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,
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.
don't know what a 'row home between the homes' is.
William Deans wrote:
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
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?
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.
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.
tom @ www.WorkAtHomePlans.com
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