How do I go about installing a subpanel on wood paneling that's mounted to a concrete wall?

Question

I've been reading, researching, watching, and preparing to install a sub-panel (which I'll be having a licensed electrician inspect and sign off on before going live) and I think I know how to go about doing it, but I have great respect for this very dangerous work and always make sure I know what I'm doing before I do it.

I'm not sure the significance of my situation here, but it is different enough from most information I've found across the interwebz that I wanted to ask about this specifically. I'm in VA btw, and VA still uses NEC 2011.

Here's what I think I know: Panels can't be mounted directly against a wall in a damp location (basements can be considered damp); the ground and neutral are bonded in the main box but MUST be isolated from each other in a sub-panel; the actual box of the sub-panel should be bonded to the ground bar; if a panel is installed in the same structure as the main breaker, it does not need it's own grounding rods (but if it's a detached structure, 2 copper grounding rods 6' apart are required); if a panel is installed in the same structure, it does not require it's own main breaker (and if it is in a detached structure, a separate breaker to power down the sub-panel is required); for 80 AMP panel, 3 Gauge NM-B, or 4 Gauge copper THHN is required (this is the size that I'm planning).

And here's how I'm thinking this should be done (packchures for reference below):

The wood paneling is secured to "studs" that are nailed to the concrete with masonry nails. Between the "studs" (I'm not sure if they can actually be considered studs so, quotes) is Styrofoam insulation. And since Styrofoam catches fire if you just look at it the wrong way, that worries me just a little. I'm not sure how secure the "studs" are so I figured it would be best to simply go right through them, and into the concrete with some masonry screws to secure the plywood to the wall. I'm thinking 4 screws on each side. Then mount the sub panel onto the plywood, and run metal or plastic (not sure if it matters) conduit out the top of the sub-panel, 90 degree it to the left, then 90 degree it down and then one more 90 degree to go left again towards the side of the main breaker box. There are two 2 x 4s on either side of the breaker box, and I am guessing it would be okay to just drill through it as needed to install the conduit into one of the punch outs. Where I do this will depend on where an appropriate size punch out is, but I'm hoping to do it near the bottom of the box since there's no clutter there.

Once all the hardware is mounted and ran, I'd then pull whatever wire I'm using (probably THHN, would be best?) and connect the two hots to either bars, and then neutral to the bar that's floating and the ground to the grounding bar (I believe I might have to buy one). Some of the videos that I've watched mentioned a green grounding screw that might be on the bar one would use as a neutral bar and that it needs to be removed (I think it bonds the bar to the box). And then I simply put whatever breakers I want/need in the sub-panel and run that wire as needed (which won't be far, this panel is just for the room that it's in).

I feel like I'm either doing something wrong, forgetting something, or maybe doing something harder than it needs to be. But my goals are to do this safe, and up to code. So please advise on what I'm looking at and what I'm planning so that the electrician will only have to come once!

Thanks!

Answer 1

Putting the plywood up makes a great base for a panel , I usually only use 1 anchor in each corner about 1" in from the edge. You can screw directly into the stud or furing strip since you plan on spanning 2 studs you don't have to go into the concrete wall. Then use screws to anchor the panel, your plan sounds good to me.

Answer 2

Since these studs are firring strips on concrete, I would tend to be suspicious of them and would want to anchor directly into the concrete. Those firring stips are often anchored using hard nails driven with a gunpowder-fired ("Ramset") nail gun. The problem is that with stress and movement on any particular anchor, they can loosen up and come out. If you hammer nails into them, you'll loosen them. If you drive a screw that goes all the way through and hits the concrete, and you keep driving, you might pull them up.

Check with your inspector, but you should be fine drilling directly into the concrete and using concrete anchor screws, like Tapcon, Hilti, or Redhead anchor screws. Don't use a hammer drill to drive these (the hammering will pulverize some of the concrete around the screw threads and make for a loose anchor).

Having the box in contact with concrete is not an issue. In most new installations you're supposed to tie your electrical ground to rebar set into the concrete anyway, and the panel itself is required to be grounded. This isn't an issue.

I'm not trying to promote any particular brands here. All three of these anchor brans are well-known and respected. You might actually have a hard time finding other brands. I'm including these links in no particular order to illustrate the concept. These are screws, you can drive them fairly tight into the concrete, and the threads will hold them in even with a little abuse.

http://www.tapcon.com/products/concrete-screw-anchors

https://www.hilti.com/anchor-fasteners/screw-anchors

https://www.itwredhead.com/