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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Art Nadelman on September 06, 2019, 11:40:00 AM
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I am NOT an electrician. So I need to ask you experts for some advice here.
I have an outdoor show next weekend where the power source is about 125 feet from the stage. That's a lot of new expensive extension cords to purchase to run this show. So I recommended to the promoter that they get a generator for the show.
They contracted for a 6500 Watt generator. The 5-piece band I'm running sound for is going direct with all instruments (no amps) and all use IEMs, so no wedges. The only major power consumers on the stage will be my 4 main speakers and 2 subs. So this is plenty of power.
My concern comes from my mixer/wireless rack. I always run it on a UPS, but I remember that UPS's and generators don't always play nicely with each other.
So I've considered 3 different options. Option 1 is to run everything off of the generator as I normally would.
Option 2 is to run everything off of the generator, but instead of running a UPS in the rack, use a power conditioner in the rack.
Option 3 is to run the rack off of city power and run everything else off of the generator. By doing this, am I causing a grounding or other issue?
What are your recommendations?
Thanks.
Art
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If you are using a digital snake, with no analog interconnects between the stage and the mixer, you probably don't have much to worry about.
If you do have analog interconnects between the components on different power sources, then I recommend bonding the ground of the generator with the ground of the utility power. The reason is that, if you don't, you could get undesirable currents on the shields of your interconnects; this is especially problematic if any of them are not true balanced. The other option is to use isolation transformers in the interconnects.
Bonding can be done with a #6 AWG wire connected between the grounding point of the generator and a grounding point of the utility power supply. That probably means buying at least some wire.
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I agree with Jonathan on bonding the grounds-but the main reason being safety anyone in the area. Undesirable currents may or may not occur-but they are easier to fix than people.
Technically, bonding wire size is determined by breaker size involved-so if only 20 amp circuits, a #12 is Ok, 30 amp breakers need a #10. Larger is better-and obviously more robust/dependable.
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Option 1a: Bypass UPS and run off the generator.
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You don't know till you try, but If you are that worried about the UPS then nix it and run off the Genie power. If you are using a digital snake and no analog connections are going to anything connected to the Genie, you should have little to worry about. I would use a network cable that has its shield lifted though as a fully shielded network cable can carry ground current between the two halves of the systems.
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You don't know till you try, but If you are that worried about the UPS then nix it and run off the Genie power. If you are using a digital snake and no analog connections are going to anything connected to the Genie, you should have little to worry about. I would use a network cable that has its shield lifted though as a fully shielded network cable can carry ground current between the two halves of the systems.
Uh... If there is an X/M32 involved you're inviting problems without shield continuity.
I think once the OP hears how loud the genset is, he'll wish for extension cords.
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Uh... If there is an X/M32 involved you're inviting problems without shield continuity.
I think once the OP hears how loud the genset is, he'll wish for extension cords.
Yes what do you mean "contracted for a 6500 watt generator", who is providing it and who wrote the specs? If not done properly you will end up with a noisy ass construction generator as Tim pointed out as nobody understands power. You have to spec Honda (or equivalent) inverter generator with a continuous operating capacity of 6500watts. It must be inverter, period. Unless you spec a larger diesel generator that have large mechanical flywheels to maintain regulation.
Don't buy extension cords. They are not rated for this use and very unprofessional. Nothing screams amateur like an orange extension cord. No matter what you do you should have a small case of 12GA SOOW rated feeder cable. I keep ours cut to 50's lengths, you may find 25' more convenient. Make your own with this cable using high quality industrial ends. Don't forget to gaff the couple. You need to route to not have trip hazard. Rubber matts, Gaff tape and cable trough are all tools that you may need.
If there is a 240v volt 4 conductor (2 hots, neutral and ground) with 125' of the stage just rent 150' of feeder, cable ramps and a lunchbox distro from a local production house. You should have a relationship with a local house to rent additional capacity as needed anyway.
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Yes what do you mean "contracted for a 6500 watt generator", who is providing it and who wrote the specs?
My Inner Cynic translates that to: "6500 watt contractor generator." The kind the snow cone concession trailer uses to drown out the ice grinder...
I'd love to be wrong about this. Art N. has been around long enough but his client may not have been. ;)
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Option 4-
Ensure it’s a whisper INVERTER generator or the show is ruined. Straight up.
Once you’ve formed that up, just put mixer near stage and go with a tablet.
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How much available power is 125ft away? 1 20amp circuit or 200amp 3phase? Or something in between?
Maybe you could borrow or rent a distro. with a long enough feeder cable that will handle your application.
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/Snip...
Don't buy extension cords. They are not rated for this use and very unprofessional. Nothing screams amateur like an orange extension cord. No matter what you do you should have a small case of 12GA SOOW rated feeder cable. I keep ours cut to 50's lengths, you may find 25' more convenient. Make your own with this cable using high quality industrial ends. Don't forget to gaff the couple. You need to route to not have trip hazard. Rubber matts, Gaff tape and cable trough are all tools that you may need.
\snip
Note that the NEC limits the number of connections in a cable run.
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Note that the NEC limits the number of connections in a cable run.
Can you expand please Rob?
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I have Socapex cable that I use for situations like this. I gives me six runs of 12 gauge wire in one easy to run cable. Even if I don't need that many circuits this allows me to distribute my loads across the runs, dropping my current draw on each. Even if I don't have 6 circuits available, I can double up the runs on a duplex, still distribute my loads across multiple runs and the net result is really the same as running larger gauge cords. Socapex still remains fairly expensive to buy but, should be easily rented from any company that does lighting.
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Can you expand please Rob?
Sure. In the case where you are feeding a distribution box (such as a spider box from a generator), 2008 NEC 520.53J defines the number of connections for various lengths.
For example, a 100’ length could be made with 2 50’ cables as it meets the 3 connections for 100’ or less. That would be one at each end and the one in the middle. Two 25’ & a 50’ would not meet code.
A 150’ could be made with 3 50’. A 200’ would require at least one 100’ so a collection of 50’ cables can’t do a 200’ run and be within code.
On the output side of a distro, I am not aware of the rules as I don’t run long runs of short cables.
Does that explain?
Edit to include version of NEC
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Rob,
In the 2017 Code that reference is 520.54 (h).
Doug,
Keep in mind with , multi conductor cables you need to derate for the number of load carrying conductors. If you have individual circuits, that is 12 conductors which translates to a de-rate of 50%. It is still useful to reduce the voltage drop by spreading the load, but once you gave over 9 conductors the penalty is pretty severe.
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You should have a relationship with a local house to rent additional capacity as needed anyway.
This is the advice that comes with experience or hanging out here. Better to set this up before you need it. It can also result in some work if they send some smaller work your way.
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Well darn, I thought sure that this had something to do with a vintage Hammond organ!
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My Inner Cynic translates that to: "6500 watt contractor generator." The kind the snow cone concession trailer uses to drown out the ice grinder...
I'd love to be wrong about this. Art N. has been around long enough but his client may not have been. ;)
Well, I requested a large, quiet diesel. And this is what I got. They tell me that is is a whisper inverter generator. It's arriving sometime today. I may go check it out just to make sure of what I have. It's being supplied by one of the event committee members whose company rents generators.
Thanks for all of your input.
Art
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That was enough info to practice the google-foo.
MQ has a Studio series with a 7kW gas unit. Appears to be a Honda eu7000 with entertainment-grade transportation and handling accessories.
http://www.multiquip.com/multiquip/PSG7AT1.htm
MQ also has a diesel 7kW rig in their Super Silent line:
http://www.multiquip.com/multiquip/DA7000SSA2.htm
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If I'm running any generator, I'd expect that it would need to be placed at least 75 feet from the stage. So all you're eliminating by having a generator is 50 extra feet of feeder cable.
While feeder cable certainly is expensive, it's something that you buy once and is good until you die. If power is available 125' away, I'd pretty much always prefer that over a generator!