Single phase dwellings and feeders.

[2xcheck]

I've read some other post of people saying the 83% rule only applies to single family dwellings. But I see in 310.15 b7 it says or 2 family or multifamily of supplying entire load wether . So for instance if I had a 2 100 amp panels in each dwelling and a service rated 200 amps x 83% =166 A could i use 1/0 cu for the service according to 310.12?

 

[ramsy]

1) See Example D7 in Annex D.
2) Submit plans to AHJ, before inspection.

i) Some combination inspectors don't use that exception, without their planning department forcing them to allow it.

ii) In my state only building zone R3 qualifies for that exception, but some dwellings are not zoned as such, and inspectors may not recognize R3 in a mixed commercial district.

iii) If inspector challenges you, and you can't provide stamped plans, or recognized authority, like a deer in the headlights they will mow you down.

 

[2xcheck]

Thanks for your input . I've done multifamily installs years ago but never planned them out . Thanks again.

 

[curt swartz]

The 83% only applies to the individual dwellings not the service conductors feeding multiple dwellings.

Also, 1/0 is only rated for 150 amps.

 

[2xcheck]

What I'm I missing 310.15 b7 1 says service conductors for a one family or the service conductors supplying the entire load in a 2 family or multifamily dwelling. Then b7 2 talks about the feeders.

 

[2xcheck]

Also according to the 2020 nec 310.12 says 1/0 cu is good for 175A

 

[augie47]

I think you are missing out on the word "individual" as in "the individual dwelling units" qualify.
So on a 6 unit dwelling, the conductors to each individual unit could employ the 83% but the overall service for the entire structure could not.

 

[petersonra]

i) Some combination inspectors don't use that exception, without their planning department forcing them to allow it..

How does any inspector get to pick and choose what he allows if the code allows it?

 

[jaggedben]

How does any inspector get to pick and choose what he allows if the code allows it?

By not having someone reasonable for a supervisor. Because suing AHJs isn't something most contractors are gonna do.

 

[infinity]

What I'm I missing 310.15 b7 1 says service conductors for a one family or the service conductors supplying the entire load in a 2 family or multifamily dwelling. Then b7 2 talks about the feeders.

Also according to the 2020 nec 310.12 says 1/0 cu is good for 175A

As Augie stated you cannot use that table for the feeder or SEC's to multiple units so the ampacity is 150 amps. Something else to consider with 2-100 amp panels you do not necessarily need 200 amp conductors. They can be sized according to the aggregate of the load calculations for the two units so your #1/0's with an ampacity of 150 amps may be large enough.