Does a panel need airspace behind it on a block wall?

In a damp or wet location, yes. NEC 312.2 requires at least 1/4 inch of airspace between the enclosure and the supporting surface.

Is strut an acceptable way to mount a panel?

Yes. Strut mounted to masonry anchors gives the required standoff and lets you plumb the enclosure on an uneven wall.

Can I mount an electrical panel on a cinder block wall?

Q: Can I use wood plugs to anchor a panel in masonry?

No. NEC 110.13(A) specifically prohibits wooden plugs driven into a hole in masonry, concrete, or plaster as a support means.

Yes, and it is a normal installation. The two things that get flagged are the airspace behind the can and the anchors holding it.

The quarter inch airspace

Section 312.2 covers cabinets and cutout boxes in damp and wet locations. It requires the enclosure to be placed or equipped to prevent moisture or water from entering and accumulating within it, and it requires the enclosure to be mounted with at least 6 mm, or 1/4 inch, of airspace between the enclosure and the wall or other supporting surface.

Whether that applies to your block wall depends on the location classification in Article 100. A basement or garage wall below grade, an exterior block wall, or any masonry surface subject to moderate degrees of moisture is a damp location. Concrete and masonry wick moisture, and a panel can bolted flat against damp block will rust through from the back in a few years.

An interior masonry wall in a genuinely dry, conditioned, above grade space is a dry location and the airspace is not required by 312.2. Inspectors vary on how they read a garage or an unfinished basement, and adding the spacers costs almost nothing, so build in the airspace by default.

How to get the airspace

Three common methods. First, mount two lengths of strut vertically to the block with masonry anchors and hang the panel on the strut. That gives you 1 5/8 inches of standoff, room for a bonding jumper, and an easy way to plumb the panel on an out of square wall. Second, use listed panel standoff spacers or a set of stainless washers stacked behind each mounting hole. Third, use a treated plywood backboard fastened to the block, which is common but only satisfies 312.2 if the plywood itself is furred off the wall.

Section 110.13(A) requires electrical equipment to be firmly secured to the surface on which it is mounted, and it prohibits wooden plugs driven into a hole in masonry, concrete, plaster, or similar materials as a means of support. Use sleeve anchors, wedge anchors, or masonry screws rated for the load, and land them in the solid web of the block rather than the hollow cell wherever you can. If you must anchor in a hollow cell, use a toggle style masonry anchor.

What does not change

Working space under 110.26(A) applies exactly the same. 36 inches deep for equipment at 150 volts or less to ground under Condition 1, 30 inches wide or the width of the equipment, and 6.5 feet high. Section 110.26(E) requires the dedicated space above the panel to be clear of foreign piping and ducts up to 6 feet above the enclosure or to the structural ceiling.

Section 110.26(C) adds entrance to and egress from working space, and if the panel is rated 1200 amperes or more and over 6 feet wide, you need an entrance at each end.

Conduit entries into block

If raceway comes up through or out of the block, remember 300.5(D)(4) for conductors emerging from grade and 352.10(F) for PVC in concrete. Where a raceway passes from a warm interior into a cold masonry chase, 300.7(A) requires the raceway to be sealed to prevent circulation of warm air, which is what causes condensation to collect in the panel bottom. Duct seal at the fitting handles this.

Field takeaway

Block wall mounting is fine. Stand the enclosure off the masonry by at least 1/4 inch under 312.2, use proper masonry anchors and never wood plugs under 110.13(A), keep the 110.26 working space, and seal raceways that cross a temperature boundary under 300.7(A).