continuous load/computer stations

[Wireit]

I have what is simple question regarding branch circuit laods for a computer work station in an office.

I have four work stations in a single office. Each work station has a computer tower drawing 6 amps and a monitor drawing 2 amps. At 120 volts I see this as a 960va load. The work station will run continuosly so I believe I must have a 125% X 960 demand or 1200va. Is this correct? If so It appears I can not supply any two work stations with the same 120v/20amp circuit, is that correct?

Thanks for any input

 

[adavey]

i believe you calculate this way:

Maximum design ampacity / ckt is 80% of OCPD (over current protection devic; ie 80% of 20A ckt bkr = 20A*0.8=16A)

Branch ckt wire is 125% (for cont. loads) of your calculated load .

FYI. Make sure you check the computer spec's to see if an " Isolated ground" is required and also excessive heat on the neutral conductor caused by harmonics (if you decide to utilize the neutral as a return path for the A,B and C phase).

 

[iwire]

I Each work station has a computer tower drawing 6 amps

It is drawing 6 amps or has a power supply capable of drawing 6 amps?


For what it's worth 2 or 3 work stations per circuit is about typical here with dedicated circuits located at laser printer locations.

 

[hdpeng]

It is drawing 6 amps or has a power supply capable of drawing 6 amps?


For what it's worth 2 or 3 work stations per circuit is about typical here with dedicated circuits located at laser printer locations.

I agree, Bob. Usually computer stations draw much less than their maximum rating for 100%. I don't recall now where I read it, but several years ago I saw a study of workstation computer demand loads & the actual demand load for each computer came out to be around 65% (for monitor & computer) in an active office environment. It is very seldom that several typical workstation computers will draw their max. demand.

 

[Wireit]

Nameplate/ Service load calculations

Nameplate/ Service load calculations

The nameplate on the computer tower is showing 6 amps at 120 volts and 2 amps on the monitor nameplate.

For doing the load calculation for the service, will the VA be 100% of the nameplate or 125% of the nameplate for its continous duty cycle?

 

[charlie b]

To be honest, I would do the formal "load calculation" on the basis of 180 VA per duplex receptacle. That is all that is required, and you don't use a 125% factor. I would go beyond the minimum NEC requirements, by limiting the number of workstations on a single circuit to 3 (or perhaps 4), as others have already said. Keep in mind that calculating load and designing a circuit for the convenience of the user are two separate tasks.

 

[infinity]

IMO a computer is not a continuous load. It will not run at it's maximum ampacity for more than 3 hours. So you can forget about adding 25% to any calculation.