Sub panel questions

[NEpowerandlight]

I have an EG4 split phase inverter and I want to run the AC out to a main lug 125A sub panel so I can power up some new branch circuits. But I'd also like to be able to power some critical loads in my main panel when the power goes out. I think it would be easier and cheaper than moving critical loads to a sub panel?

So my question is: Can I run a 40 A circuit from the sub panel to a 40 A breaker in my main panel with an interlock? Is there any issues with having 2 breakers on the same circuit, one at the sub panel and one at the main panel?

As I ramp up my solar I'd like to be able to run a small mini split and 1 or 2 20 amp branch circuits from the inverter off grid through the sub panel and be able to have back up power.

 

[MurphyGuy]

So long as the main breaker is interlocked with the 40 amp breaker being fed from the sub panel, you should be fine.

There is no issue with having 2 breakers on the same circuit if they are in series. Putting them in parallel would be bad as the combined amperage of both breakers could set the conductor on fire.