Update on Romex in Spray Foam Insulation

[Rjryan]

1. The NEMA Bulletin " The Thermal Effects of Residential Installed Type NM-B Encased in Spray Insulation " is in the process of being pulled.
2. Goodson Engineering Publication " Effects of Polyurethane Foam Systems on Wiring Ampacity " raises serious questions of ampacity in NM-B cables in Spray Foam. This was brought before the CMP, which I was told was the reason they are asking for a comprehensive study. Such a study apparently does not exsists, at this time on spray foam insulation.
3. A qualitative definition of "Spacing" with reguard to NM-B cable is not coming in the next code cycle I was told, because no one wants to define it at this time.
4. Because the "R" factor of spray foam increases with depth, the depth of the spray foam will probably increase the temperatures of conductors.
5. Older Houses that have heavily loaded circuits and in all probably NM-B cable not spaced, spray foamed could pose a problem.

 

[petersonra]

Seems to me that if it's a problem with NM it may well be a problem with other wiring methods, especially PVC conduit, Smurf, and MC.

They should investigate all wiring methods embedded in foam.

 

[ptonsparky]

Well, my basement should be a test lab for them. All exterior walls are foamed. My office computers being the largest load that run most of the day. 12-16 hours. I'll let you know it was problem if I live thru the smoke.

 

[BarryO]

What makes spray foam more problematic than other insulation? Higher R-per-inch? Better sealed against air movement? Most NM in outside walls are encased in insulation of some sort.

 

[ptonsparky]

What makes spray foam more problematic than other insulation? Higher R-per-inch? Better sealed against air movement? Most NM in outside walls are encased in insulation of some sort.

No air movement would be my guess.

 

[Fred B]

What makes spray foam more problematic than other insulation? Higher R-per-inch? Better sealed against air movement? Most NM in outside walls are encased in insulation of some sort.

The curing process creates a large amount of heat that actually exceeds the rating of NM, approaching 1.5 - 2X the rating, can be high enough to cause 3rd degree burns on humans skin. Not sure how long NM will tolerate such high of level, the Temp quickly rises and depending on how thick it is sprayed can last minutes to 15 or more. The other issue would be bundling and such encapsulation in foam gives the same effect that passing bulk of wire thru a stud and should need to be derated, IMO. On the other hand Fiberglass batting in wall allows air movement thus the insulating inefficiency compared to foam.

 

[infinity]

The other issue would be bundling and such encapsulation in foam gives the same effect that passing bulk of wire thru a stud and should need to be derated

Doesn't NM cable already require derating when bundled? How does the foam encapsulation change that?

 

[Rjryan]

Doesn't NM cable already require derating when bundled? How does the foam encapsulation change that?

According to the Goodson Engineering Study even derated, the temperature of the wire can exceed the limit of the wire, if there is a heavy load, especially if cables are bundled. There is no standard for "spacing" which makes any study not bundling questionable. What is good spacing?
In the You Tube video the quoted a ULC 1998 study "Spray Foam Insulation and Electrical Wiring" suggested that a heavy load could possibly
pose a problem. The NEMA bulletin on spray foam, is based on a study by the University of Toronto in 1985. I talked to an engineer that sits on the CMP, while the results of this study are quoted, she was unable to get a copy of the actual tests preformed. The NEMA Bulletin is questionable and in the process of being pulled. Under normal conditions or loads, without bundling, there probably not an issue. I think the real
problem arises at the extremes of loads and bundling. Spray foam seems to do a better job of trapping heat.
The CMP is asking for a comprehensive study to make reasonable assertion of NM-B cable in spray foam.

 

[infinity]

According to the Goodson Engineering Study even derated, the temperature of the wire can exceed the limit of the wire, if there is a heavy load, especially if cables are bundled.

So what you're saying is that for bundled cables within the foam the standard NEC adjustment factors are inadequate. I'm all for testing but this doesn't seem like a real world problem.

 

[electrofelon]

So what you're saying is that for bundled cables within the foam the standard NEC adjustment factors are inadequate. I'm all for testing but this doesn't seem like a real world problem.

Rest assured, the NEC will make it into a problem

 

[infinity]

Rest assured, the NEC will make it into a problem

I'm having a flashback to the conductors above a roof debacle that they put in, amended for XHHW, and then took out of the NEC.

 

[Rjryan]

I'm having a flashback to the conductors above a roof debacle that they put in, amended for XHHW, and then took out of the NEC.

Go to Goodson Engineering - Publications - "Effects of Polyurethane Foam Systems on Wiring Ampacity" Chart #1
Compares loads, Type of insulation, number of cables and temperature developed. It shows if accurate, show that you are treading on the
safety margins of the wire. New installations are to have spacing, along with derating and probably lesser loads, but not as serious as spray foaming older homes that probably lack all three of these factors.

 

[Rjryan]

Go to Goodson Engineering - Publications - "Effects of Polyurethane Foam Systems on Wiring Ampacity" Chart #1
Compares loads, Type of insulation, number of cables and temperature developed. It shows if accurate, show that you are treading on the
safety margins of the wire. New installations are to have spacing, along with derating and probably lesser loads, but not as serious as spray foaming older homes that probably lack all three of these factors.

 

[electrofelon]

I find this somewhat comical. Not that it's being discussed nor that there is concern. I am actually curious what the temperature rise would be for a bundle of conductors with a certain degree of loading encased in a super good insulator like spray foam. What is sort of ridiculous is this is a super easy experiment to do. I mean I could do it right here at home in like a few hours. You just need some ramen, a can of spray foam - ok I don't have a thermocouple handy but I could order one on amazon and have it tomorrow. Between my toaster and various space heaters and baseboard heaters I have lying around I think I'm good with loads. All seems easy to me, but I'm sure it will get made into a multi-year study and 4 code cycles of different changes over it.

 

[electrofelon]

Go to Goodson Engineering - Publications - "Effects of Polyurethane Foam Systems on Wiring Ampacity" Chart #1
Compares loads, Type of insulation, number of cables and temperature developed. It shows if accurate, show that you are treading on the
safety margins of the wire. New installations are to have spacing, along with derating and probably lesser loads, but not as serious as spray foaming older homes that probably lack all three of these factors.

Thanks for the link, well that's good this study is already been performed so that part is done. The temperatures are indeed alarmingly high. I guess the question is how likely these scenarios are.

 

[Rjryan]

Thanks for the link, well that's good this study is already been performed so that part is done. The temperatures are indeed alarmingly high. I guess the question is how likely these scenarios are.

I guess the question is how many people trust their inverse time breaker to protect the circuit and keep plugging thing in until it trips.
Recently went to my wife bosses house to check a breaker that was tripping. Tried to explain that the loads were continuous and he could only
use 80% of the circuit to be safe. He called a licensed contractor, who told him if the breaker was his safety device, until it tripped he was good.
I guess, is do people calculate loads or do they just keep plugging thing until the breaker trips?

 

[rc/retired]

I guess the question is how many people trust their inverse time breaker to protect the circuit and keep plugging thing in until it trips.
Recently went to my wife bosses house to check a breaker that was tripping. Tried to explain that the loads were continuous and he could only
use 80% of the circuit to be safe. He called a licensed contractor, who told him if the breaker was his safety device, until it tripped he was good.
I guess, is do people calculate loads or do they just keep plugging thing until the breaker trips?

How does one calculate 80% of their circuits when using every day devices and such?

 

[gadfly56]

I guess the question is how many people trust their inverse time breaker to protect the circuit and keep plugging thing in until it trips.
Recently went to my wife bosses house to check a breaker that was tripping. Tried to explain that the loads were continuous and he could only
use 80% of the circuit to be safe. He called a licensed contractor, who told him if the breaker was his safety device, until it tripped he was good.
I guess, is do people calculate loads or do they just keep plugging thing until the breaker trips?

If you mean Joe Homeowner, no. If you mean an electrician or electrical engineer, also no. No one is calculating the loads on a particular branch circuit to see whether the temperature is going to be an issue for some particular combination of appliances. After this, some of us here may wind up doing that, but by no means all.

 

[electrofelon]

The more I think about it and look at those figures, something just doesn't seem right. A single 12 cable with NO insulation and 24 amps reaches 237 degrees F???

 

[gadfly56]

How does one calculate 80% of their circuits when using every day devices and such?

You probably have a limited number of possible devices you can put on any one circuit. Let's say 6. You can have any combination of devices on at the same time, from only one to all six. If you assume there is some critical threshold of devices that definitely gives higher temperature than the cable rating, the trick is to know when a combination is excessive. How may combinations do you need to look at? The general formula for combinations is:

_nC_r = n!/(r!(n-r)!)

The formula means "If I have n objects and choose r at a time, and order does not matter, how many different ways can I choose those objects?"

You actually have to sum up six instances, since you can choose sets of from one to six objects at a time. I've used an online calculator to give the following:

6 + 15 + 20 + 15 + 6 + 1 = 63

There are 63 cases to calculate for 6 devices on one branch circuit.