Residential Design - Power Needs

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Samardas

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Location
Los Angeles, CA, United States
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Electrical Engineer
Hi, I usually design commercial and light Indutrial facilities. However, this 40+ year residential apartment complex has utility power @400A x 2, 240V, 1 Phase, one for the east side and the other for the west side of the facility. It is fed from a utility transformer on a pole. There are 24 and 23 apartments respectively (altogether 47) that have 90A panel in each. Each apartment has an electric oven, an electric heater and a window air-conditioner. Based on my rough calculations, the incoming power should have been much more than 400A each. Also, there is a disconnect switch for the house power and laundry room. Although I saw somewhere that the residential complex can have a multiplier of 0.4. This also does not satisfy the existing incoming power in my opinion. The contractor was asked by the building owner to replace the old distribution panels one for one because of some insurance renewal issue. The utility comapny might need a premise based transformer for higher curent capacity, but the building owner is hesitant to go with that.
Any thoughts, pleaase? Thanks a Lot!
 
Samardas said:
This also does not satisfy the existing incoming power in my opinion.
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Typical when NEC 220.87 is used to Determine actual Existing Loads.

Samardas said:
The contractor was asked by the building owner to replace the old distribution panels one for one because of some insurance renewal issue
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Typical diagnosis when panel flippers are asked to conduct building inspections for insurance renewals.

Give me a bone, so I wont notice defective smokes, unit wiring defects installed by remodel monkeys, and layers of paint clogging unit outlets & sub-panel switches.

Flipping the service is a typical bone requested, regardless if its clean, dry, and serviceable, or the last unmolested electrical system in the building.
 
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Thank you both Ramsy and Ptonsparky for your expert comments. They have not talked about EV chargers or solar panels yet.

The issue is that the utility company said they cannot give any demand or peak load because there are 48 meters in the facility. As we all know, once you touch any old installations you have to comply with the new prevailing code. I got one of the readings for the electric heater which itself is 13.3A (@240V, 20A CB). I understand the electric oven does not get fired up all at the same time, I may be able to consider a minimum of, say 15A (@240V, 50A CB), and the air conditioner of 5A (@240V, 20A), plus other 120V loads. The panel at each apartment is with a 90A breaker. I do not understand and cannot justify why, if the load needs to be that low to meet the 400A feeder (@240V, 1Ph for 24 apartments), the breaker size is so high. In the meantime, I am having the contractor put a power meter for the utility incoming cables and one of the apartments.

I would really appreciate more suggestions that will be really very helpful to design the proposed system.

Thanks a LOT!
 
I just went though similar thing. Utility was able to give me the percentage that each transformer in my situation was loaded up. If you do any NEC. Calcs even with your own diversity factor .4 or whatever it will still seem like it shouldn’t work. I’m adding air conditioning to all the units and utility still want concerned even with one that was loaded up at like 80%. I verified what work the contractor would need to do in the event they replaced the transformer and luckily it’s the same size vault. So we’re rolling the dice !
 
I should clarify I did do a 220 calculation for each unit panel. Basically the dwelling square foot va divided out between the lighting /receptacle circuits and adding in the known equipment loads. So in your case if you have a main you need to track that though then maybe apply the demand factors for number of units. My situation was different because my service laterals were directly from transformer to the meter bank
 
texie said:
You should do a complete Art. 220 calc for the premises. That is the only correct way and most AHJs would require ti.
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I should add that that the POCO transformer sizing is their responsibility. You need complete 220 calcs on all the premises side equipment and conductors if you changing most/all of the equipment.
 
Thanks, Texie and Charged. I appreciate your advice! Like I said, I am going to ask the contractor to put a power meter to check the load on the main incoming and one of the occupied apartments. Thanks again. Regards
 
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