Thanks for everyone's input. My main panel has 30 spaces and allows a maximum of 40 circuits. It already has the full complement of slimline breakers and is maxed out at 40 circuits by the builder. I'll need to move a few circuits over to the sub-panel to make room for the double pole 70 amp feeder breaker. By the way, the main panel is rated at 200 amps and I calculate my current home load at 87 amps, so I have plenty of capacity available for the 40 or so amps I intend to use from the sub-panel. Dave

Any comments on inspection from the OP? Not anything I can say from personal experience... yet. This project is the first change to the electrical since we moved into the house, so no one but the purchase home inspector has looked at it and told me anything. While he was not an AHJ, he didn't say anything about it. But I'm not sure that's worth much. I'm assuming it did pass an AHJ inspection when the house was built, or if the previous owner(s) had any work done. (The house was built in the mid-80's and we moved in about 8 years ago.) We all know what assumptions are worth. Will update when we do get an AHJ inspection.

"i do not get the seperation between n/g when they are bonded in the main panel, anyway. the code writers are idiots." I find is extremely humorous that you call the code writers "idiots" when you don't even understand basic electrical theory. Because you don't understand why neutrals and grounds are separate in sub panels, the code writers are idiots? You don't even own a copy of the NEC to even look at the codes, yet you're calling code writers idiots. If anyone is the idiot here I bluntly say it's you, smithy. Your posts are a joke. You give blatantly wrong information and advice, and you act like you're some electrical God. Stop posting before you get someone hurt.

I suspect he's out of poles in the original panel so the two circuits he's moving are ones he is getting out of the way so he can put the feeder breaker in. There's no such conduit called bendable conduit. FMC, LFMC, and FMT are restricted in their use in the Chicago code and I do not believe they can be used here. LFNC is not permitted at all. If you run RMC or EMT, you can likely use the conduit itself as the equipment ground. The THHN 12g Copper is fine for 20A even after derating. For a 50A feed, derated, you could get away with 8g Cooper. To stuff 4 #6 and 4 #12 in EMT, 3/4 is too small, 1" would be the smallest allowable size.