.
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Matt Greiner

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Re: single phase vs 3 phase rac pac power load for amps
« Reply #3 on: January 02, 2018, 09:39:22 am »

Will these interface with the rental distro you pictured?  Yes.

I should have clarified this part, I knew they would physically connect, but I've read so many times about balancing a 3 phase load when hooking up to a single phase.  That was/is my main concern.
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Tim McCulloch

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Re: single phase vs 3 phase rac pac power load for amps
« Reply #4 on: January 02, 2018, 12:05:24 pm »

I should have clarified this part, I knew they would physically connect, but I've read so many times about balancing a 3 phase load when hooking up to a single phase.  That was/is my main concern.

The way Motion most likely wires the L14-30 outlet  panel is X-Y, Y-Z, X-Z.

Load balancing isn't radically critical, but if you've got your single phase loading balanced the 3 phase distro, wired as above, will be reasonably load balanced.  You're in pretty good shape, Matt.
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"Practicing an art, no matter how well or badly, is a way to make your soul grow, for heaven's sake. Sing in the shower. Dance to the radio. Tell stories. Write a poem to a friend, even a lousy poem. Do it as well as you possible can. You will get an enormous reward. You will have created something."  - Kurt Vonnegut

Matt Greiner

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Re: single phase vs 3 phase rac pac power load for amps
« Reply #5 on: January 03, 2018, 01:08:10 pm »

In regards to the power linking ability of these pacs, if I use my racks as configured, I don't believe I would be able to link any of them.

Each of my main racks draws up to 36.625 amps, so between the two of them, they would draw 73.25 amp.  And that is greater than 60 amps.  Again I don't think other than the sub amps that I'd be approaching those numbers.   The music in question would be modern country or rock bands, no pre-recorded music at those levels.

Any real world thoughts on the actual draw of power amps for a situation like this?  Would I be ok to link the 2 FOH racks?  Or in the words of Molly Hatchet, would I be "Flirting with disaster?"
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Tim McCulloch

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Re: single phase vs 3 phase rac pac power load for amps
« Reply #6 on: January 03, 2018, 01:23:04 pm »

In regards to the power linking ability of these pacs, if I use my racks as configured, I don't believe I would be able to link any of them.

Each of my main racks draws up to 36.625 amps, so between the two of them, they would draw 73.25 amp.  And that is greater than 60 amps.  Again I don't think other than the sub amps that I'd be approaching those numbers.   The music in question would be modern country or rock bands, no pre-recorded music at those levels.

Any real world thoughts on the actual draw of power amps for a situation like this?  Would I be ok to link the 2 FOH racks?  Or in the words of Molly Hatchet, would I be "Flirting with disaster?"

The only time I use a "link through" is to run a drop for accessories (or chain hoist power, as the amp rack won't be in use).

If Motion is building to order they can eliminate the link connector and install a cover plate. Call them and have a chat.

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"Practicing an art, no matter how well or badly, is a way to make your soul grow, for heaven's sake. Sing in the shower. Dance to the radio. Tell stories. Write a poem to a friend, even a lousy poem. Do it as well as you possible can. You will get an enormous reward. You will have created something."  - Kurt Vonnegut

Matt Greiner

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Re: single phase vs 3 phase rac pac power load for amps
« Reply #7 on: January 03, 2018, 01:33:12 pm »

The only time I use a "link through" is to run a drop for accessories (or chain hoist power, as the amp rack won't be in use).

That makes sense thank you Tim
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Matt Greiner

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Re: single phase vs 3 phase rac pac power load for amps
« Reply #8 on: January 03, 2018, 03:52:08 pm »

10 gauge work for the 30 amps?  Or should I up it to 8 gauge?
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TJ (Tom) Cornish

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Re: single phase vs 3 phase rac pac power load for amps
« Reply #9 on: January 03, 2018, 04:09:02 pm »

10 gauge work for the 30 amps?  Or should I up it to 8 gauge?
NEC table 400.5(a)(1) gives the derating schedules.  #10 wire in a 5-wire configuration is not acceptable for 30 amps.  In fact, #8 wire is borderline.  If you have 3 current-carrying conductors (ground doesn't count and neutral may or may not count), #8 wire is good for 35 amps.  If you are using your neutral as a current-carrying conductor which is very typical in our industry (pretty much any 120v usage counts), then you need to use the supplementary derating table below, the result for 4 #8 current-carrying conductors is 28 amps.

Most houses I work with use 8-5 for 30A 3-phase loads and I haven't heard of any AHJs dinging for this, but YMMV.
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