NEC Clarification Needed

[William Deans]

Greetings,

The NEC states that you cannot put metalic boxes onto masonry which may be damp without a 1/4" airspace. Does this mean that you cannot use metal boxes / emt on masonry basement walls? What about metal boxes / emt on masonry basement walls of a row home between the homes?

Thank you for your time and energy, William

[roger61611]

i am not an electrician but one code reference i found said that the airspace (6 mm) applies to surfaces that might be wet a lot, or washed down, such as would be found in dairies, commercial laundries, etc.

i've commonly seen metal boxes screwed directly into concrete block basement walls, using pipe/conduit to carry the romex from above down to the box.

d> Greetings,

[William Deans]

Thank you for your response Roger.

Normally a basement wall is in direct contact with the earth. I have heard that masonry in direct contact with earth should be considered damp even if it isn't actually damp? Is this a requirement of the NEC?

In a row-home two of the four basement walls are in contact with the earth. The other two basement walls have another basement on the other side. (unless of course it is an end of row, in which case three of the walls are in direct contact with earth.) If you are not allowed to have emt run along masonry in contact with the earth one might reason that the walls between the row homes are not in contact with earth and therefore it is acceptable. One might also reason that it is unacceptable because it relies upon aspects of an adjoining structure. Running Romex within EMT for supplemental protection as you mention probably has different rules than using a conduit system.

You have commonly seen metal boxes screwed directly into concrete block basement walls [ without an airspace? ] Did the boxes have "supplemental corrosion protection?" Unless masonry in contact with earth is NOT always considered moist as I have been told I do not see how this could be code compliant? Please help me to understand. Is it a violation of the NEC proper but inspectors simply choose to allow it?

Thanks again, William

[Ron]

The 1/4" air gap for damp installations comes from Article 312.2(A). This article applies to cabinets, cutout boxes (disconnects), meter socket enclosures. The article is also refer to by other sections such as overcurrent protective devices, panelboards, etc. There is no similar requirement for boxes (junction or device) or raceway (conduit) to follow 312.2(A). The equipment must be placed or equipped to prevent moisture from entering or accumulating within the box or fitting. See 314.15(A) in the 2002 NEC at the library.

[The Real Tom]

For my education, where in masonry (damp) are you rquired to have the

1/4" air space. Please reply with article and section and NEC version.

Thank you very much for this.

I've 'electrified' my unfinished basement with emt and it was all done with a real inspection also. All surface mounted though, if that makes a difference in your point.

Thank you,

tom

[The Real Tom]

Ah good stuff. I was confused where 'device boxes' were in this requriment, and you cleared it up, it isn't.

Just my 2 cents: As for basements, only 'some basements' need to be treated as damp locations (ART 100), not all have to be treated as damp locations.

thx,

tom @

[Goedjn]

On the other hand, it's not all that freaking hard to mount your panel on standoffs, so why not just do it?

[The Real Tom]

I think the original question was about 'boxes'

tom