Office Computer Loads...

[robbie]

I am trying to find a good design guideline for actual computer loads for an office environment. A typical application would consist of a computer, monitor, printer, scanner and radio. Unfortunately, the computer power supply manufacturer's only state the maximum current output on their power supplies (typically 8 amps). I'm currently running an amp probe 24-hour recording right now on a computer, monitor, printer and radio. It has not gone over 3.5 amps. It tends to hover around 2.5 to 2.6 amps under normal operating conditions.

We have been burned in the past by inspectors reading the maximum amperage for the computer power supply as an actual load for the computer. We were forced to put (2) computers per branch circuit for this particular office. :roll:

I've searched all over the web for some information regarding actual amperage ratings for office computers, but to no avail.

Any thoughts???

 

[websparky]

Re: Office Computer Loads...

Hi Robbie,

The amp rating on the computer power supply is the maximum that it can produce. Typically, they never get close to this amp draw because most computers do not have all of the possible power taps used and never will.

From an inspectors point of view, maybe he is trying to cover his butt and add up the name plate ratings as if it were operational ratings.
If you follow his logic, he is justified.

I do not agree with his logic either.

For future installs, is there a way to not stipulate what is being plugged in to these receptacles?

Here in Ohio, it gets cold in the winter an often times the computer operators bring in their own little space heaters and plug them into the same circuit as the computers! We then get a call from the customer complaining that the computers have a problem! Typically, we try to figure 3 computers per circuit to be safe!

 

[nvcape]

Re: Office Computer Loads...

For a desk top computer and large monitor, have seen about 2.5 amps. I believe some units include an allowance for the auxiliary outlets. Laser printers can run 7 amps.

 

[steve66]

Re: Office Computer Loads...

I do agree, 2 computers per circuit does seem kind of bare. When you state that manufacturer's give the current output of their power supply, that can't be taken as being equal to input current. The output current is at a lower DC voltage than the input voltage. The result is that the input current will be much less than the output current.

On another note, I doubt your amp reading is very accurate. It definately won't be accurate unless you are using a true RMS meter. In addition, it has to have a frequency response high enough to catch all the harmonics present the the short pulses of current DC power supplies take.

 

[waters_rs]

Re: Office Computer Loads...

Experience says that 3 per circuit is reasonable.

Using your inspector's logic, you could not install more than 10 20A breakers with a 200A service. You might be able to change his opinion with that type of argument.

 

[bwyllie]

Re: Office Computer Loads...

In office enviroments I have put 6 receptacles on a circuit and haven't had any problems. You listed a printer as one of the loads, laser printers usually draw more current than ink jet. Also, I agree that the inspector is way off on using the nameplate rating, anyone is the business will tell you the equipment only draws 1/2 of what it is rated.

 

[bonding jumper]

Re: Office Computer Loads...

you can say that top of the line computer now a days contain 400w power supplys. That means that the output of the power supply is 400w, not the consumption. If you use an efficiency of .8 typical you recieve 500W. 500 / 120 = 4.1A. Most regular computers, not top of the line, are approximately 250 - 300 watts. 313-325watts, or 2.7 amps.