Table 220.44

[charlie b]

Two part question. I am assisting in the design of a large laboratory building. The company has created a standard spreadsheet for performing the service calculation. They also have spreadsheet for individual panel schedules, but that spreadsheet does not have the problem I am about to describe.

When the "service calculation" spreadsheet calculates receptacle load, it starts with the area of each floor, and multiplies by an assumed value of watts per square foot. The assumed value of w/ft2 might vary, depending on the types of rooms (lab versus office versus corridor versus equipment room, etc) that are contained on each floor. Then the spreadsheet takes the first 10,000 watts, and adds 50% of the remaining watts. Clearly, its author had in mind Table 220.44.

Question (Part 1): Is this a mis-application of the table? I think it is. I think you cannot use Table 220.44, if you calculate the receptacle load on a ?watts per square foot? basis.

Question (Part 2): Does anyone have any experience in comparing receptacle load, as calculated on the basis of ?watts per square foot,? to receptacle load, as calculated on the basis of counting the number of installed (or planned) receptacle outlets, and assigning 180 VA to each duplex? My initial conjecture, using an office/cubicle situation as an example, is that the ?watts per square foot? basis is going to start out low, and that applying the demand factor of Table 220.44 will end up undersizing the service.

Comments????

 

[Smart $]

I agree it is a misapplication of Table 220.44. Subpart 220.44 only permits the demand factors of the table to be applied when receptacle loads are calculated "in accordance with 220.14(H) and (I)". Therefore, receptacle loads calculated by other means cannot have the demand factors applied to them.

As I see it, the spreadsheet may serve as a best-guess preliminary design tool... but nothing more. Without having a full understandng of the spreadsheet, its development and usage, I cannot comment further. Off hand, I'd say your "gut feeling" is steering you in the right direction

 

[steve66]

I agree that the demand factor doesn't apply to loads calculated on a square foot basis.

I would do two calculations. One using their spreadsheet, and one following the NEC watts/square foot. Give them a service that meets the higher of the two.

Steve

 

[steve66]

Charlie:

In regard to question #2, I always use the watts per square foot. If you are going to count receptacles (and if you have powered cubicles), are you going to count each receptacle that is in the movable office partitions? The problem with that is you usually won't know how many receptacles they will put in these until late in the project.

I use a calculation of 1 w/ft^2 foot for office receptacles, and 3.5 w/ft^2 for lighting. However fluorsecent lighting would probably never hit much more than 1 w/sq^ft, and new energy codes limit the amount of lighting power I can have to 1 w/sq^ft. So the way I see it, the building actually has 1 w/ft^2 for lighting, and 3.5 w/ft^sq for receptacles.

 

[charlie b]

It turns out that, for the particular project that led me to ask this question, the table does apply. But that fact would not be any more evident to a plan reviewer than it was to me. I did notice that the spreadsheet included some odd values for the assumed ?watts per square foot? for receptacle load.

Late yesterday I came across an instruction from the owner. For each type of space (e.g., office or lab), it gave a minimum number of circuits, and a specific number of receptacles on each circuit, for each 100 square feet of space. I assigned 180 VA for each duplex, and then converted to units of ?watts per square foot.? Lo and behold, it exactly matched the numbers that the spreadsheet had used as assumed values. So in essence, the receptacle load was calculated on the basis of 180 VA per duplex, and therefore Table 220.44 does apply. But a person looking only at a printout of the spreadsheet would not be able to discern that fact.

I will have to take care, in other projects, to make sure that the spreadsheet is not used improperly.

Thanks for the backup and the other information you both provided.