Spray foam over romex

This is not for a client, I am just trying to get better educated:

I do not see a lot of spray foam, in the walls, in new construction around here; so I have never thought about this before. The insulator spray foamed (closed cell) all of the roughed in romex. The city inspector was giving the electrician a hard time about it, but passed it. I guess the inspector was concerned about heat buildup because of the foam encasing the wiring, which is why he was giving the electrician a hard time.

Would there be significant heat buildup? Is there anything in the current code cycle that would prevent spray foam over romex? Has anyone heard of any issues because of this?

Happens all the time since the insulation comes in after the trades (HVAC, plumbing, electrical) rough in. Highly doubt it will be an issue. Not sure though if NEC addresses this issue…it may?.. Ya could go around the house and break it all off the romex. No Fun!

The NEC doesn’t directly address spray foam insulation. If the cables are separated within the insulation there is no code issue.

I would say encasing any junctions or boxes would be the issue and not the romex per say.

The correct term would be Nonmetallic-Sheathed Cable (Type MN-B) and the NEC does address the issue of how you would have to adjust for the “in contact” aspect of the ampacity variance.

From a manufacturers perspective, the manufacturer of the “Foam” would need to make a statement regarding it’s testing for use with Type NM-B Cable. The heat generated during the curing process is documented. However, most of the makers of the foam have done the testings needed to determine that it does not cause any negative effects on the PVC sheathing, but they need to document this in their specifications.

Wire and Cable Manufacturers do not do these testings normally due to a wide variety of manufactures of the “foam” open and closed cell products. So when asked we usually tell them to consult with the foam manufacturer because we sell our products to be installed in buildings, they produce foam to encase our wires so they have the responsibility to provide the tests and documentation.

Paul if double insulating conductors is not allowed common sense tells me closed cell foam surrounding non metallic conductors is even worse. Under what law of physics can a this be a acceptable situation or is running Romex through conduit cool now?

There is no prohibition against NM in conduit. It is specifically required in some instances for protection against physical damage.

NM plastic wire has a maximum temperature rating, usually 90 C. Wires heat up when current passes through them. Sheathed wire is supposed to be installed surrounded by air, such as between studs or joists, so that heat build up can dissipate. When surrounded by insulation heat can’t dissipate. Hotter wires also have higher resistance, so heat that can’t dissipate generates more heat. In other words the conditions for the perfect (fire) storm. The better the insulation the greater the danger.

There is no danger of NM cable overheating when being installed within insulating material if done so according to the NEC.

Any modern home (built in the last 40 years) will have some type of Insulation packed between studs and joist at all exterior walls, and attic floors.

Sorry…Not sure I understand the question. And if you are inferring that sheathing counts as “insulation” then you are a bit off my friend.

The Type NM Cable in a raceway is not really the problem, it is how you would secure it to the enclosure or device box that becomes the problem except of course specific aspects of the NEC like 312.5© Exception or 314.17© Exception. However, in any case it is also very common to install Type NM-B Cable in short raceways for protection or even as detailed in 334.10(5) where applicable.

The point is…we have adjustments and corrections for a reason. They are limitations that serve to prevent damage to the conductors insulation and thus causing a potential circuit failure…or worse!

The sheathing is generally rated the same (at least ours is) as the conductor insulation but simply as a issue of product material and is kinda irrelevant to the question. The foam producing manufacturers are the individuals who have to say placing their foam in contact with our nonmetallic-sheathing and subsequent conductors is ok…not the Wire and Cable industry.

FYI- Here is a great article by Charlie Trout http://www.ecmag.com/section/codes-standards/type-nm-cable-and-more

Paul I have heard you are not allowed to double insulate or run Non Metallic through conduit do to heat build up for ages.

Go take a look at the many discussions around the internet on it.

Blows my mind it is OK.

Around here Non metallic is not allowed so is it OK to use Romex through conduit then ? hmmmm.

The NEC does not prohibit NM through conduit. Consider your mind blown. As I said above, the code specifically calls for conduit to protect NM against physical damage.

So since we are not pencil neck geek code inspectors lets talk in the real world.
When is overheating from surrounding environment a factor.
Lets take over packing junction boxes and while we are at it how about running my Christmas light extension cord through conduit.
Maybe go further and discuss extension cords running through walls.

Past code what is the difference and is it all Ok or not ?

Was I wrong ?


Do you think that an NM cable embedded in insulation and properly protected by an OCPD is really a problem?


Extension cords cannot be used as a substitute for building wiring so there is no need to talk about running them through conduit. There also are prohibited from going through walls and floors.

Junction and device box capacity is based on conductor volume.

If you are asking for non-compliant installations, where do you want to start? T-stat wiring on 15 amp circuits, overfilled boxes?

Older code also allowed NM in conduit. I think you have been under the wrong assumption, possibly for many years.

I always thought it was based on heat generated based on overly insulated conductors.Learn something new everyday however can you please elaborate what you mean by based on conductor volume ?

How and why is there a limit based on conductor volume ?

The NEC has assigned a specific volume for each conductor size and requires the box to have enough cubic inch capacity for the number and sizes of conductors installed in them. Article 314.16 has a chart of box volumes as well as the calculation factors. Table 314.16(B) has conductor volumes.

For example 3 12-2 cables plus a receptacle in a plastic box would require a box of at least 20.25 cubic inches. 6 conductors + 1 ground +2 for the device =9. A #12 is 2.25 ci.

This allows bend space and room for heat dissipation.

If the NM is reaching temperatures of 194F inside a wall, regardless of whether it is insulated or not there are bigger problems present.

We are talking about 1 or 2 strands of NM running through foam. We are not talking about 8-10-12 strands of NM being stuffed in a larger conduit. Apples and oranges.

Type NM-B cable is installed in walls, ceilings, and floors with other types of insulation and the
properties of the spray foam insulation are not different to warrant special consideration.