Max Comps on 20 amp circuit
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Thread: Max Comps on 20 amp circuit

  1. #1
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    Max Comps on 20 amp circuit

    Title says it all. My Buddy and I are having a mini lan, and he's got a 20 amp circuit at his house. How many computers are safe on that?
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    The standard household outlet circuit in the US is 15 amps, with one circuit per room to a maximum of 4 (over and under) outlets (8 plug sockets total).

    Currently in my study I'm running 2 boxes, with associated monitors, a printer, a cable modem, powered speakers, a television, a circular fan, and a dvd player on one 15 amp circuit and having absolutely no problems whatsoever. In order to figure out exactly how much load you can put on your 20 amp circuit you'll need to know the overall current draw of each machine and the electrical layout of your circuit, and some simple mathematical equations to help you.

    First, is the outlet circuit of your buddy's house set in series or parallel? Most commonly household outlet circuits are series - if so it won't hold as much current as if it were a parallel circuit.

    Here's what a series circuit looks like:

    [source]---line---outlet1----line----outlet2----line----etc-----ground.

    parallel would look like this:[edit] grrr - pardon and ignore the '...'s' in the diagram, for whatever reason the editor won't accept spaces as I put them[/edit]

    ...............|------line-----outlet1-----|
    [source]-|------line-----outlet2-----|-----ground
    ...............|------line-----outlet3-----|

    In a parallel circuit the current draw can be higher since the overall effect of the impedence of the computers is reduced.

    Another thing to look at is that you're setting up a primarily capacitive circuit. This actually helps on current draw, but only after the systems are all up and running - your best chance of tripping a breaker is at startup. Don't start your boxes all at the same time.

    For the mathematical formulae I'll give you the following:

    E=iR (voltage = current*resistance)

    That plus RC (resistance*capacitance - for calculating the capacitance of a capacitive circuit) are about all I can remember of my basic electrical engineering days. The rest of it you'll have to look up for yourself. National Electrical Code handbook, plus your computer manuals might be a decent start point.

    Given I can run an entire Christmas light display on one plug along with my livingroom stuff (also a huge capacitive circuit since I have lots of home electronics), common sense says you should be able to run 4 or so just fine, maybe even 6. Don't hold me to that tho
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    Or to simplify this alittle, Amps=Watts/Volts.
    So you have a 400 watt computer 400watts/110volts=3.6amps, 5x3.6=18amps, you could run about 5 computers, but you have to take into account any monitors or other objects plugged into that circuit. This is just a baseline to look at.

  4. #4
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    dev, correct me if I'm wrong, but if he places them in a parallel configuration, he could effectively double that, no? Since the resistance/load is shared across the parallel circuit, the current draw is lower...
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    Everything you plug into a wall outlet is in parrallel already. They share a common Hot, neutral, and ground.

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    The difficulty comes when trying to measure current draw of your computers (The max watts on a PSU are only output; for max current input you need to consider effeciency among a few other things including the actual output power). I'd estimate my computer (a nice over-clocked dual cpu workstation) draws 5-7 amps @ 120vac including monitor.

    In general you can expect to safely toss ~5-6 computers onto a 20 amp circuit without tripping the breaker. I'd recommend using several outlets though (no daisy-chaining) because I've seen powerbars melt before when 10+ people daisy-chain off of each other. You might be able to squeeze 8 people, but if your buddy has substandard electrical wiring and run close to the max current for many many hours the resistance in the wiring can build up quite a bit of heat.

    In one of the largest LAN events in my state (I think one year we approached/broke the state record with perhaps 70-80 people?) we actually had the electrical company install some *really* heavy duty wiring and a voltage transformer so we could power up ~70 machines, a projector, and concession stands. No electrical issues that time. But this setup was nothing compared to the warehouses with 200-500+ people in Texas duking it out on the weekends.


    So you should be able to support 5-6 friends on a 20amp circuit. If you need more people, bring power in from another room/circuit but put less load on it over the extension cords to be safe.

  7. #7
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    E=IR
    P=EI

    E = Volts
    I = Amps
    R = Ohms
    P = Watts

    |3lack|ce.. all you lower when you place things in parrallel is the resistance

    The power draw rises..

    Look at the labels on the devices there is your Max power for the device..

    oh how much do we have to start with?
    .... it is a 20Amp Circuit so you also have 120V supply (you poor bastard Yanks)..so you have 2400w to play with. if each pc has a 17"crt monitor you will typicly have 400w PC-PSU + 180w CRT = 580w

    so you have a typical capability of 4 systems . 5 if the monitors have significantly lower consumption.. oh.. Turn the systems on in sequence.. the surge current of 4 monitors comming on at once could cause a problemt (the degauss circuit fires at first start this current is the highest ..considerably higher than the run current)..
    ..
    now we poor Aussies.. 15amp circuits at 240V.. 3600w..lol... but some dumbass politition said 4 7.5Amp outlets per circuit for domestic GPO.. hmm silly maths.. but no worries the cable is capable of 2 15Amps outlest.. so All circuits are fuse to 30amps .... ..now I am not recommending upping the size of the fuse for a day or so.. but you never know...(warning .. overloading of circuits will lead to fires or very load bangs or Both)
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    Yes, but...

    Undertaker, you're bringing things back to perspective now. Thanks, it's been 13 years or so since I did any electrician work at all and 17 since I had BdoubleE.

    The rub with P=IE is that it fails to take into account the capacitance the electronics add to the AC circuit. At initial start the current draw will be maximum (depending, of course, on whether he starts up *everything* at once or not.) After this, the circuit will become capacitive - power supplies drawing less current than their rated need since the caps within the electronic circuits they buffer will be performing the function of acting like batteries - since the electronics need less current, the power supply will draw less. Once again it's been umpteen years since BEE, so I can't even foggily remember how the transformers within the PSU's will effect the circuit - I think once charged they rock along on a trickle.
    Even a broken watch is correct twice a day.

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  9. #9
    They call me the Hunted foxyloxley's Avatar
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    Stick with four to your circuit to be safe .............

    In the UK we can afford several outlets in each room
    and each outlet is classed as a 13A socket, wired in a ring circuit, it effectively gives us 30A capability in the ring. No use to our American cousins, but I like to gloat

    Ice is right about the capacitive effect, but in the real world application, it isn't counted in the equation.

    Get another lead in from another room for an extra system [one lead = one system] but don't go wild with numbers, just go wild on LAN..............
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  10. #10
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    Interesting thread, I thought I would chime in since the major point is lurking around the edges. Those power rating on the outside of the power supply should always meet the maximum power needed. Meaning they take into account the current draw on startup. That is why you will note a different label on the monitor versus the pc versus the laser printer, versus another PC with a bigger power supply. But here is the point... and Tim hit closely on it..

    Power supplies are extremely INEFFICIENT. The BEST ones are only around 60 - 70 percent efficient. Need proof? Well if they were 100 percent efficient that would be excellent. We could perhaps make a perpetual machine, especially at 110 percent where we would definitely have a perpetual device, generating more energy than consumed.

    But we only get 70 percent of what we put into it, at best! This is due to energy loss through heat and conversion. Lets toss out some numbers... Thus a 110 volt power supply with a 5 amp rating will have an input of 550 watts. The same labeled at 120 volts/5 Amps will have a max input draw of 600 watts. This is the factor we care about, what is being drawn from the mains. I used both because you will find different labels. Now lets use 7 amps for the dual processor motherboard. Total input from 120 volt/7 Amps is 840 rated watts total input.

    Process that through the totally inefficient PSU and convert AC to Multiple DC voltages and you have 70 percent of 840, which is 588 or 600 Watts. Big honking power supply, but you will typically find a 350 watt power supply is around 65 percent efficienct in a 5 amp circuit. Thus the magical numbers of 250, 300, 350 watts etc. What happens if the PC components draw more than the power supply is cabable of delivering, would you blow a circuit breaker? No. (unless the PSU burns and shorts due to a deffect).

    Therefore what happens on the inside of a PC and it's power ratings are not the same as what happends on the outside of a PC; with it's power ratings for the input of the power supply comming from the 20 amp circuit and looking toward your circuit breaker. In ALL cases they will be less on the inside by a large degree. Although they are directly proportional.
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