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Author Topic: Measure Current draw - Australia/230V  (Read 8374 times)

Jonathan Johnson

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Re: Measure Current draw - Australia/230V
« Reply #10 on: July 01, 2015, 06:50:18 pm »

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.
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Stop confusing the issue with facts and logic!

Callan Browne

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Re: Measure Current draw - Australia/230V
« Reply #11 on: July 01, 2015, 08:57:42 pm »

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.
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Melbourne Cover band: Rubix!

Lyle Williams

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Re: Measure Current draw - Australia/230V
« Reply #12 on: July 02, 2015, 03:20:57 am »

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.)
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TJ (Tom) Cornish

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Re: Measure Current draw - Australia/230V
« Reply #13 on: July 02, 2015, 07:13:26 am »

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
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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. 
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Mike Sokol

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Re: Measure Current draw - Australia/230V
« Reply #14 on: July 02, 2015, 08:09:41 am »

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.
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TJ (Tom) Cornish

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Re: Measure Current draw - Australia/230V
« Reply #15 on: July 02, 2015, 08:49:31 am »

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.  :)
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Stephen Swaffer

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Re: Measure Current draw - Australia/230V
« Reply #16 on: July 02, 2015, 11:36:35 am »


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. :)
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Steve Swaffer

TJ (Tom) Cornish

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Re: Measure Current draw - Australia/230V
« Reply #17 on: July 02, 2015, 11:51:58 am »

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

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Re: Measure Current draw - Australia/230V
« Reply #18 on: July 02, 2015, 06:30:14 pm »

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....
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Frank DeWitt

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Re: Measure Current draw - Australia/230V
« Reply #19 on: July 03, 2015, 06:42:12 pm »

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. 

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Re: Measure Current draw - Australia/230V
« Reply #19 on: July 03, 2015, 06:42:12 pm »


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