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Author Topic: Chasing power again....  (Read 5039 times)

Mike Monte

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Chasing power again....
« on: January 01, 2018, 08:34:33 AM »

I am in the process of putting together a bid for a small multi-band festival at my local YMCA.
I spec'd a generator to power the stage.

At a recent meeting with the Y the custodian said that instead of generator power he can supply the power from a snack-shack located 150' from the stage area.
(The custodian is a former construction worker and he says that he has 150' of 8 gauge extension cable....He told me that he could bring one dedicated 20 amp circuit at the stage..)

I also have 150' of 12/3 power cable.

I can make do with one 20 amp circuit for the FOH/monitors but would like to have one additional 15/20 amp circuit for the backline, etc.
(This is a daytime event.)

What is the power loss for a 20 amp circuit over 150' of 12/3?   (There's got to be a table somewhere...)

Here I go again....
Chasing power seems to be the bane of my existence.
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Lyle Williams

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Re: Chasing power again....
« Reply #1 on: January 01, 2018, 08:48:35 AM »

The issue isn't power loss.

The issue is fault impedance.

If you short live to ground at the stage, how long will it take the circuit breaker to trip with the extra 1/2 ohm of copper (150'x2) in the path.

12G appears very close to being ok over that distance.  In my country I'd use 4mm2.
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Erik Jerde

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Re: Chasing power again....
« Reply #2 on: January 01, 2018, 09:10:53 AM »

Voltage drop is completely dependent on your load.  150' at 20A requires 6ga to keep below 3% loss.  At 10A you need 10ga.  Only at 6A do you get to 12ga.  Of course with higher acceptable drop the numbers change.  Google voltage drop calculator and you should get results for how to look up this stuff on your own.

Keep in mind that these calculations don't take into consideration the voltage drop in the wiring to the snack shack.  If you've got 100' of 12ga just to get that 20A ckt there then things become more problematic.

Lastly I'd be concerned about the construction of a 8ga cable with 20a connectors.  Many 20A won't accept 8g 12/3 soow cable with the jacket intact.  I haven't tried it myself but I'm skeptical of the ability to fit 8ga into the terminals intended for 12ga.  Remember if strands are trimmed or otherwise not properly inserted then it's not really an 8ga cable and you've got a potential problem point.

The proper generator seems like a much better and more reliable option.  Plus it's under your control should any issues arise.
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Keith Broughton

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Re: Chasing power again....
« Reply #3 on: January 01, 2018, 09:31:34 AM »


The proper generator seems like a much better and more reliable option.  Plus it's under your control should any issues arise.

It won't hurt to have the extra shore power but use a genny for the band and PA.
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William Schnake

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Re: Chasing power again....
« Reply #4 on: January 01, 2018, 09:44:58 AM »

Here I go again....
Chasing power seems to be the bane of my existence.

Mike, IMHO it actually comes down to how much risk and reputation you are willing to sacrifice.  You might be able to get enough power out of the snack-shack or you might not.  It is one thing to be able to power the system up and play a track.  It is a totally separate game to be able to properly power a rig and then add a band playing.

example:
You are playing music everything is great.  Band with two guitars, base and drums is ready to play.  Now you are adding not only more volume through the house system, but all of the monitors are working and you have added three guitar/bass players.  Your power draw goes up significantly and the system shuts down.  The audience looks at you, the band looks at you and the people who hired you look at you and all are not happy.  All you can do at that point is wish you had a correctly sized generator.

It doesn't take long for a failure to get around to other clients.  Do me a favor and get them to get you a generator so I can sleep at night.

Bill
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Ivan Beaver

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Re: Chasing power again....
« Reply #5 on: January 01, 2018, 09:59:38 AM »



What is the power loss for a 20 amp circuit over 150' of 12/3?   (There's got to be a table somewhere...)


The "problem" is that you are dealing with a dynamic signal, NOT lights or heaters or fans.

The actual current draw will vary with the music, how loud the players are, dynamic range of the music etc.

Yes, the backline will draw some idle current, tha is easy enough to figure out IF you know the specific gear being used.

But the music signal current draw will be constantly changing, so much harder to figure out.

It also depends on the particular breaker being used, how often it has been tripped already etc.  Some trip faster, earlier than others.

YES, it gets very complicated when you try to get specific.

NOW, lets just make it a bit more complicated.

YOUR cable is 150' 12ga.  How long and what gauge is the cable from the breaker to the outlet you are plugging into?

THAT is the total length of the cable over which there will be loss, NOT just your final piece--------------

Fun isn't it :) :)
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Stephen Swaffer

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Re: Chasing power again....
« Reply #6 on: January 01, 2018, 10:00:22 AM »

One option would be to actually load test ahead of time. Take a 15-20 amp load-PAR cans or an electric heater or two (amps=watts/volts), measure the voltage unloaded and then measure it loaded.  Ideally you should be within 3-5%.

The real concern will be how low your voltage drops and what your gear will be OK with.  If the supply voltage is already on the low side, voltage drop will be more of an issue.  Also, music can be very dynamic-depending on music you may have peak draws greater than 20 amps that will drive your voltage drop even higher.

I agree a genny is still a good option-but an actual load test will get you better answers than anyone can give you sight unseen.  You don't have to string out the cable or anything just hook it up coiled load it and see how it does.

Most load tables for wire will assume worst case, full load draw and are designed for safety which is a heating issue.  Function for sound gigs is a voltage drop issue which is a different animal completely.
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Steve Swaffer

Keith Broughton

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Re: Chasing power again....
« Reply #7 on: January 01, 2018, 10:16:16 AM »

The Kill-A-Watt device (Google it)is great for testing actual current, wattage and voltage for a given device.
As Stephen suggested, put an electric heater on the end of your cable run and test the voltage.
Most heaters are in around the 1300-1600 watt range and draw about 12-14 steady amps.


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Steve Crump

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Re: Chasing power again....
« Reply #8 on: January 01, 2018, 11:35:30 AM »

If the snack shack is a point where a distribution panel is located and the proposed connection point is close to the panel, (of course that is considering that the whole system was originally installed and sized properly), you could probably use these simple voltage drop calculators.

http://www.southwire.com/support/voltage-drop-calculator.htm

Southwire also has a voltage drop app for iPhone and Android.
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Keith Broughton

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Re: Chasing power again....
« Reply #9 on: January 01, 2018, 11:40:08 AM »

(of course that is considering that the whole system was originally installed and sized properly),
There is why an real world test would be better.
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ProSoundWeb Community

Re: Chasing power again....
« Reply #9 on: January 01, 2018, 11:40:08 AM »


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