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# P=VI?

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### #1 keller

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Posted 20 June 2010 - 06:08 PM

Ok, so I was lying in bed last night thinking about power supplies, (that's right, I'm that cool) and something occurred to me. Are power supply wattages getting to a point where blowing a household circuit might be a real concern? If I remember correctly P=VI, so assuming P=1000 Watts and V = 120 Volts, I would have to be 8.3 Amps, right? Standard household breakers are 10 to 15 Amps, so that doesn't leave a lot of wiggle room.

Unless, of course, I'm over-simplifying the whole AC/DC conversion process or overlooking something I was supposed to have learned in physics class, which is why I thought I'd ask here.

Edited by keller, 20 June 2010 - 06:09 PM.

### #2 Platypus

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Posted 20 June 2010 - 06:53 PM

Your basic assumption is correct. A 120V circuit with a 10A breaker is rated for 1200W (using W=VA for general conversation). So in theory a true 1000W PSU with 80% conversion efficiency supplying 1000W to a load would be exceeding that rating, drawing 1250W. (In practice it will probably be less because of the way conversion efficiency works out in a switching regulator.)

There are some modifying factors, because a PSU draws current proportionally to its required output. For a safety margin, a PSU normally wouldn't be run to 100% of its rated capacity. So a 1000W PSU might be in a system that has a peak power requirement of 850W (theoretical 1062W consumed), and that would only occur during particularly heavy usage. Say the system used 400W doing light duties like websurfing, the mains consumption would be 500W.

These are all generalisations to convey the concept.
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### #3 strolln

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Posted 20 June 2010 - 07:29 PM

I don't know about your setup but I have a strip plugged into a wall socket with a 15A breaker. I have not only the computer plugged in to the strip but my monitor, printer, external hard drive, powered speakers and router. All of those are drawing from that same 15A circuit. Luckily, I'm not a gamer who needs 2 or 3 GPUs in my system so I doubt I'll ever have a PSU in my computer over 500W.
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