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We're hosting a LAN party (several computers, 50+) and I had some questions about power draw; I hope this is the right place to ask.

Currently, we're assuming each PC will draw an average of 3.5 amps. We have several 20amp circuits, so we're putting 5 PCs on each 20amp circuit.

How concerned should we be with extension cord quality? Our plan is to run two 16/3 cables from each circuit, and run about 3 PCs per cord. Should there be any issues with this? Would going with 14/3 cables really be any better?

We considered going with single 12/3 cables and running all 5/6 PCs from that single cord... but given that the max listed amperage on these is 15A, that doesn't give us the amount of power consumption we're going to require.

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What kind of extension cord lengths are involved. If they are less than 50 feet, 16/3 is fine for 10 amps. But if they are like 200 feet, then no. –  wallyk Sep 4 '13 at 13:49
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Do make sure that each of the "We have several 20amp circuits" is indeed a separate circuit breaker back at the power panel. Do not be misled by the fact that multiple 20A outlets in your location may be daisy chained off a single circuit breaker. Your estimate of 50+ computers implies 10 separate 20A circuits and it is not really common to find homes with 10 separate circuit breakers controlling just outlets. –  Michael Karas Sep 4 '13 at 14:35
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3.5 amps is pretty low for a gaming computer. My gaming computer draws 6 amps (it uses over 700 watts), and that's not even including the monitors, speakers, etc. Though, that's at American (120V) voltage. –  BlueRaja - Danny Pflughoeft Sep 4 '13 at 20:04
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For continuous concurrent loads, do not attempt to draw more than 80% of the total rated ampacity of any circuit or conductor. So for a 20A circuit, do not plan to connect devices that will draw any more than 16A total. When you exceed this, the chance of nuisance tripping increases. –  bcworkz Sep 4 '13 at 23:26
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Since you're renting a facility, did you ask them about providing power distribution boxes to get the power more centrally located to reduce your need to run long power cords? Often they can provide a high current (50amp is common) "spider box" with a long cord that provides several electrical outlets where you need them (and they may also supply cable ramps to help keep attendees from tripping over the cables). –  Johnny Oct 21 '13 at 20:51

3 Answers 3

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Extension cords need to be chosen according to the length and load. The following chart will guide you to the correct gauge to choose for any one extension cord. You might choose several different gauges in order to save money, properly configured you won't need to put 12awg wire everywhere:

Table, current, cord length, and cord size

To carry 20 amps any distance, you'll need 12awg cable. You could use one 50 foot 12awg cable to carry the current to a table, then three 16awg cables to carry the current to one or two computers each.

You might find it cheaper, however, to run two 14awg wires in parallel from one outlet than to run one 12awg wire.

Keep in mind that while some cable calculators and tables will indicate that 16awg can handle 20A, the cable will heat up, and if you've properly secured it to the floor using tape or channels with other cables running parallel, the heat buildup can be significant and dangerous. Particularly with long runs.

You should also consult with the hotel - many of them have requirements for extension cord use in conference areas, and in some cases you will be required to use their cables and installers in order to meet their safety and load planning requirements.

If you pay attention to the chart you'll note that each cable size and load include a voltage drop. You'll also realize that due to the wiring at the venue you'll see a voltage drop at the outlet as well. This will result in a lower voltage at each computer than 120V. However, as long as you keep your voltage drop below 10%, and the venue is wired correctly producing a less than 10% drop to the outlet under load, then it is unlikely that your computers will experience any problems. Most computing equipment that uses AC power has a universal power supply that accepts a wide range of voltage, often down to 90VAC. It will be a rare computer or peripheral that will have problems with the voltage drop presented by a suitably rated cable.

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Was this table created by you, or was it something you found? The table seems to show the voltage drop through each length of cable, though you don't mention what the acceptable drop is. Is an 8.2% voltage drop acceptable? How can somebody use this chart to determine the overall voltage drop, if they're using more than one cable? –  Tester101 Jan 14 at 11:30
    
I found it, but no longer recall the source. It's easily calculated, though. The table primarily concerns itself with cord heating and safety issues related to that. Equipment with universal AC power supplies will often have a voltage rating, such as 90-260VAC. However you'll also need to account for voltage drop in the building. However you should be fine with 8-10% drop in the cord assuming the building has less than 5%, and if it's wired to US standards it probably will be. If you use the runs and gauges according to this table it will be a rare piece of computer equipment that will suffer. –  Adam Davis Jan 14 at 14:01

I seriously advise against attempting this in what, is assumed to be, a domestic residence on single phase power. Very few dwellings have an incoming capacity over 100A. I agree with the post re current draw per computer, it seems a very low estimate. Do consult your local utility provider. The issue of extension leads is secondary to the current demands on the in house installation.

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In fact, many newer homes (2000 or later?) have a service of 200A@240VAC which could supply as much as 400A@120VAC. Mine is an example. –  Philip Ngai Sep 21 '13 at 23:54

You could try first to load up the sockets/receptacles you intend to use with appliances that overall will demand the same power all your computers will need. Make them as equal as possible.

See if breakers trip. As someone else has said it may be that all or a number of your 20 amp sockets are using the same breaker. In which case it won't work.

If you have 50 computers drawing 3.5 amps each that is 50 x 3.5 amps = 175 amps.

175 Amps @120 volts is 21 KWatts. Quite a lot of power. So plug appliances into the sockets you have available equally.

If you have 10 sockets try to load them up with 2.1 KWatt loads. This is equivalent to 2100W/120V = 17.5 amps load each.

I really expect you will get trouble- they will most likely cause tripping because some sockets will be using the same breaker.

At least you'll know.

If all should be well use the excellent chart given elsewhere on current carrying capacity and voltage drop.

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