Nope, it's a huge flaming code violation, because a misstep (or miscommunication between two people) will wind up with the generator "backfeeding the grid" and killing line workers.
"But I'm sure we'll do great." No, you won't. The military has studied this: in a crisis, people don't rise to the occasion, they sink to chaos - or, the level of their training. That's why the US military trains so hard.
"But the linemen are supposed to ground out their work area" After they've been up for 2 days straight, and/or called in from a neighboring state and working on systems they're unfamiliar with? Don't count on it.
"But they'll never know" actually smart meters can tell them.
"But there's no penalty" LOL... read your service agreement. They can shut your power off and tell the city, and that hole will cost you at least $10,000 to dig out of, even if there's no $10,000 fine per se.
Run two cables but wait! Relax.
"You can't believe how expensive 6/3 is, I'm not buying TWO of them!" That's right. You're not. Or even one, for that matter.
Look inside your panel, see the neutral bar? That's not tin-coated copper. That's tin-coated aluminum. See the main lugs? Those are aluminum too, and most likely so are the fat wires. Turns out, aluminum lugs play well with copper wire (the reverse was the problem), and aluminum wires work great on them. Large feeders have never been a problem.
The generator feeder.
So, we need two cables. First, the generator feeder. Because of problems with NM and UF jacketing, they are limited in their amperage. Aluminum wire isn't sold in those types, so they get a higher amperage than you'd expect. #6Al is good for 50A if the terminals are rated for aluminum and 75C wire. And the stuff is dirt cheap.
Make sure to get inlet terminals rated for AL wire and 75C thermal - that shouldn't be a problem.
If you want 65A wire, use #4Al. If you want 90A wire, use #2Al. They're not that much more money, really, but just watch what will fit on your inlet terminals. (I'm afraid this does nothing to reduce the cost of the inlet).
"That wasn't so bad!"
The subpanel feeder.
OK, now the subpanel feeder. You can use #6Al there too, but since the stuff is so cheap, I say use #2Al so you get 90A to your subpanel. It lives in a happy price point because of its frequent use for mobile home feeders (in fact "MH Feeder" is the perfect stuff outdoors, or indoors in conduit).
"But I don't want a 90A subpanel, only 60A". Do it anyway, the cost difference is pretty small, and it will give you loads of expansion room for anything you do in the future. Spaces are cheap - and nobody ever came in here saying "Help! I want to add something, but my panel has plenty of extra spaces in it."
And in 2021 we're even seeing people get higher offers, by thousands of dollars, simply because their garage is ready for EV chargers... because they're thinking there might be an EV in their future. (For the homebuyer it's easier to add $11/month to their mortgage today, than lay out $2000 cash right after buying a car). Anyway, for that you'll want 90A (to run an EVSE and whatever else, or dual EVSE's using Share2 protocol).
The bill of materials:
- Interlock for main panel (US$30-70 typically)
- 2 breakers
- Inlet with terminals rated 75C and AL wire.
- X feet of #6 AL 4-wire cable
- X feet of #2 AL 4-wire cable
- 24-space, 200A rated subpanel. Main breaker required only if it's an outbuilding. It can be rated down to 100A - point is, there's no reason to avoid a 200A panel if it's available.
- various conduit, trenching, cable clamps etc. as needed. Don't put both cables in the same conduit without doing a derate calculation (we can help).
Note that Rigid Metal Conduit only requires 6" of cover). Expensive, but you can trench it with a garden trowel, so saves calling DIG-SAFE to get the utilities marked, and saves ditcher rental.