Derate calcs for underground feeder wires

[Designer101]

I have encountered a situation, where we have to do feeder tap into the existing sub panel 225A, and its connected to split phase 400A rated main panel onto one of 200A main disconnect. the existing feeder wires are rated 2/0 ONLY, But the temperature where PV is installed is usually high and we have to used derate factor of at least 0.87.
. my question is how are under ground feeder wires derating calcs done? is it separate for the wiring calcs thats are done on ambient temperature? Or the calcs is done similar.
example 200 A of Service wires ( ambient temperature is usually 100 degrees F ON MOST OF SUMMER)
THWN-2
(3)Awg #3/0
ampacity 225*0.87=195.75A, i used 0.91 instead of 0.87 and choose 204.75A as final derate ampacity, to argue 3/0 is sufficient.
but the existing feeder wires have only 2/0 pre wired .
I am not sure how the 2/0 wire was used as feeder between sub panel and main panel? and the inspector was okay with that. Or I m missing some major information here, I am not an electrical engineer by profession my back ground is from mechanical.
thank you

 

[Rock86]

Is the soil super warm where you are? Normally for underground conductors, you would do an ampacity adjustment for # of conductors in a given conduit but not because of temperature. 2020 NEC 310.15(C)(1)(c) excludes ampacity adjustment for conductors underground, 10ft above emergence of earth, and 4 or fewer conductors.

Not sure I can explain the existing #2/0s. Was the system always rated for 200A?

 

[Dennis Alwon]

2/0 is only good for 175 amps-- is this a dwelling?

 

[Designer101]

Is the soil super warm where you are? Normally for underground conductors, you would do an ampacity adjustment for # of conductors in a given conduit but not because of temperature. 2020 NEC 310.15(C)(1)(c) excludes ampacity adjustment for conductors underground, 10ft above emergence of earth, and 4 or fewer conductors.

Not sure I can explain the existing #2/0s. Was the system always rated for 200A?

Umm, Its riverside county in southern California, It hardly rains may be 7 times per year, and temperature can go high as 115 degrees , thank you for the code reference I was not sure which section of code should I refer to for the underground cables. Any ways looks like Inspector was too lenient and didnt care for the wires ran underground for the feeder, so what ever the temperature column you looks, 75 degrees or 90 degrees the minimum conductor should have been 3/0 or above if the temperature derating is not needed and only conduit derate factor is applied..

 

[Designer101]

2/0 is only good for 175 amps-- is this a dwelling?

yes this is dwelling, now I am confused how to know the which temperature column should we use for table 310.15 (b)16, nec 2017, I usually used 90 degrees column and using that too its still 195 only so 3/0 Should have been used.

 

[Dennis Alwon]

You can use 2/0 if the conductors carry the entire load of the dwelling. You can do this by using art.310.15(B)(7)

 

[jaggedben]

Yeah I see a lot of confusion in residential installations where people seem to have internalized the ampacities implied by 310.15(B)(7) without understanding the specific circumstances under which it's allowed to be applied.

If the 2/0 needs to stay, maybe you can do a load calc and downsize the breaker protecting it.