310.15(B)(7) clarification 3 unit apt

[dm9289]

Lets say multiple dwelling unit. 3 units calculated load 80 amps each so 240 amps all 3 combined.

So I'm assuming the service conductor size must be capable of 240 amps so 350kcm aluminum at 75C at 250 amps. I think what code says is i cant use 83% rule
do you agree?
So for main feed for all 3- 310.15(B)(7) rules do not apply?

All 3 units even though the calculated load is 80amp minimum service size is 100amps so for these individual dwelling unit feeders should be based on 100amp OCP not 80 amp calculated load but can be done at 83% of 100amp due to 310.15(B)(7) so 83 amps so #2 aluminum at 90amps.
Is this correct application of this section?

 

[Dennis Alwon]

Lets say multiple dwelling unit. 3 units calculated load 80 amps each so 240 amps all 3 combined.

So I'm assuming the service conductor size must be capable of 240 amps so 350kcm aluminum at 75C at 250 amps. I think what code says is i cant use 83% rule
do you agree?
So for main feed for all 3- 310.15(B)(7) rules do not apply?

All 3 units even though the calculated load is 80amp minimum service size is 100amps so for these individual dwelling unit feeders should be based on 100amp OCP not 80 amp calculated load but can be done at 83% of 100amp due to 310.15(B)(7) so 83 amps so #2 aluminum at 90amps.
Is this correct application of this section?

That is correct. Each units feeder, assuming they carry the entire load of the dwelling, can be sized at 83%. The service conductor cannot be sized at 83% but look at T. 220.84

220.84(A) Feeder or Service Load. It shall be permissible to calculate
the load of a feeder or service that supplies three or more
dwelling units of a multifamily dwelling in accordance with
Table 220.84 instead of Part III of this article if all the following
conditions are met:
(1) No dwelling unit is supplied by more than one feeder.
(2) Each dwelling unit is equipped with electric cooking
equipment.
Exception: When the calculated load for multifamily dwellings without
electric cooking in Part III of this article exceeds that calculated under
Part IV for the identical load plus electric cooking (based on 8 kW per
unit), the lesser of the two loads shall be permitted to be used.

 

[dm9289]

That is correct. Each units feeder, assuming they carry the entire load of the dwelling, can be sized at 83%. The service conductor cannot be sized at 83% but look at T. 220.84

Wow so that would be a 45% demand factor. So would that make the 240amp calculated load 108 amps and you could use 1/0 aluminum 120 amps at 75c?

 

[Dennis Alwon]

I think I would leave it 200 amps..... Do the apts have electric ranges? Is there hvac in each unit, washer dryer.... ?

Anyway you can't do 120 amp service it would have to be 125 amps but put 200 amps in case they use more. Also is there a house panel?

 

[dm9289]

I think I would leave it 200 amps..... Do the apts have electric ranges? Is there hvac in each unit, washer dryer.... ?

Anyway you can't do 120 amp service it would have to be 125 amps but put 200 amps in case they use more. Also is there a house panel?

This was more theoretical than actual just working thru some material in my books/DVDS and trying to make sense of it.
But yes I see where the dreaded Range table would come in, and Dryers. Yes and a house panel i left out of my made up example.

I do see the 200amp as practical but given the conditions 125 amp service would be compliant its correct just doesn't seem correct.