Calculating Main Disconnect

[tkb]

We are finishing up a 31 unit project in Boston, and the service size is 1200 amps three phase.
Sounds similar by the OP description.

 

[kwired]

I haven't gotten involved in anything else other than the main disconnect, 600 amps just seemed too little for 20 all electric units. With the chart 220.8 we see its actually 38% which pushes us to 612 amps. That would've been a huge and costly mistake to replace an entire main disconnect. I will find out where he got the 32% from and show him the chart.

Thanks everyone for the help will report back.

single phase or three phase?

I have a low income housing complex I have done work off and on at over the years, 600 amp 208/120 three phase, all electric, 20 units (spread out via 5 different buildings) plus the admin laundry and community center in an additional building. I never done an actual load calculation on the facility, but would be willing to bet you seldom if ever could come in and measure more then 400 amps on the main supply.

 

[profT]

single phase or three phase?

I have a low income housing complex I have done work off and on at over the years, 600 amp 208/120 three phase, all electric, 20 units (spread out via 5 different buildings) plus the admin laundry and community center in an additional building. I never done an actual load calculation on the facility, but would be willing to bet you seldom if ever could come in and measure more then 400 amps on the main supply.

It is actually single phase. How new/well insulated is the complex?

 

[kwired]

It is actually single phase. How new/well insulated is the complex?

1965 give or take a year or two.

Probably not insulated nearly as well as it would be if constructed today.

Cove heaters in most apartments, those that aren't will be converted eventually as original boiler systems have been failing but there is only maybe 3 or 4 at the most that haven't been converted to electric heat. Admin building still has gas boiler right now.

They all still do have gas water heater in each building.

Looked at what heaters I have sold them - each apartment only has about 4650 watts of heat. That puts us at about 260 amps if they all would be running at one time and is balanced across all three phases. Regardless of what NEC says for the rest of the load - this place is mostly older retired people with little money, there isn't room to have a lot of things most just don't have much for load and I would bet you seldom would ever see more than 15 kVA being drawn (outside of the heating) from all 20 apartments. Not sure what sizes are but they are only one bedroom, a bath room and living/kitchen are sort of in one room.

 

[Ingenieur]

What is the avg sq ft per unit?
387000/1200/20
assuming 1200 that is 16 va/sq ft
not an unreasonable number