Definition of Basement

T

tajalir

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Engineer
What section of NEC defines a basement? Is the first floor of a ranch-style house considered a basement? The first floor opens directly to the outside.
 
It can be hard to define, and may be a building department question. In your example, I'd say no, because it's not substantially below grade. One clue is whether that floor contributes to finished square footage.

I have a finished floor, an English basement, with its own front door. The middle floor's back door is about four feet above the back yard. The basement, although finished, is not included in listed floor area.

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The NEC doesn't define a basement, just what is required in one. You would go by what the plans say and whether or not the space is below ground. I would not consider a concrete space with its floor at grade level below a wood framed structure to be a basement. So, really, it's up to opinion in some cases.

-Hal
 
There was a time (ie: Art 336 in the '93-99 Code) the NEC defined the 1st floor as the one with 50% or more of the exterior wall at or above grade.

That wording was dropped from the NM Section in 2003.
 
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It can be complicated to figure out from the building code side because of the definition of "Grade Plane". You have to get an accurate elevation from many points around the house to get an accurate average, and a lot of the time you will not know what finished grade is actually going to be until the final inspection time - maybe.

From the 2021 International Residential Code.

[RB] BASEMENT. A story that is not a story above grade plane (see “Story above grade plane”).

[RB]STORY ABOVE GRADE PLANE. Any story having its finished floor surface entirely above grade plane, or in which the finished surface of the floor next above is either of the following:
  1. More than 6 feet (1829 mm) above grade plane.
  2. More than 12 feet (3658 mm) above the finished ground level at any point.
[RB]GRADE PLANE. A reference plane representing the average of the finished ground level adjoining the building at all exterior walls. Where the finished ground level slopes away from the exterior walls, the reference plane shall be established by the lowest points within the area between the building and the lot line or, where the lot line is more than 6 feet (1829 mm) from the building between the structure and a point 6 feet (1829 mm) from the building.

So, walk-out basements are almost always considered to be basements by the building code. If you are in a position where you are arguing about whether it is a basement or not, it is very likely to be a basement.

In my experience, this argument has only come up in the past when an electrician did not want to GFCI protect the whole basement as called for in the 2020 NEC and newer. The problem that they usually run into is that before we bother to measure elevations, we just ask the general contractor if he wants to change his basement on the plans to be above grade space, which means adding more $$$ the permit fee because the standard valuation system from the ICC charges much less per square ft. of basement space than above grade living space. They always say no to that - the GFCI's are always cheaper than updating the permit and, consequently, updating the permit fee.
 
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NEC Inspector said:
It can be complicated to figure out from the building code side because of the definition of "Grade Plane". You have to get an accurate elevation from many points around the house to get an accurate average, and a lot of the time you will not know what finished grade is actually going to be until the final inspection time - maybe.

From the 2021 International Residential Code.

[RB] BASEMENT. A story that is not a story above grade plane (see “Story above grade plane”).

[RB]STORY ABOVE GRADE PLANE. Any story having its finished floor surface entirely above grade plane, or in which the finished surface of the floor next above is either of the following:
  1. More than 6 feet (1829 mm) above grade plane.
  2. More than 12 feet (3658 mm) above the finished ground level at any point.
[RB]GRADE PLANE. A reference plane representing the average of the finished ground level adjoining the building at all exterior walls. Where the finished ground level slopes away from the exterior walls, the reference plane shall be established by the lowest points within the area between the building and the lot line or, where the lot line is more than 6 feet (1829 mm) from the building between the structure and a point 6 feet (1829 mm) from the building.

So, walk-out basements are almost always considered to be basements by the building code.
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Sounds like some of the NEC CMP members are moonlighting for the IRC or had a few extra beers while writing :)
 
Does it really matter?? For our purposes why not just say that any space built on or below grade on a concrete slab must have GFCI protected receptacles. The whole reason for GFCIs is for personal protection because the concrete slab is conductive. Who cares what anybody decides to call it??

-Hal
 
hbiss said:
Does it really matter?? For our purposes why not just say that any space built on or below grade on a concrete slab must have GFCI protected receptacles. The whole reason for GFCIs is for personal protection because the concrete slab is conductive. Who cares what anybody decides to call it??

-Hal
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Better to be safe than sued. I do marvel at the concrete slab in a basement is considered an area that needs a GFCI, but a slab floor house does not. Both are equally conductive.
 
I expect slabs at grade level to require GFCI soon too; I suspect that the people who proposed GFCI's in basements were working toward that goal by taking several small bites rather than one big bite.

The only difference in a basement is there might be more moisture in some cases - I remember seeing that in the changes to the NEC book when we went through the 2020 changes.
 
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