Multi-Family Dwelling Calculation

[jnreimer]

I'm working on a project that has 160 residential dwelling units and there is some confusion between several engineers in our office on whether or not we can use Table 220.84 (Optional Calculation Demand Factors for Three or More Multi-Family Dwelling Units).

We meet the first two provisions for using the table. 1) No dwelling unit is supplied by more than one feeder and 2) each dwelling unit is equipped with electric cooking equipment. The confusion lies within the third provision. We can't decide if our units have "air conditioning" or not. Our HVAC system is a four-pipe fan coil system. Heating water is provided by a central boiler plant and chilled water is provided by a chiller plant. The boilers, chillers, and associated pumps are common to all dwelling units and are fed electrically from the main switchgear. The only HVAC load that is fed from our dwelling unit panel is a fan coil that consists of a fan, filter, and hot and cold water coils.

Is the fan in the fan coil sufficient to be considered "air conditioning"? Or does "air conditioning" require that the entire dwelling unit air conditioning system be powered from the dwelling unit panel (e.g. heat pump, furnace w/ condensing unit, etc.)?

Any help is appreciated.

 

[charlie b]

You are going to have to get a ruling from your local AHJ. But my inclination is to say that you do not meet the third requirement. My reasoning is that the purpose of any demand factor is to take into account the probability that not all equipment will be running at the same time. If everyone had their own air conditioners, then the various compressor units would be cycling on and off all day, and not all would be on at the same time. But if all you have is a fan, it is more likely that all fans will be running at the same time. Thus, using a demand factor would impose an unrealistic condition on the sizing of the service.

 

[bdarnell]

I agree with Charlie. I just finished a design of a 10-story 50 unit condo building with a 4-pipe system and did not use the optional method for that very reason. We met the first two provisions as you do, but it would be a stretch (and, IMO an inadvisable one) to say we met the third.

 

[jnreimer]

Thank you for the responses. I have calculated the service size using both calculations. The optional calculation yields 1850 amps at 208/3, while the standard calculation yields nearly 3500 amps. Needless to say, this will have a significant impact on the design.

 

[charlie b]

With that much power needed, I would suggest (if thisis at all possible) that you consider bringing in power at 480 volts, and placing transformers and 120/208 volt panels at strategic locations.

 

[George Stolz]

I agree, you can't use Table 220.84. (Not to hijack the thread or anything, but I am curious about the reasoning of this requirement. If someone has a fact or opinion on it, please share it. I don't get it.)

How many units per building? (This isn't one building, right? )

Sounds as though multiple services per building might be the ticket, too, depending on the circumstances.

Can you provide more details?