NEC - Diversity for computer based plug loads

nfurgason

Member
Location
Detroit, Mi
Occupation
EE
Looking for opinions\interpretations on how to calculate diversity for computer based plug loads. One school of thought is that a receptacle in an education environment intended for a PC should be calculated without a diversity, for the assumed actual load of the PC (200W or so). The other thought is that the receptacle is simply a convenience outlet (180VA) and is subject to the diversities in table 220.44. This is a big picture question, at the service level, not the branch circuit level. I have a lab building with several computer labs and with the shear number of PCs in the building (offices, classrooms, labs, etc), it actually affects the distribution system sizing quite a bit, and thus cost...
 

ron

Senior Member
My position is that you need to be consistent on the Permit drawings.
If your panel schedules show the branch circuits with convenience receptacles, feel free to take that all the way up to the service calcs. It is just your liability if overload occurs and the CB's do their job and the client comes after you for E&O.
Practically, I treat them special because they are.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
my inclination is to use what the code says for doing the load calcs as ron suggests.

On the whole, your actual load will likely be far less than the calculated load as a computer with a 200 W power supply rarely uses even half of that.

I would be mindful of the potential for overloading individual branch circuits on occasion though. You might want to have more branch circuits than the bare minimum. maybe allow enough BCs that you only have perhaps < 1000 VA worth of receptacles per BC.
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Engineer
This is difficult, as the code doesn't really specify. Even if you knew exactly what was going into the room today, in 5 years it will most likely be different. We used to have a PC and a CRT monitor. Then it was an LCD monitor. Now two LCD or LED monitors or a single huge one. PCs can be weenie ones with 200W power supplies, or monster ones with 800W power supplies. Or they switch to laptops and who knows what percentage are charged when the users shows up.

From a code point of view, I'd treat it as general use but add provide additional capacity as 180VA per outlet is probably not enough. A server room is more difficult, as insufficient capacity there tends to imply insufficient HVAC capacity too and both systems are already rather large compared to a classroom.
 
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