if I have a sub panel in my outdoor shed is it possible to back feed my generator through the subpanel and if so how and where would i put the interlock. help
I'm afraid not. No way to do it.
This has been asked a lot, and we've searched at length, and there's no legal or safe way that we have found.
And it's exactly the problem you're seeing. You >>NEED<< a way to have the interlock work; so the generator can't turn on except when the utility main is off. And there's no way to implement that.
So the conventional way to do that is just put more wires in the conduit, or trench a new cable if you're doing direct burial.
At least in the cheap, simple, "cost IS an object" universe.
The PowerWall option
Now of course everything is changing in the 21st century with demand-side management, EMS, and all that. Let's imagine you're installing a Tesla PowerWall, which is a sophisticated inverter that can be grid-tied (to sell solar production back to the utility) or off-grid (during grid-down conditions). It also needs an interlock.
The way the PowerWall and competitor systems do it, is to have a power-operated disconnect - not a transfer switch, a disconnect - on the utility side of the main. Some of them are "meter collars" that sit underneath your electric meter. On command from the PowerWall, that disconnect opens, and then, an interlock (mechanical switch) on the disconnect closes, signaling "the disconnect is open". At that point the PowerWall knows it's safe to spin up.
I mean if you were putting a Tesla PowerWall in the shed, that's just how you'd do it, but you'd still be having to wire that control cable, so unless the existing run is in conduit, you're still trenching.
So for your generator, I suppose something like that could be done. And the disconnect switch could be manual. But I really can't point you to any products on the market that would do that achievably.
And even if such equipment was readily available, we can't say it would be cheaper than trenching a second set of wires for the generator, e.g. 6-6-6-6 aluminum is a bargain... which is the straightforward approach.