Service Entrance Conductor Sizing

[⚡Shmn]

HELP PLEASE!
I have a question about an ongoing discussion in my area about proper practice. I will give an example and hopefully clearly display the issue.

Situation :
200A Service 120/240
SER Cable from exterior disconnect to loadcenter.

Common practice seen in the area is a 200A OCPD feeding a MLO panel with 4/0 Aluminum Service Entrance Conductors.

230.42 States ampacity shall be determined by 310.15.

230.42(B) States additionally the minimum ampacity for ungrounded conductors for specific installations shall not be less than the rating of the service disconnecting means specified by 230.79(A)-(D)

310.15(B)(16) shows
4/0 Aluminum at 75° is only rated for 180A

Question 1:
Is this wire sized 1 size too small?


Question 2:
Is this allowable because of the 83% feeder rating for single phase dwelling services in 310.15 (B)(7)?

Question 3:
Would the same be allowed to feed a 100A Sub panel with #2AWG rated 90A at 75°?

Question 4:
Even with the load calculated, how can a OCPD that supplies Amperage have conductors hooked to it not rated to handle the maximum possible amperage at the point of overload of that OCPD be code compliant?


I know this is dense but I want to make sure beyond a shadow of a doubt that I am installing the right thing. As well as have the proper knowledge to guide others in the right direction. I would like to know the answer by code not opinion please.


Thank you so much,

 

[don_resqcapt19]

See 310.15(B)(7) in the 2017 code or 310.12 in the 2020 code

 

[⚡Shmn]

So if I understand that 83% could be used on the 200A service with SER from disconnect to loadcenter?

Would #2 on a 100A sub panel be a violation because it does not contain ALL loads for the dwelling?

 

[augie47]

It would be a violation

 

[infinity]

So if I understand that 83% could be used on the 200A service with SER from disconnect to loadcenter?

Would #2 on a 100A sub panel be a violation because it does not contain ALL loads for the dwelling?

Yes the #4/0 qualifies for using the 83% rule the #2 does not. If you use the #2 then you would need to use a 90 amp OCPD at the origin of the feeder.

 

[⚡Shmn]

This is great! Thank you so much for the clarification. I really appreciate this community as a resource.

 

[retirede]

Shmn" data-source="post: 2654466" class="bbCodeBlock bbCodeBlock--expandable bbCodeBlock--quote js-expandWatch">

Would #2 on a 100A sub panel be a violation because it does not contain ALL loads for the dwelling?

Not if the #2 was protected at the main panel by a breaker corresponding to its ampacity (probably 90A). A subpanel is not required to have a feeder that supplies the full ampacity as the panel rating.

Oops - already answered above.