PA System Power - Overloading Circuits

S

Stephen Findlay

Freshman
Aug 17, 2012
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Sault Sainte Marie, Ontario, Canada
I have a simple (and maybe not SO stupid) question regarding powering a PA system for a venue:

When do you have to start worrying about overloading a circuit at a venue? I have never encountered that problem, but as I get more and more high-powered stuff, I do not want to have a breaker go during a live event. I am a DJ and am now considering purchasing a JBL SRX System (2 SRX 715's, 2 SRX 718's, and powered by a Crown MacroTech 9000i). Should I be concerned about too much amp draw since I am pushing it down to a 4-ohm load?

I heard Crown makes some of the most efficient amps out there and would love to cut weight by using only one amp.

Comments, suggestions on other power options. Thanks!
 
Re: PA System Power - Overloading Circuits

First get one of those little "Kill-a-Watt" meters (or similar) and get a handle on how much current you are using. When going to a new venue, never assume anything about it's AC power. A receptacle may be wired backwards or the may have a coffee pot on your circuit that they turn on after you set up.
 
Re: PA System Power - Overloading Circuits

Well using one amp would mean you be in mono as far as sound, not stereo. 1 side of the amp for subs, the other for mid highs, so i don't think the loss of stereo imaging would be worth the ease of using 1 amp.

I use the crest pro -200 series amps on everything now. I can tell you this much. I run two of the 8200's bridged on 1 20amp circuit running two double 18's. I have tripped a 20 amp circuit once with a guy running a double kick, and i was leaning on it pretty hard.

I run a 9200 in stereo on my mids, 2 15's per side, a 8200 on the horns, all my processing, along with the band, on another 20 amp, never tripped it yet.

I would look into 2 amps, Crest 8200 bridged into 4 ohms would give you about 4500w for low end. A 9200 in stereo on the top into 8 ohms would be 1300w into each box.

Are you buying new or used?
 
Re: PA System Power - Overloading Circuits

Alan Sledzieski said:
Well using one amp would mean you be in mono as far as sound, not stereo. 1 side of the amp for subs, the other for mid highs, so i don't think the loss of stereo imaging would be worth the ease of using 1 amp.

I use the crest pro -200 series amps on everything now. I can tell you this much. I run two of the 8200's bridged on 1 20amp circuit running two double 18's. I have tripped a 20 amp circuit once with a guy running a double kick, and i was leaning on it pretty hard.

I run a 9200 in stereo on my mids, 2 15's per side, a 8200 on the horns, all my processing, along with the band, on another 20 amp, never tripped it yet.

I would look into 2 amps, Crest 8200 bridged into 4 ohms would give you about 4500w for low end. A 9200 in stereo on the top into 8 ohms would be 1300w into each box.

Are you buying new or used?
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Hi Alan,

Thanks for the input and stereo would definitely be a plus with the two amps! I have a DriveRack PA+ so I am sure it will be put to good use with the new amps. I am unfamiliar with Crest but will look into it. At this stage of the game, I am just looking into options since one of the venues I do is rather large and am looking to push more air for events. So used/b-stock would be my best option since I don't know how often I would use this setup, but who knows, I may love the sound of a bigger system. Your real-world example has given me a great benchmark and probably would not push it that hard. Thanks again.
 
Re: PA System Power - Overloading Circuits

Put each of those amps on it's own circuit within the same leg and ease your worries. Or, if you are going Stereo 3 way: put the mids and highs on one circuit and the subs on it's own. It's cheaper than a purchasing a power distro and less liability. You should need to know where the circuits are located amongst the outlets and be certain that just because you are using two different outlets that they are indeed not on the same circuit. Fortunately those amps have a power switching supply and (someone correct me if I'm incorrect) they are built to perform based on the power they receive.

These days I only carry a volt-ohm meter to check the amount of wall power and one of those little simple circuit testers that quickly show the wiring of the outlet. I would love to have one of those circuit sniffers for finding circuits but haven't gotten that far yet.
 
Re: PA System Power - Overloading Circuits

We don't know the places you work in, so a "3 Edison duplex outlets on the wall" in the club or event center might be on the same breaker... or separate breakers. You don't know what else might be on the same breaker.. the ice machine, the blender, the frozen drink dispenser, coffee maker... and that makes it even more fun.

If you can find 3, 20 amp circuits your audio life will be good.

There are choices in lightweight, efficient amplifiers these days. We've had good luck with original I-Tech and XTi-6002. The QSC PL380 is a good amp, too. No direct experience with the Crest *200 series but another regional has over 100 of them and are happy with them.

As for the occasional work in a large space, I have this business thought: excess capacity is a 100% expense when it is not being used. Owning gear to use only a couple of times a year isn't prudent if that gear can be rented locally when you need it. Invest your funds in things that customers are willing to pay extra for on a consistent basis, or that give you a significant competitive edge that results in additional work or revenue streams you would not have gotten otherwise.
 
Re: PA System Power - Overloading Circuits

Kevin Graf said:
First get one of those little "Kill-a-Watt" meters (or similar) and get a handle on how much current you are using. When going to a new venue, never assume anything about it's AC power. A receptacle may be wired backwards or the may have a coffee pot on your circuit that they turn on after you set up.
Click to expand...
What is the response time of the Kill-A-Watt? Audio power draws are spikey - during kick hits or bass notes the draw goes up a lot - much higher than the average power. Depending on how the Kill-A-Watt is designed, it could significantly under-report the short-term usage if it's too slow to catch the peaks.
 
Re: PA System Power - Overloading Circuits

Alan Sledzieski said:
Well using one amp would mean you be in mono as far as sound, not stereo. 1 side of the amp for subs, the other for mid highs, so i don't think the loss of stereo imaging would be worth the ease of using 1 amp.

I use the crest pro -200 series amps on everything now. I can tell you this much. I run two of the 8200's bridged on 1 20amp circuit running two double 18's. I have tripped a 20 amp circuit once with a guy running a double kick, and i was leaning on it pretty hard.

I run a 9200 in stereo on my mids, 2 15's per side, a 8200 on the horns, all my processing, along with the band, on another 20 amp, never tripped it yet.

I would look into 2 amps, Crest 8200 bridged into 4 ohms would give you about 4500w for low end. A 9200 in stereo on the top into 8 ohms would be 1300w into each box.

Are you buying new or used?
Click to expand...
Advice such as this is about as good as it gets - anecdotal experience in combat audio situations.

My situation is similar - I have a PLX3402 running a stereo 4-ohm load and a PL236 running a bridged mono load into 4 ohms. I know from experience that under moderate load - no clipping, leaving a little on the table, I'm fine on 1 20A circuit for these 2 amps. If I'm running hard, then a 20amp circuit is marginal - 2 circuits is a lot safer - especially if there's a band with monitor wedges and backline.
 
Re: PA System Power - Overloading Circuits

TJ Cornish said:
What is the response time of the Kill-A-Watt? Audio power draws are spikey - during kick hits or bass notes the draw goes up a lot - much higher than the average power. Depending on how the Kill-A-Watt is designed, it could significantly under-report the short-term usage if it's too slow to catch the peaks.
Click to expand...
No where near as fast as an audio signal. But a thousand times faster than a circuit breaker.
 
Re: PA System Power - Overloading Circuits

Kevin Graf said:
No where near as fast as an audio signal. But a thousand times faster than a circuit breaker.
Click to expand...

Breakers work based on heat generated by the load. A very short burst above 20 amps may not trip the breaker, but with enough of them, over time, it will trip. In my small racks, I like to have 15 amp breakers and design my current load based on that. If a breaker does trip, it's likely going to be in my rack, which is easy to reset. If I push right to the 20 amp load limit, and something else is on the circuit that I'm not aware of, then I have to go track down the breaker to get back up and running.

Now, you have to look at what amps you are using to know if they are going to trip a breaker or not. Reality is, you can't create watts. A 20 amp breaker means at 120 volts, you have 2400 watts available to you. With a single amp of that size, it's possible to blow the breaker on its own.

QSC has a good current consumption chart on its amps.
QSC - PLX3602 Specifications

This amp is roughly half the power rating of the Crown 9000. Both are fairly efficient amps, so I'm guessing the crown power draw would be roughly double. If you're running at 1/8 power, the PLX, at 4 ohms draws 11.5 amps. 1/8 power is generally just tickling the clip lights. 1/3 power would draw 19.5 amps, which is generally a solid clip light. If you go balls out with the amp, it can pull 40 amps from the wall, which will undoubtedly sound like crap and blow the breaker.

I often do run 2 3602's off of a single 20 amp breaker with no issue, as long as I stay completely out of clip territory. However, if its a show where I need to crank things up, I'll make sure to have 2 circuits available.

With the 9000i at 4 ohms, I'd want to make sure i have a good solid 20 amp service available for just that amp if I'm expecting to push any sort of real power with it. While it's a nice powerful amp, it really limits your ability to split your power draw if you don't have the full 20 amps available to you.
 
Re: PA System Power - Overloading Circuits

You can run a pretty decent setup on a single 20 amp circuit but any time you can split it to two circuits will be better.

Want to talk about real world measured experience, I ran a large rig a couple months ago with 8 X 3-Way tops, 16 X 2X18" subs plus 2 single 18"s and 2 12" tops as texas headphones. I was running single phase and if my memory serves me I think I was at ~50 amps on one leg and 45 on the other. This was with 6 Camco Vortex 6 amps, 3 QSC PL380s, 4 TCS TA2400, 3 Mackie 1400i amps. And this was for an Electronic Dance Music Concert with a very high profile headliner.

We just did a show last weekend with 12 3-Way Line array boxes, 4 2X12" subs, 18 2X18" subs, 4-way sidefills, and 10 wedge mixes. We were three phase and were pushing 65 amps on two of the legs and 35 on the 3rd leg. We had approximately 40 amplifiers in use on that show with a very high profile EDM DJ as the headline and about 12,000 people in front of the stage. That is a ton of gear on the equivelant of nine 20 amp circuits.

Even though the amplifier manufacturers will give you 1/8 and 1/3 power current draw figures, I highly doubt you'll ever get close to those figures in real world use unless you are just being plain stupid and destroying your gear.

I don't really think you need all the power of a MA9000i for the little system you are considering but you might want to look at a pair of Itech5000hds or a couple used Itech4000s and you would be good to go with plenty of horsepower, plenty of DSP and a system that will be very efficient with wall power.
 
Re: PA System Power - Overloading Circuits

I can only speak specifically on Homeline, which we've done tests on with a few different ammeters.
(working on getting better test equipment)

It's a fairly recent development. (2 years at most)
 
Re: PA System Power - Overloading Circuits

Philip LaDue said:
I can only speak specifically on Homeline, which we've done tests on with a few different ammeters.
(working on getting better test equipment)

It's a fairly recent development. (2 years at most)
Click to expand...

That's interesting - obviously Homeline has nothing to do with what pro-audio guys should be dealing with at shows.

I'm a fan of Square D or Siemens QOB bolt-in myself.

At shows when I think I'm short on power, I crank the heck out the system before the show starts and I go around and meter all my circuits. They are usually surprisingly low. The only issue I've ever had with wall power was with IT12000s running full-tilt.
 
Re: PA System Power - Overloading Circuits

Theoretically only after 3 hours of a continuous 20 amp current, a 20A breaker should trip. But that's at room temperature, like when you do your setup in the morning. Later in the evening when everything in the building is running full blast, the inside of the breaker box heats up and the trip current number goes down.
 
Re: PA System Power - Overloading Circuits

Silas Pradetto said:
That's interesting - obviously Homeline has nothing to do with what pro-audio guys should be dealing with at shows.

I'm a fan of Square D or Siemens QOB bolt-in myself.

At shows when I think I'm short on power, I crank the heck out the system before the show starts and I go around and meter all my circuits. They are usually surprisingly low. The only issue I've ever had with wall power was with IT12000s running full-tilt.
Click to expand...

While that's a good test, it's not always going to be accurate. If there is something on the same circuit that isn't turned on yet, you can still be in for surprises. Also, with more stuff on, breaker panels get warmer inside, lowering the trip point. And, depending on your system, you may be dealing with power compression by the end of the night, meaning you have to crank it even more than you did during setup to get the same output. If you're right on the edge, all of these things can result in blown breakers.

The amount of stress and effort to locate an extra circuit before the show starts is way less than scrambling when the only circuit you found trips mid show.
 
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