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[Callan Browne]

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,

[Mark Cadwallader]

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.

[Kevin Graf]

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

[Lyle Williams]

http://www.jaycar.com.au/Power-Products-Electrical/Power-Monitoring/Power-Meters/Mains-Power-Meter/p/MS6115

[Lyle Williams]

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.

[Mark Cadwallader]

http://www.jaycar.com.au/Power-Products-Electrical/Power-Monitoring/Power-Meters/Mains-Power-Meter/p/MS6115

We have a winner!

[Guy Holt]

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

[Lyle Williams]

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.

[Mark Cadwallader]

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.

[Guy Holt]

..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

[Jonathan Johnson]

As you said yourself: "Good tools are expensive, but cheap tools are damned expensive."

As long as the user understands the limitations and risks of using cheap tools, and is able to determine that the cheap tool can do the job adequately, then a cheap tool is acceptable. A cheap cordless drill from Harbor Freight is probably fine for Jill Homeowner who is hanging pictures in drywall a few times a year.

It's when people expect cheap tools to do things that they cannot (or make invalid assumptions of precision or accuracy) that things get "damned expensive". That Harbor Freight drill is going to be "damned expensive" for the electrician that is using it all day long in his trade.

[Callan Browne]

Great, thanks for all the replies.
I'm not familiar with PFC and Non-PFC and will try and read up more on that over the weekend.

[Lyle Williams]

The cheapish "Watts Up?" meter I have seems to track broadly correctly against a $20,000 Dranetz when I tested against moderate loads (fan heater, kettle, computer.)

[TJ (Tom) Cornish]

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

Guy, once again I will argue that being aware of harmonic issues is nice, but the larger issue is the dynamic nature of audio itself. A small increase in output volume of 3dB, which corresponds to one or two clicks on your TV remote, requires TWICE the input power. In the heat of the gig it can be very easy to end up louder than you realize due to environmental factors. This is the reason for the 1/8 power and 1/3 power specs listed on audio gear. 1/8 power represents an estimate based on empirical testing of real music run with the clip light occasionally being hit.  1/3 power is what people who run solid into the red should use (during the brief interval between speaker repair visits).

Other factors of course are if the circuit is a 15A circuit, a shared 20A circuit, a dedicated 20A circuit (or the Australian equivalents) as well as the condition of the wiring, nominal voltage of the supply that day, etc. 

OP, your test is interesting in theory, but I am not sure how effective it will be in reality, as due to the above factors you need to leave some headroom on your circuits.  I would use the 1/8 power rating of your amps and only fill your circuits to 75% of nameplate capacity. As long as you stay out of clipping, this should be pretty safe. Your own empirical testing will give you the best data for how you tend to use things. 

[Mike Sokol]

OP, your test is interesting in theory, but I am not sure how effective it will be in reality, as due to the above factors you need to leave some headroom on your circuits.  I would use the 1/8 power rating of your amps and only fill your circuits to 75% of nameplate capacity. As long as you stay out of clipping, this should be pretty safe. Your own empirical testing will give you the best data for how you tend to use things.

It's the reality of the music Crest Factor that often gets you in trouble. For instance, acoustic oriented music typically has a very high Crest Factor (peak to average power ratio), so I can run a rather large sound system from a single 20-amp service. However, EDM, Rap and Reggae music typically have a lot of sustained bass, so unless I have a dedicated service of 50 amperes or more, I'll be constantly tripping circuit breakers with the same sound system. Also, while adding dynamic compression to the mix sounds great for a lot of music styles, it does tend to lower the Crest Factor, thereby increasing the average power requirements that the AC distribution system needs to supply. One of the things I like to do during sound check is push the sound system to its limits so I can see if the breakers will trip. If they do, then I may trim back the gain to the main amps to reduce the power a bit when my mixer hits 0 dB VU (or whatever your reference level may be). After a while you'll get to know what your sound system can supply from the breakers in a particular venue. If the circuit breakers trip, then you need to turn it down.

[TJ (Tom) Cornish]

It's the reality of the music Crest Factor that often gets you in trouble. For instance, acoustic oriented music typically has a very high Crest Factor (peak to average power ratio), so I can run a rather large sound system from a single 20-amp service. However, EDM, Rap and Reggae music typically have a lot of sustained bass, so unless I have a dedicated service of 50 amperes or more, I'll be constantly tripping circuit breakers with the same sound system. Also, while adding dynamic compression to the mix sounds great for a lot of music styles, it does tend to lower the Crest Factor, thereby increasing the average power requirements that the AC distribution system needs to supply. One of the things I like to do during sound check is push the sound system to its limits so I can see if the breakers will trip. If they do, then I may trim back the gain to the main amps to reduce the power a bit when my mixer hits 0 dB VU (or whatever your reference level may be). After a while you'll get to know what your sound system can supply from the breakers in a particular venue. If the circuit breakers trip, then you need to turn it down.

Mike, while this is true in one sense, it also works the opposite way.  As loudspeaker drivers make pretty lousy toasters, their capacity decreases as the crest factor decreases.  For example, a 4000w peak capacity subwoofer driver may have a "toaster mode" capacity of only maybe 200-400 watts. 

How this all shakes out in the end largely depends on whether the operator in question is an idiot or not (making no judgment about the OP or anyone else in particular here).  If the operator has a clue, the 1/8 power number should be pretty accurate for any musical genre.  The number of amplifiers and speakers required to do the show will heavily depend on the desired output and music content, but that should be fairly easy math:  Number of amps * 1/8 power rating.

If the operator is clueless, all bets are off, and it may become a race between which component fails first - the breaker or the speaker coils. 

[Stephen Swaffer]

If the operator is clueless, all bets are off, and it may become a race between which component fails first - the breaker or the speaker coils. 

If the breakers fail you could have a very big problem.  Tripping when overloaded is perfectly normal and expected behaviour.

[TJ (Tom) Cornish]

If the breakers fail you could have a very big problem.  Tripping when overloaded is perfectly normal and expected behaviour.

True. 

[Lyle Williams]

Every electrical component is a little bit of everything.  It might look like a wire on the diagram, but it also is a little inductor and a little capacitor.  And a big fuse.  And the place that smoke is coming from....

[Frank DeWitt]

The OP was worried that he was getting close to tripping a 10 amp breaker.  so wire up a male plug on a cord, a box with a fuse holder in it, and a cord with a female connector.  Put a 7 or 8 amp fuse in the fuse holder, turn every thing on, turn it up and find out.  The fuse will check for total current, crest factor, power factor, and everything else. 

[Stephen Swaffer]

Unfortunately, it is not that simple-fuses and breakers can and do have very different trip parameters.  At a former employer, a common failure mode in some of the welders commonly tripped 800-1200 breakers in the switch gear without blowing 60 amp fuses at the welder.  Admittedly, we had poor coordination of over current protection-still would you use a time delay fuse?  fast blow? is the breaker "inverse time delay"-the most common? or magnetic or thermal? If you are borderline, with the right crest factor you could have peaks over the rated protection all night long and be fine-but that might change completely with a minor change in the trip parameter.

[Frank DeWitt]

I have noticed over the years that any problem can be made so complicated that it can not be solved, In fact so complicated that it isn't even worth trying. I have also noticed that those same problems can be solved by others who simplify them.

Certainly there are hundreds of reasons why that particular circuit breaker might not trip EVER, or why it might trip at 5 amps but if I found that my sound system didn't trip a 7 amp fuse and my sound system wasn't used for life support I would feel quite confident.

I have made a good living solving problems that can't be solved.

[TJ (Tom) Cornish]

The OP was worried that he was getting close to tripping a 10 amp breaker.  so wire up a male plug on a cord, a box with a fuse holder in it, and a cord with a female connector.  Put a 7 or 8 amp fuse in the fuse holder, turn every thing on, turn it up and find out.  The fuse will check for total current, crest factor, power factor, and everything else.

I think that's as good a method as anything.

[ProSoundWeb Community]