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I read online that a plywood backer may or may not be required behind a breaker panel, and that it must be painted black. What are the rules for this? And why must it be painted black? I'm in MI, USA for reference.

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Plywood is usually used when mounting a panel to masonry walls for a few reasons.

Isolation

Masonry is not a great insulator, which can lead to problems especially where the wall is below grade. The cabinet has to be bonded to the grounding electrode system. Because of this, if you mounted the cabinet directly to a below grade masonry wall, you could end up with a ground loop.

Moisture protection

Masonry walls are also not great at stopping moisture. If the cabinet was mounted directly to the wall, there's a possibility of moisture in or on the cabinet, which can lead to problems.

Insulation

Because masonry is not a great insulator, a ground-fault could cause the wall to be energized.

Ease of Installation

It's much easier to attach electrical devices, cables, and raceways to plywood than masonry.


As for the color of the plywood, I've never heard of a requirement to paint it black.

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  • I would think bonding to especially below grade concrete would be an improvement to the grounding electrode system, hence the ufer ground. @Tester where am I going wrong?
    – Rand
    Commented Apr 24, 2014 at 10:49
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    @Rand Not all "ground" is created equal.
    – Tester101
    Commented Apr 24, 2014 at 11:31
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If this helps, I just had my service upgraded to 200A in Lansing, Michigan. Here is a before and after photo of a 1959 breaker panel vs. a 2020 installation. The old breaker box was mounted directly to the concrete wall. For reference, this is an exterior wall built in 1911 that tends to "sweat" during periods of heavy rainfall, so some physical separation from the wall makes sense. I didn't think to check the back of the old panel for rust (too late now). What you can't see here is that there is a (PVC, I think) tube that goes through the concrete to connect to the service meter, so it is waterproof the entire way to the meter.

Michigan Breaker Boxes, 1959 vs. 2020

I've found some evidence that there is something in the Michigan Electrical Code that addresses this, but I'm not certain which part:

Dec 26, 2014 - In Michigan we have to mount panels to pressure treated lumber. Plain old plywood is no longer legal here.

And then another comment specific to painting the plywood (Ontario, Canada):

Jan 18, 2013 - Last time I put Fire rated plywood up some fool came in and painted it.. I had to replace it.. (not allowed to be painted and label must be visible [in Ontario anyway])

Additionally, some rationale for this change to the Code (Pennsylvania):

Dec 27, 2014 - It may not be in the code, but there are decades of experience in this part of the country that tend to indicate that panels and metallic conduits/boxes in contact with basement wall will corrode and fall apart. It's not the moisture per se, but galvanic corrosion as a result of the ground current in those metallic objects and the steel sacrificing itself to protect the galvanized coating. Depending on the moisture levels, it can take from just a few years to a few decades, but the stuff WILL fall off the wall at some point, especially in older homes in developments where water runoff wasn't planned and high moisture is an everyday thing.

Finally, I found some direction in the Consumer's Energy Electric Service Metering Guide (2010), which adheres to the NESC and NEC:

3.11 Interior meter boards of adequate size for the meter installation shall be furnished by the customer to provide a smooth and dry surface for mounting the metering equipment in those cases where the customer’s walls or structure are not suitable for direct mounting.

3.11.1 Interior meter boards are to be constructed of ¾-inch plywood or other approved material, painted on both sides with good quality paint, and mounted rigidly on the wall or structure in a true vertical position.

3.11.2 The location and size of the meter board shall be such as to allow at least a 12-inch clear space above the meters, and all vertical conduit runs to and from the service equipment shall be kept at least eight inches from the sides of all meter troughs to facilitate the installation and testing of the meters.

From the above resources, the relevant codes appear to be:

  • NEC 300.6 Protection against corrosion and deterioration
  • IBC Section 603 Combustible Material in Types I and II Construction
  • IBC Table 803.11 Class B wall finishes (flame spread 75 or less) vs. Class C (flame spread 76-200, e.g. untreated plywood).

In my case, my electrical company is not Consumer's but another electrical company that operates in the same region of Michigan (Lansing Board of Water and Light). Apparently, the requirement for painting is specific to Consumer's Energy's interpretation of the Electrical Code, with the caveat that the Code in Ontario, Canada specifically disallows that.

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    While this is an excellent example of a panel mounted to (unpainted, thus refuting that point) plywood on a masonry block wall, it does not answer the question of whether or not the plywood was required, if it's just a "good idea", or happens to by your electrician's SOP.
    – FreeMan
    Commented Sep 28, 2020 at 17:01
  • @FreeMan Got it, I revised my answer.
    – Parker
    Commented Sep 28, 2020 at 19:59
  • Wow... MUCH better!!! Well done.
    – FreeMan
    Commented Sep 29, 2020 at 10:21
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It is up to the local authority what to require. The NEC only says that work be neat and workman like and that equipment be firmly attached to the surface to which it is mounted. It's generally understood that it's impossible to firmly mount anything to drywall, thus a more substantial surface is required. What that is is up to the local authority.

I've never heard of any requirement for color but I'm not too surprised. In particular if the panel is in public view I could see some requirement beyond unfinished plywood. A requirement such as it be painted, but a specific color is beyond typical requirements.

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In SE Pennsylvania and the surrounding area, I don't think I've ever seen a panel mounted to a residential masonary wall without a piece of wood between the panel and the wall. I started reading NEC around 1973 and would bet money (not a lot) that in one or more of the code cycles then or since stated that a painted piece of wood could be used for installing a panel in a damp location, but it's possible I just dreamt that.

And besides being a, maybe just a traditionally accepted, way of protecting the panel from the damp wall, if you have enough board above the top of the panel it also gives you something to get that first staple in within 12".

For the color, I like battle ship gray for a gray panel, a shade of brown for a brown panel or whatever color you have a half a gallon of laying around.

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