NEC code specifies that my 200amp main will only be allowed to handle a 40amp backfeed from a PV solar array. I need more than 40amps. I want to upgrade my main panel to 400amp and derate the main breaker to 200amps. Is this an acceptable approach? Are all 400amp panels 3phase. I believe my current service is 2phase, this is residential.

Residential is single phase. A 400amp panel is still single phase.

The code section is NEC 705.12(D)(2)...
You are limited to 120% of bus rating of the panel. Check the label on your panel, some are rated at 225 amperes...
You could install a 200 ampere main circuit breaker panel with 225 ampere rated bus that would allow a PV input of 70 amperes...

Single phase panels are available in many different sizes and ratings from different manufacturers.

I have a picture of the label, There is no way to put that into a reply. How can I get the label picture to you? I think it is stuck at 200amps but I would like a trained eye to verify it.
Thanks

Post a picture. Click on the "Go Advanced" button.

Label attached
47118

See the top left corner... "200 amperes maximum"
How much do you want to connect to the panel?

See the top left corner... "200 amperes maximum"
How much do you want to connect to the panel?
80 amps from a PV solar array.

Have you read the electrical code to consider any of the alternatives?
by the way...if my math is close, do you have around 100 solar panels?

Have you read the electrical code to consider any of the alternatives?
by the way...if my math is close, do you have around 100 solar panels?

No, they are 315watt panels, there are 60 panels.

I could degrade the main breaker to 175amps. I cannot find a 175amp main breaker for a Siemens panel. Does anyone know where to get one? Or does a 175amp even exist?