NM-B vs. MC cable vs. EMT

[jfrog]

As a specifying engineer, our company's spec does not include NM (or NM-B) for wiring. Our typical spec requires EMT, but in some cases allows MC.

I've had a couple of multi-family residential projects lately where the owner, architect or contractor is asking why we're not allowing NM-B. Obviously there are some cost-saving opportunities there, but we feel EMT or MC provides a better product.

My question is: Is there a good rationale for not using NM-B? Or are we "over-spec'ing" residential projects?

As an example, my latest project is a 6-story building where the top 5 floors are residential. It used to be, of course, that NM-B would not be allowed in that case, but not anymore.

I'd love to hear some opinions

John

 

[iwire]

When the wiring systems are enclosed in the walls what are you really gaining by prohibiting NM?

 

[retire09]

Is this a building permitted to be of type III, IV or V construction?

 

[MasterTheNEC]

As a specifying engineer, our company's spec does not include NM (or NM-B) for wiring. Our typical spec requires EMT, but in some cases allows MC.

I've had a couple of multi-family residential projects lately where the owner, architect or contractor is asking why we're not allowing NM-B. Obviously there are some cost-saving opportunities there, but we feel EMT or MC provides a better product.

My question is: Is there a good rationale for not using NM-B? Or are we "over-spec'ing" residential projects?

As an example, my latest project is a 6-story building where the top 5 floors are residential. It used to be, of course, that NM-B would not be allowed in that case, but not anymore.

I'd love to hear some opinions

John

Each product has their own specific advantages and as iwire stated, when installed within walls why would you say you feel EMT or MC is better. Obviously each product has it's limitations but if it is permitted then why exclude it or why bring personal bias into it, let the people who are buying the building or paying for it decide based on the full scope of options. If the building falls within the scope of permitted uses in 334.10, and not excluded per the uses not permitted in 334.12 then spec it and let the customer decide.

Just my two cents on the issue..Also check out Annex E for additional clarification on the buildings construction type III,IV or V more specifically....

 

[growler]

As a specifying engineer, our company's spec does not include NM (or NM-B) for wiring. Our typical spec requires EMT, but in some cases allows MC.
My question is: Is there a good rationale for not using NM-B? Or are we "over-spec'ing" residential projects?

When the wiring systems are enclosed in the walls what are you really gaining by prohibiting NM?

I think I would check with the insurance carriers and see if they charge different rates for such structures that are wired in NM. Just to be on the safe side.

I don't know why it works like this but I have found that often when you do something cheaper you end up paying somewhere else.

 

[Dennis Alwon]

The buildings I have seen where there were commercial spaces on the first floor had the entire building done in MC. As stated above it would depend on the classification of the structure whether or not you can use nm cable.

 

[Npstewart]

In my opinion you should never put that type NM cable is permitted on your drawings. I have worked on many projects where the construction type of the building changes and most times the Architect doesen't bother to tell anyone because they are not aware it effects the electrical. Our standard spec is for EMT with branch circuits permitted to be installed in MC. At some point most larger projects will get an RFI during bid time requesting if type NM is allowed at which point we will write a letter reciting the code section and referring to the architectural drawings for construction type. This letter can be used over and over and only takes a second to print.

Wiring a building in MC is always cheaper then wiring a building in NM and then re-wiring it in MC.

 

[jfrog]

When the wiring systems are enclosed in the walls what are you really gaining by prohibiting NM?

That's the heart of my question.



BTW, The current project in question is type 3B construction.

 

[Dennis Alwon]

That's the heart of my question.



BTW, The current project in question is type 3B construction.

Not sure what the "B" means but Type III construction allows NM cable art 334.10

 

[MasterTheNEC]

Wiring a building in MC is always cheaper then wiring a building in NM and then re-wiring it in MC.

Only when the contractor is too ignorant to actually find out all of his/her options....it's their building so they have a say in it.

 

[MasterTheNEC]

Not sure what the "B" means but Type III construction allows NM cable art 334.10

Here is an example of both A and B for Type III Construction. The A versus B has to do with protected and unprotected combustible.

TYPE III-A--Protected Combustible (Also known as "ordinary" construction with brick or block walls and a wooden roof or floor assembly which is 1 hour fire protected).
2 Hr. Exterior Walls*
1 Hr. Structural Frame
1 Hr. Floor/Ceiling/Roof Protection


TYPE III-B--Unprotected Combustible (Also known as "ordinary" construction; has brick or block walls with a
wooden roof or floor assembly which is not protected against fire. These buildings are frequently found in
"warehouse" districts of older cities.)
2 Hr. Exterior Walls*
No fire resistance for structural frame, floors, ceilings, or roofs.

 

[Npstewart]

his/her options

Im yet to come across a lady electrician but I bet they are out there


I find contractors (at least around here) are typically pretty sharp when it comes to cutting cost however staying code compliant is the trick.

I dont see any issue with using NM in general. Honestly I dont even know why its not allowed in certain types of construction. I know the construction type has to do with the combustibility of materials permitted and usually NM has a paper wrapping the ground wire. I always assumed that was why it was prohibited but not sure. Maybe the insulation too?

 

[GoldDigger]

Both, flame and smoke emission can compromise fire safety and increase the odds of a low grade short or arc igniting other things in the wall, like studs.

 

[SceneryDriver]

I used to live and work in Chicago. By local amendment, NM isn't allowed to be used anywhere. Conduit for everything. MC is OK too. The suburbs have followed suite, and don't allow NM either. I'm pretty sure the big box stores in the Chicago area don't even stock it, as no one should be buying it.

The stated reason for no NM is that rodents chew on the jacket and wires and that causes fires. The cynical part of me believes that, since conduit takes more skill and time to install, the "safety" amendment was pushed through by the locals. Not that I mind though; I got really good at bending conduit while I was working for a kitchen and bath remodeling contractor.


SceneryDriver

 

[MHElectric]

Wiring a building in MC is always cheaper then wiring a building in NM and then re-wiring it in MC.

No it's not. Verifying the building type, wiring the place in romex, and then passing your inspections is the cheapest thing to do.

Over-building your jobs never serves a justifiable purpose if it's not what the customer wants.

 

[GoldDigger]

No it's not. Verifying the building type, wiring the place in romex, and then passing your inspections is the cheapest thing to do.

Over-building your jobs never serves a justifiable purpose if it's not what the customer wants.

Look carefully at what was actually said: not that MC was cheapest, just that it was cheaper than wiring twice, ending with MC because you did not do the homework.
You also have a valid point that does not contradict what you were replying to.

 

[don_resqcapt19]

I used to live and work in Chicago. By local amendment, NM isn't allowed to be used anywhere. Conduit for everything. MC is OK too. The suburbs have followed suite, and don't allow NM either. ...

As I recall the use of MC is very limited. I think only for fishing into a wall for existing buildings and no more than 12 or 15'.

 

[don_resqcapt19]

...Over-building your jobs never serves a justifiable purpose if it's not what the customer wants.

I just did an inspection (regular inspector was on vacation) for a condo where our code would have permitted NM. The contractor installed EMT because he is from the Chicago area and his guys don't know how to "rope" (slang term for installing NM in our area) a house. This was a bid job so there was no increase in costs to the owner.

 

[SceneryDriver]

As I recall the use of MC is very limited. I think only for fishing into a wall for existing buildings and no more than 12 or 15'.

I believe you're right. We only used MC very sparingly where EMT just couldn't fit. Inspectors never had a problem where we used it. I got pretty good at using the "Chicago" bender. The occasional short radius bend meant there were few places we couldn't get a piece of 1/2" EMT.

I actually prefer conduit; it's easy to alter the conductors if you want to change something later, provided it was laid out with some forethought. Try changing wires after the fact with NM or MC. I also like the fact that you don't deal with lots and lots of ground wires; the conduit is the ground, or if you like, there's one green wire in the pipe, not multiple bare conductors you have to terminate and find room for.



SceneryDriver

 

[iwire]

This was a bid job so there was no increase in costs to the owner.

You cant say that, all the contractors could have bid it as pipe. Heck the plans likely indicated pipe.