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Author Topic: Service/distro sizing  (Read 585 times)

Stephen Swaffer

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Service/distro sizing
« on: December 22, 2013, 10:36:11 pm »

Is there a "rule of thumb" you use to determine service/distro ampacity?  Or, how many amps do you run on a 20 amp circuit before you start to get concerned?  As an electrician, I know that I can put 2400 watts of heat/lighting on a 20 amp circuit before I trip a breaker and I should only load the circuit to 80% for a continuous load, but I routinely run a trio of QSC 1450's plus mixer/processor/etc on a 20 amp circuit and have never tripped one-even when I push things.

Just curious how to gage for future use-I am often in a position to install what is needed-but just doing more and throwing money at a project isn't the right way either-nor do I really like the idea of plugging stuff in until it starts to trip breakers and backing off!

I assume that given the variety of program material there is no hard and fast rule...
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Mike Sokol

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Re: Service/distro sizing
« Reply #1 on: December 22, 2013, 11:08:59 pm »

I assume that given the variety of program material there is no hard and fast rule...

Now I'm not an expert on power amp design, but I'm a student of tripping circuit breakers. I'm sure others here will pitch in with more details, but here's my spin on this.

I would say that your two main factors to consider are #1) amplifier power supply technology and #2) music crest factor.

#1) Old school amplifiers used full-wave bridge rectifiers on a big transformer with big storage capacitors. These full-wave bridge rectifiers only drew power from the 60-Hz power line on the peaks of the sine wave. And those peaks could trip a circuit breaker pretty easily. Plus the output transistors were often biased in class AB2 mode (if memory serves), which causes a lot of output transistor heating as a trade-off to better linearity. However, modern power amps using Class-D circuity draw power during more of the 60-Hz sine wave, and thus can avoid the peak amperage draw of old-school linear amplifiers. Plus transistors in Class-D (On/Off) mode have less waste heat produced, drawing less power for the same program material.

#2) Different music will have wildly different crest factors (ratio of the peak level divided by the average level). So if you're doing reggae or heavy metal, the crest factor can be quite low, and the average level of the music will be very close to the peak level your amplifiers can produce. That's when you'll be tripping circuit breakers. This crest factor is also reduced by processing such as dynamic compression, which ups the average of vocals and bass guitars. Other music, such as jazz, will have a very high crest factor with lots of transients. In that case you could run many thousands of watts worth of amplifiers from a single 20-amp service. The peak power is there to give that style of music its dynamics, but doesn't need much average power from the wall outlet. In any event, a power amplifier reproducing music will rarely be subjected to producing full RMS power, except for perhaps some club tunes. Your mileage will vary, as they say in the auto biz.

I did a few experiments with amplifiers and clamp-meters in my youth, but not enough to really learn anything useful. However, with today's modern clamp meters from Fluke and Amprobe (and others) it would certainly be interesting to monitor the RMS amperage draw from your amp rack. Just remember that you can't simply clamp an ammeter around the power cord. You need a special cable with the hot and neutral separated so that the line amperage doesn't cancel itself out. I don't have time to do such a study right now, but it sure would be interesting. 

TJ (Tom) Cornish

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Re: Service/distro sizing
« Reply #2 on: December 22, 2013, 11:13:35 pm »

Is there a "rule of thumb" you use to determine service/distro ampacity?  Or, how many amps do you run on a 20 amp circuit before you start to get concerned?  As an electrician, I know that I can put 2400 watts of heat/lighting on a 20 amp circuit before I trip a breaker and I should only load the circuit to 80% for a continuous load, but I routinely run a trio of QSC 1450's plus mixer/processor/etc on a 20 amp circuit and have never tripped one-even when I push things.

Just curious how to gage for future use-I am often in a position to install what is needed-but just doing more and throwing money at a project isn't the right way either-nor do I really like the idea of plugging stuff in until it starts to trip breakers and backing off!

I assume that given the variety of program material there is no hard and fast rule...
Lighting is generally sized at 100% capacity, using fully-rated breakers in the dimmer rack - i.e. 4 575w bulbs fit on a 20A circuit.  Several NEC sections relate to sizing for dimmers.

Normally audio is specified at the 1/8 power rating of the amp, which represents using the amp to occasional clipping - plus or minus a guesstimate of the application for the system. 
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Lyle Williams

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Re: Service/distro sizing
« Reply #3 on: December 23, 2013, 03:30:57 am »

"Green" power meters are only about $20 here.  The will tell you peak and average power, current, PF, dollar cost of the electricity, etc...  I wouldn't leave a cheap consumer device in circuit all the time, but a few measurements during system setup will give you an understanding of how your system behaves.
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Mike Sokol

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Re: Service/distro sizing
« Reply #4 on: December 23, 2013, 07:36:25 am »

"Green" power meters are only about $20 here.  The will tell you peak and average power, current, PF, dollar cost of the electricity, etc...  I wouldn't leave a cheap consumer device in circuit all the time, but a few measurements during system setup will give you an understanding of how your system behaves.
Something like this Kill-A-Watt could work. I don't know if they're smart enough to give you a true RMS value with a Class-D amp, but they should give you a pretty good circuit breaker trip threshold. I've got one in my NSZ rig, so the next time I have a PA system running I'll plug in a few amps and see how much power draw they show at various room levels. Would be some interesting data.
 

TJ (Tom) Cornish

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Re: Service/distro sizing
« Reply #5 on: December 23, 2013, 08:39:36 am »

Something like this Kill-A-Watt could work. I don't know if they're smart enough to give you a true RMS value with a Class-D amp, but they should give you a pretty good circuit breaker trip threshold. I've got one in my NSZ rig, so the next time I have a PA system running I'll plug in a few amps and see how much power draw they show at various room levels. Would be some interesting data.
I'm curious how fast the peak measurement is, and/or if it integrates the current draw over time, like the heating element in the breaker.  Amps with power factor correction are a lot easier to measure than conventionals, though I'm not sure if PFC always correlates with being Class-D - IIRC a few old dogs like the bigger QSC Powerlights had PFC correction with some kind of class H topology, and I don't know if the new cheapies like the Peavey class D amps have PFC functionality.
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Mike Sokol

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Re: Service/distro sizing
« Reply #6 on: December 23, 2013, 08:42:54 am »

I'm curious how fast the peak measurement is, and/or if it integrates the current draw over time, like the heating element in the breaker.  Amps with power factor correction are a lot easier to measure than conventionals, though I'm not sure if PFC always correlates with being Class-D - IIRC a few old dogs like the bigger QSC Powerlights had PFC correction with some kind of class H topology, and I don't know if the new cheapies like the Peavey class D amps have PFC functionality.
I have a high-end clamp-meter with true RMS capability, so it would be really interesting to compare it against a Kill-A-Watt on a few different power amp technologies. Now if I can only find the time....
« Last Edit: December 24, 2013, 02:12:05 am by Mike Sokol »
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Cailen Waddell

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Re: Service/distro sizing
« Reply #7 on: December 23, 2013, 12:41:02 pm »


Lighting is generally sized at 100% capacity, using fully-rated breakers in the dimmer rack - i.e. 4 575w bulbs fit on a 20A circuit.  Several NEC sections relate to sizing for dimmers.

Normally audio is specified at the 1/8 power rating of the amp, which represents using the amp to occasional clipping - plus or minus a guesstimate of the application for the system.

An important note is that while lighting loads on the dimmer side are typically setup for 100% capacity, that is rarely the case on the service feed side.  For a small example, there are many 4 x 1.2k dimmer packs that only have a 20A inlet total. In touring land, an ETC 96 rack is typically fed with a 400a 3 supply, but if fully loaded would need a 640a 3 supply. On the large side in an install, a theater with 2 96 racks, a 48 rack, and 48 relay circuits, all 20a would in theory need a 1920a 3 supply but in reality is set up with a 1200a 3 supply.  Generally on installations the consultant, installer, and electrical engineer work together to determine what would be appropriate.

In sound land, sometime inrush current makes the decision.  One place I worked had 6 20a circuits at amp world however 3 would of been fine if everything wasn't turned on at the same time, but that couldn't be guaranteed, and it was easier and cheaper to sequence it together, apparently.

The last portable system I setup, we used manufacturer 1/8th power, stereo, 4 ohms, power draw.  Probably overkill, but maybe not.  We sized power distribution to be able to accommodate this load and ended up with 2 L14-30 distros. Which makes for a 60a 120/240v split phase supply.   We did some further testing and determined, with no subs, and just a pair of stereo monitors onstage, we could easily daisy chain the 2 l14-30 distros and run off of one  30a 120/240v plug. 
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Lyle Williams

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Re: Service/distro sizing
« Reply #8 on: December 23, 2013, 10:06:04 pm »

The unit I have (which is from before the current wave of green initiatives) samples 8000 times per second.
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