Power Supply for Multi-Unit Apartments

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frflaspo

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Location
Fayette, Missouri
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General Contractor
I am working on a multi-unit apartment building, containing 8 individual units on the second floor above a commercial retail space (the lower level is completely independent, and has it's own electric service). Each unit will be approximately 450-500 sq feet, with individual mini-split heat/air, hot water heater, stove, & refrigerator. Utility service is supplied by the local municipality. I believe I have my demand load calculations complete, but I was looking for someone to verify them. These are based on the standard calculation under the NEC Article 220:

Each apartment is approximately 450 sq ft (450 * 3vA = 1350vA), plus the presumed two small appliances (3000vA). Additionally, each will have its own mini-split A/C & heat system (15A x 240v = 3600vA). Each unit will also have its own stove range (40A x 240v = 9600vA). Water heaters will be 30gal tanks, 3.8 kW (3800vA). I am not including any washer/dryers, which will be in a communal area on a separate panel, which already has its own service line (as will hallway and exit lights).

As I understand the calculation, for the general load, the first 3000vA gets calculated at 100%, with the remaining 1350vA at 35% (3000 + 473). The A/C unit and range are at 100% (3600vA + 9600vA) and the water heater at 75% (3800 x .75 = 2850vA). That brings me to 19,523vA per unit (3000+473+3600+9600+2850). As I see it, that will allow me to run each unit on it’s own individual 100 amp panel.

Multiplied by 8 units, that's 156,184vA demand. The formula I have is Demand=vA/(240 x 1.732) for single phase service, which give me a 376 amp demand. In order to keep it at 80% usage, a minimum of 500 amp service would be required (which is significantly lower than I was imagining, so I may have gotten lost in the numbers).

If those numbers are correct, then my next question is about 3-phase power. I only want single phase service in the apartments, but there is currently 3-phase power coming to the building. It appears to be coming in on 3/0 copper stranded wire. Can that line provide 500 amp service? Would I need to convert the power over to a single phase system?

I am also looking at several different residential meter stacks, and it appears that Eaton makes several 4-gang units that take three-phase in, and then single phase out at 125 A. If that eliminates the requirements for a transformer, then I think I could use that to feed at least one of the meter gangs.

I would appreciate any thoughts, suggestions or recommendations...
 
Place close attention to phase loading on the three phase side using single phase for eight units.
One phase will have higher load.
2x8=16/3= 5.33
 
As stated, there are multiple issues with your calculation. You didn't seem to take advantage of the range demand factors, I'm not sure what the 80% usage is about, and other things. Additionally, presumably this would be fed from a 208 three-phase system, in which case some of your appliances would have lower wattage than nameplate so you could adjust for that (typical 4500W water heater would be 3400 watts)
 
There are several flaws in your calculation and thought process about 3 phase. Strongly suggest the services of a electrical contractor or engineer.
Yeah, I've sent the specs to an electrical engineer for them to look over as well. Just wanted to see what I could figure out in the interim.
 
As stated, there are multiple issues with your calculation. You didn't seem to take advantage of the range demand factors, I'm not sure what the 80% usage is about, and other things. Additionally, presumably this would be fed from a 208 three-phase system, in which case some of your appliances would have lower wattage than nameplate so you could adjust for that (typical 4500W water heater would be 3400 watts)
Thanks! I'll look back at the cooking demand loads, and see if I can recalculate that part.
 
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