Dave, I think you're OK with that placement, but I suggest you confirm with your local building dept. The plywood is for insulation, and it's indeed a very good practice.

Insulation? Plywood isn't a particularly good thermal insulator nor is it tremendously useful electrically.

The main reason for the plywood backing behind panels is to give a convenient place to secure the wires coming in (as well as mounting some ancillary things like doorbell transformers, etc..).

Have you considered replacing some of the standard sized breakers in the existing panel with the slim breakers that fit two breakers in the space required for one standard sized breaker? In my own panel I have 10 double breakers.

You also use shoddy wiring practises, Texasredhead. The OP has the right mindset to install a subpanel rather than add further circuits to his main panel.

If your panel is rated to accept the slim breakers there is nothing shoddy or wrong with using them.

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

Sounds like someone went circuit happy if you have 40 circuits and only use 87 amps. I think lonesparky answered your original question.