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Service/distro sizing

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TJ (Tom) Cornish:

--- Quote from: Mike Sokol on December 23, 2013, 07:36:25 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.

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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.

Mike Sokol:

--- Quote from: TJ (Tom) Cornish on December 23, 2013, 08:39:36 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.

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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....

Cailen Waddell:


--- Quote from: TJ (Tom) Cornish on December 22, 2013, 11:13:35 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.

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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 3Ø supply, but if fully loaded would need a 640a 3Ø 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 3ø supply but in reality is set up with a 1200a 3ø 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. 

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

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