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Author Topic: What do do when the AC power sucks  (Read 4303 times)

(Brian) Frost

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What do do when the AC power sucks
« on: December 21, 2004, 01:45:12 am »

I did a gig last weekend at a newer built ballroom which just had crappy power.  It initially read 112V.   I plugged in 18 amps worth of light to a circuit.  Power dropped to 101.  My mains amps were on a 20 amp reading 114, under load it was dipping to the high 90s.  I ended up putting all the amps on their own circuit and still couldnt see more than 105 Volts on a big vocal or bass note.  It sounded ok I guess but I saw clip lights come on a lot more often than Im used to.

What do you do in this situation.  I had 8amps max on each circuit and it still wasnt stiff enough to give full voltage.

Crappy power sucks
Frost
Owner, Narnia Producions
Chicago IL
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Frost

Owner, Narnia Productions

www.NarniaProductions.com

Chicago IL


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Dave Lowum

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Re: What do do when the AC power sucks
« Reply #1 on: December 21, 2004, 10:03:15 am »

Couple of questions for you...

A) Which venue were you in?  I'm familiar with a bunch of them in town...

B) What was the power distribution scheme?  Wall outlets?  House spider box?  Your panel?  What size feeder?  Lots of orange extension cords (I know, but I have to ask...)?

There are lots of reasons for excessive voltage drop, but the root cause is too much distance on too thin a wire...  Newer construction  is generally better, since 15A circuits are almost never installed, you're at least looking at 12ga wire, but an older venue with sub-panels all over the place on heavy feeder will react better than the newer venues with the "electric room" in the basement feeding a 20 story building...



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Dave Lowum
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Jason Phair

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Re: What do do when the AC power sucks
« Reply #2 on: December 21, 2004, 10:24:40 am »

I'm not sure I've ever dropped below 100 without tripping a breaker, and I've been in some BAD venues.
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Jason Phair

Dave Stevens

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Re: What do do when the AC power sucks
« Reply #3 on: December 21, 2004, 03:25:31 pm »

Lower voltage or greater voltage drops in and of themselves doesn't necessarily mean "crappy power".  As another poster indicated, if you are using 16AWG extensions and drawing current toward the max rating, you'll get a voltage drop greater than you are acustomed to having.  It's Ohm's Law, the higher the load and greater the resistance, the more pronounced the voltage drop.  NEC recommends wire sizing a branch so that there is only a 3% drop(NEC 210.19).  General rule in AC power is the bigger the wire, the better.

How did you measure the voltage?  If it was with one of those rack mount power strip/ light units, forget it, those things are notoriously innaccurate.  I would expect some voltage drop and you are running the wall circuits to almost max, which is never a good idea on a shared local circuit.  If you have your own distro it's one thing, if you are using shared provided outlets it's difficult to ascertain what the real issue might be.  It could be that on the supply side the wiring or other factors in the ballroom were causing that and if you combined that with small gauge orange AC extentions and power strips it's only compunding the problem.

Using wall power for the kind of load you wish to draw can work at some or even most places, but not every place.  There comes a time when one has to look at the big picture and invest in a small distro and use it in situations where the wall power is suspect or where a disconnect is available.

If your AC cablling and config is good unless you can get a genset or other alternate power source there is not much you can do when you come across suspect power.

Dave
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Charles Johnson

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Re: What do do when the AC power sucks
« Reply #4 on: December 21, 2004, 05:47:27 pm »

Dave wrote on Tue, 21 December 2004 15:25


How did you measure the voltage?  If it was with one of those rack mount power strip/ light units, forget it, those things are notoriously innaccurate.



Dave,

While a Furman/Juice Goose/etc rack-mount power strip units isn't the tool to use when it comes to Officially Metering The AC Voltage, I'd just like to point out that I think that their tolerances are close enough to allow for relative metering - ie the original poster noted that one of the circuits initially metered >110V, but under load metered <100V. Even allowing for a +/-2V tolerance rating (on my Furman PL-PLUS units, the LEDs are in 2V increments), one can clearly see that something funky is going on with the power! (or shall we say "overall power distribution system"? Smile)

Anyhow, just wanted to politely point out that the AC voltage meters on the rack-mount power strips aren't totally useless when it comes to acting as a measurement tool (of sorts) Smile

Respectfully,

Charles Johnson
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(Brian) Frost

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Re: What do do when the AC power sucks
« Reply #5 on: December 22, 2004, 04:11:44 am »

The venue was actually a radison in green bay WI,  Ive never seen power this bad in Chicago, except at the Baxter Labs campus.  

The did have a distro but we were on a riser stage on the other side of the room.

There was no other twist lock distro.

There were 3 circuits on the wall close by, so I used them plugging in with my own 12 guage.  Between sets I borrowed a bunch of long orange extensions to add 3 more circuits.  That way I never had a really hard load on any circuit.   I still had voltages which dropped by 10 volts on big bass notes or big vocals.  

To be honest, it didnt sound bad, but I hate seeing clip lights and I know the system can do a lot more, especially the subs.

Frost
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Frost

Owner, Narnia Productions

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Chicago IL


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Jim Brooks

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Re: What do do when the AC power sucks
« Reply #6 on: December 22, 2004, 09:13:49 am »

I run across this quite often.
First of all, all of my equipment is designed to run on separate 110 circuits with Edison connectors. The amp rack requires two circuits, the dimmer packs that need two circuits have two 110 cables for example
.
All of the sensitive equipment, FOH, monitor equipment, wireless equipment, etc.(no power amps or dimmers) are powered from a Furman voltage regulator. No brown out to worry about this way.

I typically run my 50 amp ,125/250 volt distro. I have all kinds of adapters and 200 feet of 6/4 SO cable. I try to be as prepared as possible so if 220 is avail. I can use it.

If that's not avail, i look for 110 house circuits.
I'll test the circuits with four 1000 watt lights (4000 watts total).
A typical 110v house circuit is designed for a load of just under 2000 watts. I plug all four lights into a house circuit.
I wait to see how long it takes the house breaker to trip.
If it takes a long time, it must not have much of a load on it. That's where I plug the amps. Now I look, for the next active circuit and do the same test. I also now know I'm on a second separate circuit.
If it doesn't trip, and I've had that happen too, move on and don't use that circuit.
Once I have all the circuits I need, I get the house guy to reset his breakers.  This gives me a chance to know where the breakers are, and to mark them if I want.

Your racks do have to be wired correctly both electrically and with your inputs and out puts. If they aren't you'll get ground loops.
For example, the input to my amp rack is transformer balanced and wired so it doesn't matter where the amp rack gets it's power.

Hope this helps,

Jim Brooks



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Dave Stevens

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Re: What do do when the AC power sucks
« Reply #7 on: December 22, 2004, 03:18:05 pm »

I've seen some of those rack units vary by as much as five volts and/or not responding to transients and peaks making the problem look much worse than it is or masking short duration spikes or dips.  It does give you an indication that the power is changing at which point you should break out your voltmeter and check it out.  Another thing the rack units won't do is meter potential between neutral and earth or earth and hot.  That can also be handy info to have as well.

They're OK as a reference but the accepted practice in the biz would be to use a DVM to troubleshoot any issues.  At the point where one was to think there was an issue would be the time to start using a DVM.

Dave
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Re: What do do when the AC power sucks
« Reply #7 on: December 22, 2004, 03:18:05 pm »


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