How to calculate the load of a Home Office?

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Old 02-04-08, 11:39 AM
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How to calculate the load of a Home Office?

I will have to check but am assuming the house was wired where the office is all on a single circuit.

Currently there are two computes (and two monitors), a printer, modem, router, laptop, phone with answering machine along with ceiling fan and light all running in the office.

How can I calculate the amps being pulled in this office to see if I am cutting it close? Is there a formula of sorts to do this?
 
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Old 02-04-08, 12:01 PM
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Originally Posted by JimInFLA View Post
I will have to check but am assuming the house was wired where the office is all on a single circuit.

Currently there are two computes (and two monitors), a printer, modem, router, laptop, phone with answering machine along with ceiling fan and light all running in the office.

How can I calculate the amps being pulled in this office to see if I am cutting it close? Is there a formula of sorts to do this?
Anything that plugs in should have a nameplate that shows its power consumption, for example INPUT AC 120V 60 Hz 4W.

Add up all the wattages. For a 15A circuit you should try to stay under 1440 watts, I think. 20A, 1920 watts. Something like this:

120 volts * 15 amps = 1800 * 80% = 1440 VA, 1440 watts at a power factor of 1.0. Power factor can be a big issue with switching power supplies, though, and it's rarely on nameplates of consumer electronics.

If your equipment is rated in amps, you just add the amps. Some stuff will have a VA rating. Most likely you'll have something of each. Just do your conversions and watch your units, just like your teachers told you in chemistry and physics.

You might want to read up a bit:
http://www.power-solutions.com/watts-va.php
 
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Old 02-04-08, 12:53 PM
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As long as that's not a laser printer, I'm confident that you're not pushing the limit of a 15A circuit (1800W). Are you just curious or have you been tripping the breaker?

The 80% rule only applies to continuous loads (defined by being on for more than 3 hours at a time).

Add up the rated watts of each device. If the rating is not listed on the computers, you can assume about 200W for a typical computer, 100W for a CRT monitor, and 50W for a flat panel. Modems, routers and the like are usually under 50W each. A ceiling fan is usually around 100W.

You can get a handy little meter called the Kill-A-Watt at the hardware store or online which can measure the real-time power consumption of any 120V plug-in appliance. It's only about $25 and is easy to use.
 
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Old 02-04-08, 01:13 PM
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Originally Posted by ibpooks View Post
As long as that's not a laser printer, I'm confident that you're not pushing the limit of a 15A circuit (1800W). Are you just curious or have you been tripping the breaker?

The 80% rule only applies to continuous loads (defined by being on for more than 3 hours at a time).
I have found that despite ratings, equipment often does not consume as much as rated.

My old LaserJet 4M has a pretty low standby power consumption but pulls something like 10 amps for one second every 30 seconds or something like that. You can easily see an incandescent bulb dim when that happens, even on a 20A circuit. In an office I used to work in, somebody had a space heater plugged into the same circuit as a LaserJet 2 and it would trip every time somebody tried to print. The newer energy star ones did much, much better in terms of power-hogging.

Regarding continuous loads, is there anything in an office that isn't on for more than 3 hours, besides the vacuum cleaner and the shredder?
 
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Old 02-04-08, 02:26 PM
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Originally Posted by ArgMeMatey View Post
I have found that despite ratings, equipment often does not consume as much as rated.
Certainly true with computer and audio equipment. Typical use is well less than 50% of rated max.

Regarding continuous loads, is there anything in an office that isn't on for more than 3 hours, besides the vacuum cleaner and the shredder?
I suppose it depends on the person's habits. I usually leave my PC on, but shut the monitor off. I'm sure other people leave everything on and some people turn everything off.
 
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Old 02-04-08, 04:27 PM
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Thanks for the replies. There haven't been any issues, but I recently rearranged the office and majority of items are running through a surge protector which is connected to a single outlet.

I couldn't believe all the wires and when you see the web of wires it made me wonder if I was overloading the circuit.
 
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