Remodel / Service Calc

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Hoyt2020

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Electrical Design
Good day everyone. Service Calc question.

Working the electrical design for the remodel of a multi-story apartment (1960 original build) (not a high rise). 150 existing resident rooms (dwelling units) (range/cook top and existing dedicated electrical panel to the room).

Dwelling Unit Rooms:
Central boiler system for heating. Baseboard hot water in each DU.
Central Laundry
Central hot water

Original design of the building did not include cooling for the resident rooms. At some point (not sure the year) they allowed the residents to utilize thru wall / window, 120v residential plug in AC units. (Staff removes them each Fall and reinstalls each spring) (about 75 of the residents utilize them... so half). The resident supplies them, so they are all different. Nameplate was 6.5a - 120v on one I noted.

Project Scope - or the DU rooms: paint up, carpet, new casework, 1 for 1 electrical device replacement, resident room electrical panel replacement, FA upgrade, and permanent cooling (condensing unit mounted on the exterior wall outside the room - 1 for each room) w/ high wall fan inside. 208v/1P - 15 FLA (fan powered via outdoor unit). Stated as a Level 1 remodel.

Existing electrical building service is a 1600 amp 208/120v 3P service. Peak load was 180 KW - highest in the last 24 months.

Question:
I need to verify the existing 1600-amp service is not overloaded.

I believe I should apply - NEC 220.83(b). I arrive at 37 amps at 208v / 3P per this calc. for each resident room.

I believe I then can apply Table 220.84 Optional Calc - Demand Factor. 62 Dwelling units and over is a Demand Factor of 23 percent. 37a per room x 150 rooms x 23 percent equals 1277 amps, without House loads. Per this direction the 1600 is overloaded... if I applied everything correctly?

Code possible to attack the service calc. in another manner, since we have the 24-month peak load? Possible to apply the new AC DU loads (15a at 208v/1P) in each room (150) x the 23 percent in Table 220.84 plus peak load at 125 percent? Options?

Your input is appreciated. Thank you.
 
Hoyt2020 said:
I believe I should apply - NEC 220.83(b). I arrive at 37 amps at 208v / 3P per this calc. for each resident room.

I believe I then can apply Table 220.84 Optional Calc - Demand Factor. 62 Dwelling units and over is a Demand Factor of 23 percent. 37a per room x 150 rooms x 23 percent equals 1277 amps, without House loads. Per this direction the 1600 is overloaded... if I applied everything correctly?
Click to expand...
You can't apply the results of 220.83(b) to 220.84.
 
I can understand that. Is calc process, in this scenario, simply applying the service calc thru 220.84.... even if we have an existing building with a 24 month peak utility load of 180 KW and the only additional load is 150 new dwelling unit AC units (15a FLA @ 208/1p per room)? No Demand factor to apply on 150 room AC units? Has to be a way to apply an optional demand factor to new AC load on an existing service with multi family dwelling units.
 
You're increasing the load by too much. You're going to need a bigger service. I have a spreadsheet that compares Article 22 Part III vs. Part IV methods. Part IV always beats Part III for more than about 30 dwelling units. I assumed (2) 1500 VA small appliance circuits and a 32A @ 208 cooktop in 150 units and then plugged in different average square footages to see what got me down below 1600A. I stopped when it got down to 300 ft² and I was still over. I never even got to guessing the house load.

That's just looking at the service entrance. What kind of distribution is between there and the units? Is it still in good shape after 62 years?
 
Thank you @JoeStillman. Yes, I agree on the service, yesterday I began the process to upgrade. New generator / ATS, etc. as well.
(the units are around 400-475 square feet).

The existing distribution to the units is as follows:
(12) vertical 3 phase 200 amp feeder runs, served directly from the service entrance distribution section in MDP (each of the 12 feeder is served with a 200/3 OCPD).
Each vertical feeder run serves either 8 or 15 unit panels. The unit panels are single phase - 100a - 208v/1P with a 100/2 mcb. They utilized a 3 phase feeder system to feed the single-phase unit panels, as stated 8 or 15 on each vertical feeder run.
 
You definitely come out ahead with the 220.84 method when you apply it to the 200A risers. You get a 43% diversity factor for 8 units and 40% for the 15. But with that, the 15 unit riser doesn't make it at 200A even if they were all 400 ft². This assumes no fixed appliances like a dishwasher or microwave.
 
There are single bed units on one side of the corridor that are smaller - 365 square feet. With that said, the 200a still won't make it on the 15 unit riser... even if that riser had all the smaller single units on it (dont have that verified yet). The 8 unit riser does appear to work even with all larger 475 square foot units.

No dishwasher, no fixed Mic. I do see some units the resident will utilize a counter top unit.
 
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