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# Power Calculation Question ## Recommended Posts  Hey all,

Hopefully, this is an easy question for someone out there.

I am running a 100% pixel based show.  I've got quite a few controllers and lots of the 12v power supplies.  (12v, 350w, 29A)

I know how to calculate my actual load on all my different power supplies.  For example:

- I have 270 pixels (evenly distributed) running off one of my controllers.  Each pixel draws .54 watts and .045mA

- 270 x .54 = 145.8 Watts

- 270 x .045 = 12.15 Amps

So, this is where I get confused.  How do I then calculate how many of these 12v power supplies can I safely put on one single 20Amp branch circuit?  Is there a calculation for this?

Any help would be appreciated.

Thanks,

Tim

##### Share on other sites  It's pretty close if you do the 12v goes into 120v 10 times so 12.5 amp dc divided by 10 is 1.25 amps ac.

##### Share on other sites  For each power supply, assume the input watts = output watts x about 1.2 (they are not 100% efficient).   Add up the input watts for each supply and divide by 120 to get the amps.  For example, lets say that you have three power supplies that have the 270 pixels each that you used in your post.  Each one is outputting 145.8 watts so 145.8 x 1.2 = 175 watts input.  Three of them would be 525 watts divided by 120 = 4.37 Amps.  A single 20A circuit should not have more than a 16 amp constant load.

BTW, I used a power supply efficiency of 80%.  In reality, they are likely better than that, but it's a good working figure.  Better to be conservative...

##### Share on other sites  Then once you have things plugged in get yourself a Kill-A-Watt meter and you can verify the wattage and amps on everything you have plugged into a circuit and see what they are actually pulling.

##### Share on other sites  22 minutes ago, k6ccc said:

For each power supply, assume the input watts = output watts x about 1.2 (they are not 100% efficient).   Add up the input watts for each supply and divide by 120 to get the amps.  For example, lets say that you have three power supplies that have the 270 pixels each that you used in your post.  Each one is outputting 145.8 watts so 145.8 x 1.2 = 175 watts input.  Three of them would be 525 watts divided by 120 = 4.37 Amps.  A single 20A circuit should not have more than a 16 amp constant load.

BTW, I used a power supply efficiency of 80%.  In reality, they are likely better than that, but it's a good working figure.  Better to be conservative...

Ditto on all of the above

And they should not be fed from a single cord because the plug is 5-15 or 12A cont per cord plug.

Also INRUSH current  is going to be pretty nasty plugging in a bunch at the same time

##### Share on other sites  ##### Share on other sites  Perfect!

That's exactly what I was hoping for.  Thank you all very much!

I already ordered my Kill-a-watt meter Thanks,

Tim

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