I just ordered some electric snow-melting cable that requires 40A/220V GFCI protection.

We have 200A service to our main panel, and there is only one slot left open. We have two C-H subpanels: one (fed by an 80A breaker) was recently installed for a renovation, and the other is older and fed by a 70A breaker. The older one only has 6 breakers in it, and only feeds our 3 A/C units.

So I thought, no problem, I'll put the 40A two-pole breaker in the subpanel for the A/C, since they will never be used at the same time, and the cover has 5 slots/side. However, I take the cover off and the buss does not extend beyond the existing 3 breakers/side. So I wonder if there is a way I can just install a longer buss in the case to accommodate the new breakers, since the case cover can cover 10 breakers (5/side). Of course, then I wonder if there is something in the NEC about what size feed I would need to accommodate more breakers (even though we will not exceed the 70A feed.

There is plenty of space in the newer subpanel, so that is another option: I could put the new double-pole breaker in there, but then would I need to worry about having a 35A load on one circuit when the whole subpanel is 80A? Yet another option is moving one circuit from the main box to the new subpanel, thereby opening up room for the new two-pole breaker (which would be SquareD rather than C-H).

Any advice is appreciated.