310.12 & 230.42 for Single Dwelling 400A

[regnar]

Any help explain this please.

310.12 says "the service conductors supplying the entire load associated with a one-family dwelling, shall be permitted to have an ampacity not less than 83 percent of the service rating"

230.42 says "Service-entrance conductors must have an ampacity not less than the maximum load to be served"

So my service is 400A and my load is 355.
Do just apply the reduction to the whole service 400A x .83 and size the wire for 332A with parallel 4/0 AL, or do i have to use the whole load 355A and use parallel 250Kcm AL?

 

[infinity]

So my service is 400A and my load is 355.
Do just apply the reduction to the whole service 400A x .83

That's all that matters. 355 amps calculated load tells you the minimum service size. 400 amps exceeds the minimum size required. You can apply 83% for condcutor sizing to the 400 amp service size so 400*83%=332 amp minimum conductor size with 400 amp OCPD.

 

[regnar]

That's all that matters. 355 amps calculated load tells you the minimum service size. 400 amps exceeds the minimum size required. You can apply 83% for condcutor sizing to the 400 amp service size so 400*83%=332 amp minimum conductor size with 400 amp OCPD.

Ok so that means that 310.12 supersede 230.42?. Meaning pretty much regardless of the load, for a 400A service I can always size it parallel 4/0 AL or parallel 2/0 CU (without taking in consireration distance VD etc)

 

[LarryFine]

Seems like you can either use the calculated number or use 0.83, your choice.

 

[regnar]

Seems like you can either use the calculated number or use 0.83, your choice.

Well, management will want us to do minimum.
Just got confused because I was doing a quick calc using AI (And both ChatGPT and Gemini were telling me that I needed to size based on the load calc and use 355 and do 250KCM because was bigger than the .83 reduced 332A. This isn’t the first time those tool give me the wrong answer in my face and I have to manually do the calc and research)

 

[infinity]

Meaning pretty much regardless of the load, for a 400A service

Yes, providing that the 400 amp service is adequate for the calculated load you can size the conductors using 83% of the 400 amps and meeting all of the requirements of 310.12(A) (2023 NEC).