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Author Topic: AC Transformer Math/Theory/Logic - Help  (Read 2145 times)

Casey Sharp

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AC Transformer Math/Theory/Logic - Help
« on: January 23, 2018, 08:21:16 am »

I need some help understanding how to make some calculations. Let me first explain my situation.  I occasionally do events in very small venues... like find some wall outlets and plug it in type of events.  I have an audio system in which the speakers alone have have a max amp draw of 5.2A, times 8 speakers is 41.6A. This doesn't include backline, rack gear, etc... Obviously, I can't run that all on a single circuit so many times I'm left running extensions from different circuits. Also obviously, ideally I could tie into a disconnect and use a power distro, but many times that just not an option... so.....

Here was my thought - I could power my system using a few step-up transformers, basically creating a more "power efficient" system.  I'm aiming at sub-15A per circuit, but here's where I'm not sure of my calculations.

To run each speaker at full power (624W) using 220V, my speakers would draw 2.84A meaning I could power up to 5 speakers per circuit as far as amp draw itself is concerned. The thing I'm wondering about is wattage because with a higher voltage means higher wattage. To power the 5 speakers on one circuit I would be generating 3120W.  But this is where I'm a little confused because this would all begin at a 120V outlet, but at 120V it would take 26A to generate 3120W.

Standard outlet - 120V x 15A = 1800W

So the conclusion I've came to is that I'm bound to the 1800W rating because that's what a 120V circuit is rated for.  Am I right?

So with stepping up to 220V, but being bound to 1800W will only leave me with about 8A to work with which leaves me exactly where I started because -

At 120V - 15A / 5.2A(per speaker) = 2.88 speakers per circuit AND

At 220V - 8A / 2.84A(per speaker) = 2.81 speakers per circuit




Is my logic and math right on this? Or have i completely missed something?

Thanks for your help!

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Steve Crump

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Re: AC Transformer Math/Theory/Logic - Help
« Reply #1 on: January 23, 2018, 09:34:56 am »

I need some help understanding how to make some calculations. Let me first explain my situation.  I occasionally do events in very small venues... like find some wall outlets and plug it in type of events.  I have an audio system in which the speakers alone have have a max amp draw of 5.2A, times 8 speakers is 41.6A. This doesn't include backline, rack gear, etc... Obviously, I can't run that all on a single circuit so many times I'm left running extensions from different circuits. Also obviously, ideally I could tie into a disconnect and use a power distro, but many times that just not an option... so.....

Here was my thought - I could power my system using a few step-up transformers, basically creating a more "power efficient" system.  I'm aiming at sub-15A per circuit, but here's where I'm not sure of my calculations.

To run each speaker at full power (624W) using 220V, my speakers would draw 2.84A meaning I could power up to 5 speakers per circuit as far as amp draw itself is concerned. The thing I'm wondering about is wattage because with a higher voltage means higher wattage. To power the 5 speakers on one circuit I would be generating 3120W.  But this is where I'm a little confused because this would all begin at a 120V outlet, but at 120V it would take 26A to generate 3120W.

Standard outlet - 120V x 15A = 1800W

So the conclusion I've came to is that I'm bound to the 1800W rating because that's what a 120V circuit is rated for.  Am I right?

So with stepping up to 220V, but being bound to 1800W will only leave me with about 8A to work with which leaves me exactly where I started because -

At 120V - 15A / 5.2A(per speaker) = 2.88 speakers per circuit AND

At 220V - 8A / 2.84A(per speaker) = 2.81 speakers per circuit




Is my logic and math right on this? Or have i completely missed something?

Thanks for your help!

If you are plugging in to a 120 volt receptacle rated at 15A, then you are correct, you are limited to that 1800VA. Boosting the voltage on the load side will not give you any additional power.

Nathan Riddle

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Re: AC Transformer Math/Theory/Logic - Help
« Reply #2 on: January 23, 2018, 09:42:18 am »

You will also loose power through the conversion process(es).

What higher voltage does help with is long runs of cord via less voltage drop.
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Scott Helmke

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Re: AC Transformer Math/Theory/Logic - Help
« Reply #3 on: January 23, 2018, 09:51:06 am »

Your math is correct. Ultimately what you're using is power, measured in watts.  The only thing you might gain with a higher voltage is less line loss on long cables.

The more interesting question is how much power your speakers are actually using, not their max rated number. If it's quiet acoustic music you're probably using a tiny fraction of what you think you're using.
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TJ (Tom) Cornish

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Re: AC Transformer Math/Theory/Logic - Help
« Reply #4 on: January 23, 2018, 09:53:30 am »

I need some help understanding how to make some calculations. Let me first explain my situation.  I occasionally do events in very small venues... like find some wall outlets and plug it in type of events.  I have an audio system in which the speakers alone have have a max amp draw of 5.2A, times 8 speakers is 41.6A. This doesn't include backline, rack gear, etc... Obviously, I can't run that all on a single circuit so many times I'm left running extensions from different circuits. Also obviously, ideally I could tie into a disconnect and use a power distro, but many times that just not an option... so.....

Here was my thought - I could power my system using a few step-up transformers, basically creating a more "power efficient" system.  I'm aiming at sub-15A per circuit, but here's where I'm not sure of my calculations.

To run each speaker at full power (624W) using 220V, my speakers would draw 2.84A meaning I could power up to 5 speakers per circuit as far as amp draw itself is concerned. The thing I'm wondering about is wattage because with a higher voltage means higher wattage. To power the 5 speakers on one circuit I would be generating 3120W.  But this is where I'm a little confused because this would all begin at a 120V outlet, but at 120V it would take 26A to generate 3120W.

Standard outlet - 120V x 15A = 1800W

So the conclusion I've came to is that I'm bound to the 1800W rating because that's what a 120V circuit is rated for.  Am I right?

So with stepping up to 220V, but being bound to 1800W will only leave me with about 8A to work with which leaves me exactly where I started because -

At 120V - 15A / 5.2A(per speaker) = 2.88 speakers per circuit AND

At 220V - 8A / 2.84A(per speaker) = 2.81 speakers per circuit




Is my logic and math right on this? Or have i completely missed something?

Thanks for your help!
There is no free lunch - stepping voltage up reduces the current after the transformer - trading lower current for higher voltage, however the current draw before the transformer will be the same as without the transformer, and actually slightly worse due to the power consumption of the transformer.

HOWEVER...

I highly doubt your speakers draw 5.2A.  Look in your speakers' spec sheets for the 1/8 power value - it will be more like 1 or 2 amps, not 5.  The reason for this is audio is a dynamic signal, and there may indeed be moments where you are putting out 600 watts into your drivers, but that is a small fraction of the time, which averages to a much lower number.
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Stephen Swaffer

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Re: AC Transformer Math/Theory/Logic - Help
« Reply #5 on: January 23, 2018, 12:45:07 pm »

And a 15A Square D QO could handle a peak of 45 amps for up to 3 seconds before it trips.  That is a thermal trip-so peaks are cumulative, so it's not somthing you want to take to the bank-but an occasional peak somewhat over 1800 watts won't trip a breaker.  If the run from the panel to the amps (whether building copper or extension cords) is long, it is likely voltage drop will become an issue first.  That is where a step-up/step down transformer could, in theory, help.  In practice it is usually more expense/trouble than it's worth for typical sound provider setups.  (Keeping in mind it only helps on the portion of the circuit run at a higher voltage.)
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Steve Swaffer

Mark Cadwallader

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Re: AC Transformer Math/Theory/Logic - Help
« Reply #6 on: January 23, 2018, 01:35:21 pm »

You might also have a 20A breaker on your circuit, even if the outlets are all NEMA 5-15R devices.  (As allowed by the NEC in various circumstances.)
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Casey Sharp

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Re: AC Transformer Math/Theory/Logic - Help
« Reply #7 on: January 23, 2018, 01:57:39 pm »

You might also have a 20A breaker on your circuit, even if the outlets are all NEMA 5-15R devices.  (As allowed by the NEC in various circumstances.)

Let's say someone (not me of course) were to buy a small distro like this one - LINK



Then running from two different 15A circuits into a Y-Split into the input of it... giving 30A total. Maybe buying 2 to give 60A.

Again... just in theory ;)
« Last Edit: January 23, 2018, 02:30:10 pm by Mac Kerr »
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TJ (Tom) Cornish

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Re: AC Transformer Math/Theory/Logic - Help
« Reply #8 on: January 23, 2018, 02:16:18 pm »

Let's say someone (not me of course) were to buy a small distro like this one - https://www.powerdistributiononline.com/rolling-distribution-racks/enclosed-rack-mount-power-distribution/lex-3ru-rack-mount-power-distribution-l14-30-in-thru-to-duplexes/?utm_source=google&utm_medium=shopping&utm_campaign=shopping&gclid=EAIaIQobChMI9vD9suDu2AIVkGV-Ch2PJwwaEAQYAiABEgIABfD_BwE&ad=47468018097



Then running from two different 15A circuits into a Y-Split into the input of it... giving 30A total. Maybe buying 2 to give 60A.

Again... just in theory ;)
NO!!!!!!!  This is not how this works.

Figure out how much power your gear actually draws, and split up to multiple circuits around the venue if necessary.

There is no magic box that fixes this.
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Tim McCulloch

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Re: AC Transformer Math/Theory/Logic - Help
« Reply #9 on: January 23, 2018, 02:24:45 pm »

Let's say someone (not me of course) were to buy a small distro like this one - https://www.powerdistributiononline.com/rolling-distribution-racks/enclosed-rack-mount-power-distribution/lex-3ru-rack-mount-power-distribution-l14-30-in-thru-to-duplexes/?utm_source=google&utm_medium=shopping&utm_campaign=shopping&gclid=EAIaIQobChMI9vD9suDu2AIVkGV-Ch2PJwwaEAQYAiABEgIABfD_BwE&ad=47468018097



Then running from two different 15A circuits into a Y-Split into the input of it... giving 30A total. Maybe buying 2 to give 60A.

Again... just in theory ;)

DO NOT DO THAT.  Seriously.
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Steve Crump

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Re: AC Transformer Math/Theory/Logic - Help
« Reply #10 on: January 23, 2018, 02:55:05 pm »

Let's say someone (not me of course) were to buy a small distro like this one - LINK



Then running from two different 15A circuits into a Y-Split into the input of it... giving 30A total. Maybe buying 2 to give 60A.

Again... just in theory ;)

Man that sounds scary.
Sounds like something off an episode of The Red Green Show.
First you need a better idea of your real load, you will probably be surprised how little power you are requiring compared to what you're thinking you are requiring.
You really need to listen to Tim and Tom. No magic box...

Keith Broughton

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Re: AC Transformer Math/Theory/Logic - Help
« Reply #11 on: January 24, 2018, 06:11:47 am »



Then running from two different 15A circuits into a Y-Split into the input of it... giving 30A total. Maybe buying 2 to give 60A.

Again... just in theory ;)
And I will ad...
NO! Bad advice! :(

To the OP, as indicated, audio draws much less than the full rated power of the speakers and amps.
Audio is not like a kettle or toaster that draw fulll power 100% of the time they are on.
I would estimate your system, in real use, would run on  2x15A  breakers.
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David Kramnic

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Re: AC Transformer Math/Theory/Logic - Help
« Reply #12 on: February 05, 2018, 01:56:31 pm »

Let's say someone (not me of course) were to buy a small distro like this one - LINK



Then running from two different 15A circuits into a Y-Split into the input of it... giving 30A total. Maybe buying 2 to give 60A.

Again... just in theory ;)

In theory: too dangerous. If you're asking if it can be done then you don't know enough to do it and make it work, no offense. Something like that CAN be done, but it wouldn't be legal, since it requires utilizing equipment in a manner not intended, and that get you in jail if someone got hurt.
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