I want to run feeder wires from the main breaker in the house to the sub panel in my garage, also at the same time I want to run another pair of wires from my generator located in the garage to back feed the main breaker box when the power goes off. I already have an interlock kit installed and just want to see what the codes are the in and outs of doing it this way. I already know that you can have a branch circuit in the same conduit as the feeders, but can I put the genny's wires in the same conduit as the sub panel feeder's or do I need to run seperate conduit for the genny or not. Also I am aware of derating and conduit fill but want to know if derating can be offset by going to the next larger wire size. The genny will require 60 amp breaker and #6 wire
Sounds like you're doing everything right and side-stepping the usual mistakes we see in this arena.
Well, I can tell you the derating part. You'll have 4 live conductors (neutrals and grounds don't count for derate purposes). There are 3 columns in Table 310.15(B)(16). You must use the lowest temperature that both the wire itself and the terminations (connections) can bear. For instance if you have a 75C lug on the breaker and THWN-2 wire good for 90C, but your generator inlet is 60C rated it drags you down to the 60C column for your practical load (e.g. for #6Cu 55A rounding up to 60A).
However, when you derate, you derate based on the highest temperature the wire is possibly allowed. So that's 90C for THWN-2, for #6Cu that is 75A.
With 4-6 wires (that count) in the pipe, you derate to 80% (but off the max the wire is allowed). 80% of 75A is 60A.
Why don't you derate off the 55A number since you are terminating on a 60C lug? Because the 60C lug is not inside the conduit.
NM-B and UF-B cables have an exception. They are 60C wire through and through. But you are allowed to derate of the 90C column anyway.