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Author Topic: Measure Current draw - Australia/230V  (Read 8373 times)

Callan Browne

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Measure Current draw - Australia/230V
« on: June 30, 2015, 11:42:57 pm »

Hi all,
Is there an easy way to measure the current draw of a PA system? A quick search suggested using a multimeter with the cables exposed, which might be fine for some, but I'd rather take a risk-adverse, passive approach if possible.

We've taken a pretty relaxed view in the past, but I am starting to get some more powerful gear and want to be better understand the requirements, to ensure we're not caught short mid show and potentially pop the 10Amp circuit breaker.

Our stage now consists of;
2 x JBL Active 15's (VP7215/95DPC- 875w), 1 x JBL 18" Active PRX-618s-XLF Sub (1000w), 2 x JBL Active PRX 615's (1000w.. on paper) as Foldback, 2 x tube amps (Marshall 50 + Mesa 100w), 1 x bass amp (~300w?), a small analog mixed and some LED lights (<5).
The VP speakers are new, replacing 2 behringer foldback's.

Q; what's the best way to determine how much our system uses at it's peak and can I achieve this during our next show?

Thanks,
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Mark Cadwallader

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Re: Measure Current draw - Australia/230V
« Reply #1 on: July 01, 2015, 01:15:20 am »

The typical "rule of thumb" is to look at 1/8 of the maximum rating of the amplifiers to estimate the typical power draw for live music. The PRX615 has dual 500 watt amplifiers, but the tweeter probably is limited to about 50 watts or thereabouts.

To make measurements, you can buy commercially-available AC mains line splitters which allow you to use a clamp type ammeter to measure the load as it is being used.  A person can make one on her/his own, but it sounds like you don't want that option.

In the USA, there is a device which is inserted in-line with the AC mains cord of the device in question which reads the draw in watts. I have not looked to see if a non-NEMA version is available. The device is sold under the name "Kill A Watt".

I'm sure that others will chime in, too, with their suggestions.
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Kevin Graf

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Re: Measure Current draw - Australia/230V
« Reply #2 on: July 01, 2015, 10:19:08 am »

This paper examines 9 meters similar to the Kill-a-Watt:

"Comparison of End‐User Electric Power Meters for Accuracy"

http://www.hiit.fi/files/admin/publications/Technical_Reports/hiit-tr-2009-1.pdf


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Lyle Williams

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Re: Measure Current draw - Australia/230V
« Reply #4 on: July 01, 2015, 02:28:32 pm »

This paper examines 9 meters similar to the Kill-a-Watt:

"Comparison of End‐User Electric Power Meters for Accuracy"

http://www.hiit.fi/files/admin/publications/Technical_Reports/hiit-tr-2009-1.pdf

The takeaway from that paper is that these cheap meters aren't great at low current measurements, but do pretty well at higher current measurements.
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Mark Cadwallader

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Guy Holt

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Re: Measure Current draw - Australia/230V
« Reply #6 on: July 01, 2015, 04:54:52 pm »

We have a winner!

Not so fast. Where some amplifiers and most LED lights are not power factor corrected (non-pfc),  you will want to make sure you get a meter that reads true RMS. In an introductory workshop my local, IATSE Local 481, offers for set electrics we do an exercise where the students meter the voltage and current on a putt-putt generator (non-inverter type) while running a non-pfc 2.5kW HMI light.   Since, invariably, the meters brought by the students range in quality, the readings they get range from being 84% over to 40% under what they should be. We then so the same exercise with tungsten lights and they all read the same. The discrepancy in the results is a good jumping off point for a discussion about how meters work and that they can be mislead by the distorted waveforms drawn by non-linear loads. Since the consequences of under measurement can be significant - overloaded cables may go undetected, bus-bars and cables may overheat, fuses and circuit breakers will trip unexpectedly - it is important to understand how meters work and why only meters based on "true RMS " techniques should be used on power distribution systems supplying nonlinear loads. To see why that is the case use this link: http://www.screenlightandgrip.com/html/emailnewsletter_generators.html#anchorMeters

Guy Holt
ScreenLight & Grip
www.screenlightandgrip.com
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Lyle Williams

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Re: Measure Current draw - Australia/230V
« Reply #7 on: July 01, 2015, 05:34:59 pm »

I don't have the meter I linked to, but I do have a similar one and it does RMS.  They also measure PF, but don't tell you if the voltage leads, lags, or is just chopped to hell.  From memory the one I have takes 8000 current and voltage readings a second and calculates from that.
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Mark Cadwallader

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Re: Measure Current draw - Australia/230V
« Reply #8 on: July 01, 2015, 05:36:52 pm »

Guy, I don't disgree with your recommendation to use a "true RMS" meter.  Given that the OP expressly stated that he did not want go probing around with a multimeter, I respectfully suggest that for the purposes of the OP, a simple gadget such as Lyle linked to is probably adequate.  If the question is what is the most accurate way to determine peak current draw, then the use of suitable calibrated equipment that will log peak measurements is the way to go.
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Guy Holt

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Re: Measure Current draw - Australia/230V
« Reply #9 on: July 01, 2015, 06:39:57 pm »

..a simple gadget such as Lyle linked to is probably adequate.

An under measurement by 40% by one of these cheap “average reading, calibrated RMS” meters could lead to popping a breaker in the middle of a show. As you said yourself: "Good tools are expensive, but cheap tools are damned expensive."

Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com
« Last Edit: July 01, 2015, 06:45:53 pm by Guy Holt »
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Re: Measure Current draw - Australia/230V
« Reply #9 on: July 01, 2015, 06:39:57 pm »


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