Office Building and Computer Loads

[Platinum]

Hey guys,

Was brainstorming about this new 3 story office building. Has both enclosed offices and cubicle areas. I was thinking of putting 3 duplex receptacles per cubicle (1 computer, 1 convience, and 1 for task lighting) and feeding a bank of cubicles with a tele-power pole. There are about 30 workstations per floor so there has to be over 30 computers per floor. I would imagine harmonics would be a problem (from all those computers using switching power supplies).

Since cost is a major factor (big surprise), could I put a 480V panel with a K-13 transformer and a 208/120V panel on each floor? and just feed all my receptacle loads from this 120V panel? Ideally I would have a separate K transformer just for my computer circuits, but this would require more total panels for the job.

 

[adavey]

Are you overthinking this job ?????????? Computers do cause harmonics, but it's only 3 stories. I've heard of engineers using 2 ckts/ 3 cubicles. 1 ckt- computers, 1 - misc loads. You can include a 1 ckt (each) for laser printer, fax and copier per 15 people if you want. Also, you might want to isolate the computer ckt's (#10awg nuetral, #12 awg HOT) just in case a UPS might be installed in the future.

 

[Platinum]

Thanks for the advice adavey.
Yeah, wasn't sure about the K-transformers but it will probably be okay without them. Thats a good idea to isolate the computer recps too.

 

[infinity]

Also, you might want to isolate the computer ckt's (#10awg nuetral, #12 awg HOT) just in case a UPS might be installed in the future.

Wouldn't the computer circuits be better if they had their own grounded conductor instead of a shared #10 neutral?

 

[Platinum]

I don't think he meant sharing the neutrals.

 

[infinity]

I don't think he meant sharing the neutrals.

OK then why in a 2-wire circuit would you need a #10 grounded conductor with a #12 hot leg?

 

[Smart $]

Wouldn't the computer circuits be better if they had their own grounded conductor instead of a shared #10 neutral?

IMO, no... though there are a lot of engineers and electricians who seem to believe otherwise.

Triplen current on a neutral of a 4-wire wye mwbc never exceeds the ampacity of any one of its component circuit's ampacity, except perhaps under extreme and intentional conditions. Additionally, most circuits as such are, by design, seldom loaded to ampacity levels. You'd have to have an average of 10A of triplen current per ungrounded conductor to exceed 30A of current ampacity on a #10 neutral. To generate 10A of triplen current on a 20A circuit would require an extremely inefficient power supply design. Running ibc's instead of mwbc's simply covers that possibility, even though that possibility is improbable. The downsides of ibc's, which seem to never be mentioned, are more wire is necessary, there will be an increased voltage drop, and this part of the system will have higher I?R losses. :-?