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Two-hour floor-ceiling assembly upgrade to allow for Romex wiring.

BayPointArchitect

BayPointArchitect

Sawhorse
Joined
Oct 19, 2009
Messages
525
Location
Lincoln
Given:
Nonsprinkled, mixed-use occupancy building, wood framing, requiring two-hour fire separation between business office below an apartment unit above.
A floor ceiling assembly that is not in strict accordance with a given UL design number but is equivalent to three UL listed assemblies that have two layers of plywood subfloor.
Rather than installing MC cable within the floor-ceiling assembly to supply the residence above and the lights for the business below, the electrician installed Romex to provide power for both the residential and commercial. My thinking (based on the opinion of a different electrical inspector) is that the Romex is acceptable for the residential supply but the power for the office lights needs to be MC cable. The inspector that we are having a problem with makes it clear that he does not like TJI joists because of their poor fire performance. But then he tells the general contractor that all of the existing Romex may remain in place only if the general contractor lays down a layer of 1/2" drywall on top of the subfloor. The inspector explains that two layers of combustible plywood without any additional protection has never been an acceptable combination where a fire-rated floor-ceiling assembly is required. The inspector tells the general contractor to provide a UL number to prove otherwise.

Question:
How should we explain to the electrical inspector that the two layers of plywood on top of the floor framing is okay? How do we say that laying down a layer of 1/2" drywall onto the subfloor is a little weird?
Floor-Ceiling-Assembly.jpg

As always, thank you in advance for your advice.

ICC Certified Pan Reviewer
NFPA Certified Fire Plan Examiner
 
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BayPointArchitect said:
A floor ceiling assembly that is not in strict accordance with a given UL design number but is equivalent to three UL listed assemblies that have two layers of plywood subfloor.
Click to expand...
How close are you to being in strict accordance with those UL assemblies?

Why is the Romex/MC cable even relevant if he is concerned about the plywood? If the concern is that the plywood won't meet the 15 minute rating to protect the NM cable, how could it meet the 2 hr. separation requirement? Asking for code sections would be the first step if the inspector is sincere, if not I don't see a path to victory unless you can prove somehow that your floor assembly is equivalent to the other UL assemblies.
 
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Floor/ceiling assemblies are tested from below only. Adding a gyp layer to the subfloor would not change its performance.

Either you meet the requirements of the UL listing or you don't, there are no equivalencies without an EJ and even that requires approval from the AHJ.
 
2017 NEC
334.10 Uses Permitted. Type NM, Type NMC, and Type NMS
cables shall be permitted to be used in the following, except as
prohibited in 334.12:
(1) One- and two-family dwellings and their attached or
detached garages, and their storage buildings.
(2) Multi-family dwellings permitted to be of Types III, IV,
and V construction.
(3) Other structures permitted to be of Types III, IV, and V construction.
Cables shall be concealed within walls, floors, or
ceilings that provide a thermal barrier of material that has
at least a 15-minute finish rating as identified in listings
of fire-rated assemblies.
Informational Note No. 1: Types of building construction and
occupancy classifications are defined in NFPA 220-2015, Standard
on Types of Building Construction, or the applicable building
code, or both.
Informational Note No. 2: See Informative Annex E for determination
of building types [NFPA 220, Table 3-1].
(4) Cable trays in structures permitted to be Types III, IV, or
V where the cables are identified for the use.
Informational Note: See 310.15(A)(3) for temperature limitation
of conductors.
(5) Types I and II construction where installed within raceways
permitted to be installed in Types I and II construction.

(A) Types NM, NMC, and NMS. Types NM, NMC, and NMS
cables shall not be permitted as follows:
(1) In any dwelling or structure not specifically permitted in
334.10(1), (2), (3), and (5)
(2) Exposed within a dropped or suspended ceiling cavity in
other than one- and two-family and multifamily dwellings
(3) As service-entrance cable
(4) In commercial garages having hazardous (classified) locations
as defined in 511.3
(5) In theaters and similar locations, except where permitted
in 518.4(B)
(6) In motion picture studios
(7) In storage battery rooms
(8) In hoistways or on elevators or escalators
(9) Embedded in poured cement, concrete, or aggregate
(10) In hazardous (classified) locations, except where specifically
permitted by other articles in this Code
 
Beniah Naylor said:
It does not.
Click to expand...
So can someone point me to a code section that specifies a list of 15 minute finish materials?

My only familiarity with the term is some sections of the IRC that use language such as "1/2 inch gypsum, 23/32 inch plywood, or other 15 minute finish material." Which has led me to, perhaps incorrectly, assume that any 23/32" plywood would qualify as a 15 minute finish material, making the OP's detail obviously compliant with 334.10(3).

Cheers, Wayne
 
Beniah – If I were to take away one layer of ¾” plywood from the top side and then add a layer of 5/8” Type “X” on the bottom side for a total of four layers of drywall, then I would be closer to UL Design L556. MtLogCabin provided an illustration of that assembly which includes four (4) layers of drywall for the ceiling. But my opinion is that the 556 assembly emphasizes the protection of the wood framing from a fire below. Let me reiterate that there is only one layer of plywood on top of this two-hour fire-rated assembly whereas I have two layers of plywood. The two layers of plywood is consistent with UL Design L505 and UL Design L511 and UL Design M502 and ULC Design M503. So there are at least four UL Designs that have been tested for a two-hour fire-rating with two layers of plywood on top. And those assemblies have only two layers of drywall underneath the wood framing whereas I have a combination of three layers and hat channel. As an architect with 28 years of experience, I have liberty to prescribe an equivalent level of safety. And that two-hour fire-rated assembly has been reviewed and approved by the State Fire Marshal without having to identify a specific UL number.

Thanks to you guys, I am now more familiar with the electrical code 2017 NEC section 334.10(3). I am thinking that there is a similarity between the required 15 minute finish rating and the 15 minute thermal barrier required by IBC 2603.4 where foam plastics would otherwise be exposed to habitable areas. The condition being that we have plastic and foam materials located in a combustible concealed space. When that plastic and foam reaches temperatures over 250 degrees, those materials will emit toxic gas similar to cyanide gas – or exactly like cyanide gas. To slow down the temperature rise for at least 15 minutes while being exposed to fire, the code prescribes a protective layer of ½” gypsum wallboard, 1-1/2” thick mineral fiber insulation, ¼” thick wood structural panel, particle board or hardboard, metal-faced panels having a minimum thickness of 24 gauge, 1 inch thick concrete or masonry, 7/8” thick stucco, or equivalent thermal barrier.

So rather than commit all six or seven options to memory, the electrical inspector has told us to provide a layer of ½” gypsum wallboard down on the floor or an equivalent thermal barrier.

Still not sure how to best proceed while trying to prove that two layers of 3/4" plywood is equal or better than 1/2" layer of drywall laid down over the floor. Some inspectors don't seem to know the meaning of the word "equivalent".
 
BayPointArchitect said:
As an architect with 28 years of experience, I have liberty to prescribe an equivalent level of safety. And that two-hour fire-rated assembly has been reviewed and approved by the State Fire Marshal without having to identify a specific UL number.

Still not sure how to best proceed while trying to prove that two layers of 3/4" plywood is equal or better than 1/2" layer of drywall laid down over the floor. Some inspectors don't seem to know the meaning of the word "equivalent".
Click to expand...
As described, I wouldn't be giving you trouble over this at all. The floor assembly shown can protect NM cable and the floor all day long. Unless there is a local amendment or there is some other detail not described here, I'm not sure why they are giving you trouble.

However, in fairness to the AHJ -

(1) I don't necessarily trust our State Fire Marshal either. Never met him, don't know anything about him. All I know is the position does not pay well at all. The SFM office signed off on it means nothing to me at this point.

(2) I can't trust 70% of the people with 30 years of experience to know what they are doing. In my experience, length of tenure has not been a good indicator of expertise. With 30 years of experience, you should be an absolute guru at your job, but it seems like many people get to 3-5 years and then coast without improving for the rest of their career while gradually losing attention to detail and becoming out of touch. Sucks for the ones who are gurus because they are rare and they will have to prove it. I used to give people credit for tenure, and then I checked basic stuff just to be equitable and found big misses. Inspectors get to learn that lesson early...

(3) "Equivalent" is very hard to establish with fire resistance for items that are not in the code, because is it really equivalent? Who tested it? How do I know? How many DPs have lied to the AHJ before you came along? How many completely whiffed something and then tried to push it through anyway? We get burned by DPs all the time. Makes for a tough dynamic even when you are dealing with a good one.

Good luck on this one, hopefully they will work with you.
 
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Some follow up comments:

- APA TT-060 "Technical Topics: APA Performance-Rated Wood Structural Panels as Thermal Barriers for Foam Plastics" may be useful here.

APA Publication Search - APA – The Engineered Wood Association

Search for publications, videos, CAD details, images, and other materials in the APA Resource Library.
www.apawood.org www.apawood.org

- The full wording from NEC 334.10(3) is "a thermal barrier of material that has at least a 15-minute finish rating as identified in listings of fire-rated assemblies."

As someone more conversant with the NEC than with fire ratings, this terminology suggests that being a thermal barrier in this context is a property of the material itself, not the full fire-rated assembly. And that the way to generate a list of all such thermal barriers is to take all listed fire-rated assemblies, and for those for which the testing included temperature rise measurement on the interior face of the side exposed to fire and for which the temperature at the 15 minute mark was less than the allowed threshold, add that face layer build-up to the list of thermal barriers. (A statement based solely on https://code-authorities.ul.com/faq/what-is-a-finish-rating-found-on-some-fire-rated-assemblies/ )

Unspecified in the previous paragraph is whether being a thermal barrier depends on the orientation of the material being installed--it seems logical that being exposed to fire from below is a harsher test than being exposed from above. If orientation does matter, then presumably the installed orientation should match the orientation in the listed fire-rated assembly that provides the 15 minute finish rating. Or perhaps there is some orientation hierarchy. Either way, are floor-ceiling assemblies actually tested for fire from above?

- Given the apparent complexity of the above, and given that the building codes have dealt with a very similar issue as far as foam plastics (for which apparently the orientation of the thermal barrier is not a concern), perhaps a better phrase for NEC 334.10(3) would be "a thermal barrier of material as specified in the applicable building code for use with foam plastics, or that has at least a 15-minute finish rating as identified in listings of fire-rated assemblies."

How does that wording sound, or could it be improved? Proposals for the 2026 NEC closed 4 months ago, but if I remember I will submit a proposal for the 2029 NEC.

Cheers, Wayne
 
wwhitney said:
Some follow up comments:

- APA TT-060 "Technical Topics: APA Performance-Rated Wood Structural Panels as Thermal Barriers for Foam Plastics" may be useful here.

APA Publication Search - APA – The Engineered Wood Association

Search for publications, videos, CAD details, images, and other materials in the APA Resource Library.
www.apawood.org www.apawood.org

- The full wording from NEC 334.10(3) is "a thermal barrier of material that has at least a 15-minute finish rating as identified in listings of fire-rated assemblies."

As someone more conversant with the NEC than with fire ratings, this terminology suggests that being a thermal barrier in this context is a property of the material itself, not the full fire-rated assembly. And that the way to generate a list of all such thermal barriers is to take all listed fire-rated assemblies, and for those for which the testing included temperature rise measurement on the interior face of the side exposed to fire and for which the temperature at the 15 minute mark was less than the allowed threshold, add that face layer build-up to the list of thermal barriers. (A statement based solely on https://code-authorities.ul.com/faq/what-is-a-finish-rating-found-on-some-fire-rated-assemblies/ )

Unspecified in the previous paragraph is whether being a thermal barrier depends on the orientation of the material being installed--it seems logical that being exposed to fire from below is a harsher test than being exposed from above. If orientation does matter, then presumably the installed orientation should match the orientation in the listed fire-rated assembly that provides the 15 minute finish rating. Or perhaps there is some orientation hierarchy. Either way, are floor-ceiling assemblies actually tested for fire from above?

- Given the apparent complexity of the above, and given that the building codes have dealt with a very similar issue as far as foam plastics (for which apparently the orientation of the thermal barrier is not a concern), perhaps a better phrase for NEC 334.10(3) would be "a thermal barrier of material as specified in the applicable building code for use with foam plastics, or that has at least a 15-minute finish rating as identified in listings of fire-rated assemblies."

How does that wording sound, or could it be improved? Proposals for the 2026 NEC closed 4 months ago, but if I remember I will submit a proposal for the 2029 NEC.

Cheers, Wayne
Click to expand...
Personally, I wouldn't bring foam plastics into an NM discussion....I've googled 15 min. thermal barriers a time or two with good results....
 
wwhitney said:
- Given the apparent complexity of the above, and given that the building codes have dealt with a very similar issue as far as foam plastics (for which apparently the orientation of the thermal barrier is not a concern), perhaps a better phrase for NEC 334.10(3) would be "a thermal barrier of material as specified in the applicable building code for use with foam plastics, or that has at least a 15-minute finish rating as identified in listings of fire-rated assemblies."

How does that wording sound, or could it be improved? Proposals for the 2026 NEC closed 4 months ago, but if I remember I will submit a proposal for the 2029 NEC.
Click to expand...
Sounds good to me. You have my non-partisan vote.

Meanwhile, I will propose that the inspector read the document produced after 23/32" thick plywood was tested in accordance with the test method for evaluation as a 15 minute thermal barrier. And that test was conducted by an approved independent fire test laboratory. If one layer is good, then two layers is better. The test was conducted eleven years ago but I don't think the laws of physics have changed since then.

Thanks again everyone!
 
FYI, there is no UL(C) assembly for a two-hour fire rated floor that involves standard wood joists. Of the top of my head. I think the only combustible assembly that has a two-hour rating are three layers of 5/8 type *C* on resilient channels, with Roxul in the joist cavities.
 
Now I have been told that the electrical inspector does not care that the overall floor-ceiling assembly is two-hour fire-rated and he does not care about any testing of plywood with respect to a thermal barrier. He wants our State Fire Marshal to produce a document that proves that a fire can not burn through two layers of 3/4" plywood in 15 minutes. Unless I am wrong, that is not the State Fire Marshal's job to argue with the electrical inspector on my behalf.
 
steveray said:
As usual....The electrical inspector knows nothing about building stuff so they should stay in their own lane.....If 19/32 works, Shirley 23/32 does...
Click to expand...
Looks like you missed footnote e:

"e. The time assigned is not a finished rating."

So not useful for the OP, unfortunately.

Cheers, Wayne
 
Unfortunately there is no good commentary on that footnote.....I can only ASSUME that it is saying that it is implying that a sheet of plywood or gyp has no rating unless it is installed as a part of a "finished" assembly why else would the table have "finish" in it......As we are talking about thermal barriers but not necessarily fire ratings, that involves heat transfer on the other side of the barrier, which is part of determining the fire resistance rating of an assembly:

Fire-resistance rating takes into account the rise in temperature on the unexposed side of the membrane.

1706203455949.png


This must be like when mu electricians try to explain stuff to me....Sorry Tim!
 
After the State Fire Marshal agreed with me that the 3/4" plywood has the 15 minute value - that the electrical inspector is looking for - the electrician still insists on the 1/2" gyp.bd. Another electrical inspector recommended that I file a formal appeal to the State Electrical Board because the electrician is not seeing the difference between a 15 minute thermal barrier (test standard = NFPA 275) and a 15 minute burn time (test standard = NFPA 221 and ASTM E119). And that experienced/older electrical inspector went so far as to say that while trying another/younger electrical inspector, there was confusion while trying to explain the difference between "thermal barrier" and "burn time". So the older electrical instructor dumbed it down for the new/younger inspector by saying "Sure. Let's go with simple burn time instead".

Meanwhile, the general contractor is not wanting to fight with the new electrical inspector. So he is going to put down one layer of 3/4" plywood first. Then a layer of 1/2" drywall. Then a finish layer of 1/2" plywood subfloor.
 
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Go to the IEBC, at the back of the book. Resource A-117. The first part of Resource A describes how to use the various tables to calculate the fire-resistance rating.

Look at assembly F/C-W-8 and F/C-W-9.
 
So maybe I'm not getting this subject .... as I see it NMC is permitted for use in the R-2 and permitted in "other buildings etc" if separated by 15 min etc. As I see the drawings provided there is a fire rated assy below and plywood flooring above that meets the 15 min rating. Is the inspector looking to protect the R from the B somehow looking for that protection that is already permitted in the floor space of the R?
Are we not trying to protect the wiring from the higher hazard which we already are with gyp? Is this inspector a hold over from the inspection requirement of NMC not being in a suspended ceiling space and it's requirement of conduit or MC?
 
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