NM vs MC vs EMT/Rigid... When and where it's permissible to use, and WHY?

[EEEC]

Hello,

So I bid a job last year based on a set of drawings (they were incomplete at the time) for some friends of mine who own a dispensary here in California. I bid it based on the use of NM-B since it is only being used as a retail/office space with LED lighting and convenience outlets for phone chargers, computers, a couple fridges, cash register, etc. and it is wood framed.

Well after getting the runaround by the city for a year, they finally got an approved set of drawings, with a few changes in floor plan layout, etc., no big deal. But as I'm reading the new electrical notes, the engineer has specified that I "... must use Rigid or EMT conduit or Flex Metal Conduit (pigtails or units on direct connections or cabinets and concealed wherever possible)." Obviously this would make my price almost triple if this is the case. But I'm pretty sure this is overkill and isn't necessary.

So I know the basics like:

NM mustbe protected from elements, can not be exposed (under 8ft), etc.
MC cannot be exposed where it can be crushed, exposed to the elements, etc.
EMT can be used virtually anywhere with the proper fittings, etc.

All of these can be backed by code.

Here's what I'm confused about and can't wrap my head around:

If you look up NEC and CEC 334.10 and 334.12 it will tell you the building type in which NB can be used. I looked up the types (I, II, III, IV and V) and they only speak of fire ratings, etc., but nothing about "commercial" or "residential".

So I ask around and people say:

You can use NM for an office building if it's under XXX stories, and it's wood studs. Well I know that the "stories" is out now, and I know that you can use NM with metal studs (100% certain for residential). Doesn't seem like a good answer.

or

You have to use MC in an office building. I can't find this in the code...

I've done a lot of searching and I can't really pinpoint the answer to these questions. At least not in the the frame of reference of building use types. And I am trying to formulate a "rule of thumb" while I'm at it....

1) What/who/why determines which wire type which needs to be used with regard to building use? The engineer? The inspector? The code?

2) When do you know it's okay to use NM? When should you use MC? EMT? (We are only talking building uses here, I know the obvious code.)

3) Can anyone provide code that says you "can not" use NM on an office space?

Thanks in advance for any help with deciphering this mystery for me!

 

[Ponchik]

Ask the AHJ what wiring method they will allow. Just because the engineer has specified it does not mean that is what the AHJ will allow. Speak to the head senior electrical inspector, NOT THE COUNTER PERMIT TECHNICIAN.

And when you do get an answer from the senior/head electrical inspector, ask him for NEC 2011 references. Don't let him off the hook by him/her saying "well, it is on the drawings so you have to go with that".

 

[electrofelon]

the big killers for use of NM in "commercial" buildings are 1) if they are places of assembly and 2) they have dropped ceilings (and you are not in MA )

There is of course the building type, and it is not always straight forward to get that information.

 

[EEEC]

Ask the AHJ what wiring method they will allow. Just because the engineer has specified it does not mean that is what the AHJ will allow. Speak to the head senior electrical inspector, NOT THE COUNTER PERMIT TECHNICIAN.

And when you do get an answer from the senior/head electrical inspector, ask him for NEC 2011 references. Don't let him off the hook by him/her saying "well, it is on the drawings so you have to go with that".

So as always, AHJ has final say. So that is most likely the determining factor? So every time I bid a job in a different city I'll have to go and talk to the "senior/head electrical inspector" you think?

the big killers for use of NM in "commercial" buildings are 1) if they are places of assembly and 2) they have dropped ceilings (and you are not in MA )

There is of course the building type, and it is not always straight forward to get that information.

By place of "assembly", do you think the mean a place of "manufacturing" or a place of assembly like a "meeting hall"? Luckily this is neither, and no drop ceiling

 

[GoldDigger]

It is where people gather together.

 

[Ponchik]

So as always, AHJ has final say. So that is most likely the determining factor? So every time I bid a job in a different city I'll have to go and talk to the "senior/head electrical inspector" you think?



By place of "assembly", do you think the mean a place of "manufacturing" or a place of assembly like a "meeting hall"? Luckily this is neither, and no drop ceiling

Not every time that you bid a job. But every time you do jobs that you can not find the answers and it is not straight forward. Some AHJs will allow NM cable in commercial, some won't. Even though it may be allowed per NEC. But if you don't ask and the city's requirements are EMT for residential wiring method then you will go bankrupt just on that job because YOU DID NOT ASK, You assumed NM was OK

If you don't want to ask then do your project per engineers plans and used RMC throughout. Your call.

 

[EEEC]

Originally Posted by EEEC
So as always, AHJ has final say. So that is most likely the determining factor? So every time I bid a job in a different city I'll have to go and talk to the "senior/head electrical inspector" you think?

Edward, I certainly wasn't being facetious with the above statement, I was absolutely asking if you felt this is/was the ultimate final answer to the questions.

And I don't mind asking, I was just hoping that there was a cut and dry, definitive answer that could be backed up by code, but it looks like this one isn't so simple. Which by most accounts, none of the code is

I appreciate everyones responses...

 

[petersonra]

Well after getting the runaround by the city for a year, they finally got an approved set of drawings, with a few changes in floor plan layout, etc., no big deal. But as I'm reading the new electrical notes, the engineer has specified that I "... must use Rigid or EMT conduit or Flex Metal Conduit (pigtails or units on direct connections or cabinets and concealed wherever possible)." Obviously this would make my price almost triple if this is the case. But I'm pretty sure this is overkill and isn't necessary.

Is this the engineer that is working for the city or for the architect.

If it is something an engineer working for the owner spec'd that is above and beyond requirements found in the code or in whatever rules the AHJ has made up, than you can appeal to him to change his specs.

If it is something someone on the AHJ side is claiming is required, you might want to ask what code actually requires it. It is not uncommon for inspectors to claim things are required that are not, just because they think it should be done a certain way.

And always keep in mind that no individual is actually the AHJ. At the most, he is an agent or representative of the entity that is the AHJ and there is always some means of appeal. Sometimes it is not worth appealing because the cost to appeal is more than what you might gain.

 

[david]

And I don't mind asking, I was just hoping that there was a cut and dry, definitive answer that could be backed up by code, but it looks like this one isn't so simple. Which by most accounts, none of the code is I appreciate everyones responses...

I think the NEC does answer the question pretty directly. States it must be concealed in walls floor and ceilings.

15 min. fire rated concealment in walls floors and ceilings is language found in most building codes. The building codes in this area require access in truss (attic) spaces with a pitched roof. And this is where the building code and the NEC overlap. Think of NEC. definition of concealed in contrast to exposed.

When the building code mandates a scuttle hole to allow access above ceilings that would otherwise wise be protected by 15 min fire rated construction you now have a wiring method exposed to potential physical damage.

I would say the NEC allows NM cable where it is not exposed in buildings allowed to be Type III IV or V

Type NM, Type NMC, and Type NMS cables shall be permitted to be used in the following:
(3) Other structures permitted to be of Types III, IV, and V construction except as prohibited in 334.12. 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.

“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. “

 

[petersonra]

When the building code mandates a scuttle hole to allow access above ceilings that would otherwise wise be protected by 15 min fire rated construction you now have a wiring method exposed to potential physical damage.

If that was the case, how could you ever run NM exposed in an attic?

 

[david]

If that was the case, how could you ever run NM exposed in an attic?

well that is the case and you cannot have NM cable exposed in types III IV and V construction. The same way if you choose to run NM cable above a suspended ceiling you would have to conceal it.

 

[EEEC]

well that is the case and you cannot have NM cable exposed in types III IV and V construction. The same way if you choose to run NM cable above a suspended ceiling you would have to conceal it.

What is the best/easiest way to determine which type of construction is being used, for this purpose?

 

[GoldDigger]

You could look at the building permit on file, I suppose.
There is one more wrinkle which is the subject of occasional argument;
The exact code language does not refer to what the building type is but rather what it is permitted to be.
One can interpret that to mean that if the occupancy is one which allows any building type, then the fact that the construction happens to be more fireproof than necessary does not restrict what wiring methods you can use.
Not many AHJs are inclined to read the NEC wording in that legalistic way, but I think the record would show that the CMP chose that wording deliberately.
It is a way of basing the wiring rules on the nature of the occupancy without having to duplicate many paragraphs of the building code.
The argument against this interpretation is, of course, that the occupancy may change over time while the actual construction methods will not.

 

[mwm1752]

Engineer designed buildings are approved per plan whether even if they are above code. If you are saying the building dept changed the submitted plans then research why. If the plans submitted were change by an engineer then the building dept/inspectors use that official document as a guideline. Your only recourse with a private engineer is to get them to revise. growing facilities could have many descriptions - agricultural - factory - retail depends on the specifics.

 

[EEEC]

What is the best/easiest way to determine which type of construction is being used, for this purpose?

Since I am not an engineer, I feel as if I cannot arbitrarily determine which building type a particular building is. So I think "you" is not an appropriate answer. Is it written somewhere in the plans? What if it a design/build with no plans, who do I ask to find out which building type it is? Does this question make sense?

 

[david]

What is the best/easiest way to determine which type of construction is being used, for this purpose?

its not what type is being used it is what type of construction is allowed to be used. sometimes you may see construction more restrictive by design choice. Since you in stated no permits your in a void. You cannot ask a building Dept.

Normally if you see wood construction members non fire treated, the building would be other than type I or II in other words Types III, IV or V

kind of what was said in post # 13

 

[EEEC]

Engineer designed buildings are approved per plan whether even if they are above code. If you are saying the building dept changed the submitted plans then research why. If the plans submitted were change by an engineer then the building dept/inspectors use that official document as a guideline. Your only recourse with a private engineer is to get them to revise. growing facilities could have many descriptions - agricultural - factory - retail depends on the specifics.

The engineer in question was hired by the owners. Here is our email exchange:

Hello XXXXX,
I just left you a voice mail in detail regarding the electrical Note #6, Sheet E-0.

According to both the NEC and CEC, Sections 334.10 and 334.12, we are allowed to use NM cable. I just left the Vallejo Building department, and XXXXXXXXXXXX said that we can make this change if you write an amendment to this note (Note #6, Sheet E-0) as follows:

Email her a PDF letter (XXXXXXXXXXX@cityofvallejo.net) with your seal saying that the use of NM cable is permissible and reference the permit number (TI14-0136). That's it!
We were slated to begin work on this project tomorrow, so if there is any way to get this accomplished today, we'd greatly appreciate it!

I agree NM cable is ok for this project. I will send a letter.
Thanks

I'll let everyone know what I find out. I am taking Edward's advice though, since the lady who said this is okay is "counter person", I am going to seek the "approval" of the senior inspector before starting the job.

However, I'd still like to know how to figure out what type of building a particular structure is...

 

[david]

The engineer in question was hired by the owners. Here is our email exchange:





I'll let everyone know what I find out. I am taking Edward's advice though, since the lady who said this is okay is "counter person, I am going to seek the "approval" of the senior inspector before starting the job.

However, I'd still like to know how to figure out what type of building a particular structure is...

You need to go to your states adopted building code and read through construction types and occupant classifications

Good Luck! most drawings have the building type on the cover page

 

[EEEC]

most drawings have the building type on the cover page

This is what I was looking for. And if it's a design/build TI, the building department of the city in question should have this information, correct? I don't think I'd have to be the person to make the determination as to which building type a particular structure is... it should have been determined during the original construction, right?

 

[EEEC]

Since you in stated no permits your in a void.

I don't remember saying there was not a permit. If I did, I apologize, because there is a permit..