Update on Romex in Spray Foam Insulation

[rc/retired]

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.

Right! And average Joe homeowner is going to do just that!

Ron

 

[gadfly56]

Right! And average Joe homeowner is going to do just that!

Ron

Well, no, but you're the one who asked. My first boss told me, "Don't ask questions unless you're sure you want to hear the answers." Thank you, Skip.

 

[PaulMmn]

Earlier points in the discussion centered on over-heating the wire while the foam cured. Has this been dismissed as a non-issue, or is the heat of curing able to damage the NM cable?
.
I will assume (!) that, in most cases, cables in a wall being foamed will not be in use (at least to outlets in that wall!)

 

[Rjryan]

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.

You're right it's hard to calculate loads.
I calculated my wife's bosses because the loads were pretty well fixed, but that is my point, people plug things in until there is a problem,
the breaker trips. Inverse time breaker don't trip exactly at 15 or 20 amps can carry more depending on the overage and the time involved.
So it is not hard to put a heavy load on a circuit without realizing it. In my wife's bosses case did not matter, just keep plugging in things until he
had trouble with the breaker again.

 

[Rjryan]

You're right it's hard to calculate loads.
I calculated my wife's bosses because the loads were pretty well fixed, but that is my point, people plug things in until there is a problem,
the breaker trips. Inverse time breaker don't trip exactly at 15 or 20 amps can carry more depending on the overage and the time involved.
So it is not hard to put a heavy load on a circuit without realizing it. In my wife's bosses case did not matter, just keep plugging in things until he
had trouble with the breaker again.

Someone was questing the validity of Table 1 in the Goodson Engineering study because they used the exact amperage of the breaker and
slightly above to get to the wire temperatures they came up with, probably more realistic than you think and a scenario that is not out of the realm
of possiblity with people that just plug things in until the breaker trips.

 

[retirede]

Inverse time breaker don't trip exactly at 15 or 20 amps can carry more depending on the overage and the time involved.

Breakers are designed to carry their rating, not trip at it.

 

[electrofelon]

Earlier points in the discussion centered on over-heating the wire while the foam cured. Has this been dismissed as a non-issue, or is the heat of curing able to damage the NM cable?
.
I will assume (!) that, in most cases, cables in a wall being foamed will not be in use (at least to outlets in that wall!)

Spray foam definitely generates heat while curing. It is applied in "lifts" of no more than 2" IIRC. That is spray foam 101 and if you don't do that there are going to serious problems such as the foam not curing properly leaving nasty fumes, and gaps and voids. So yes I suppose over applying could cause excessive temperatures that could damage wiring, but honestly you're going to have much bigger problems than that in that case.

 

[gadfly56]

Spray foam definitely generates heat while curing. It is applied in "lifts" of no more than 2" IIRC. That is spray foam 101 and if you don't do that there are going to serious problems such as the foam not curing properly leaving nasty fumes, and gaps and voids. So yes I suppose over applying could cause excessive temperatures that could damage wiring, but honestly you're going to have much bigger problems than that in that case.

I don't have much experience (that is, zero) with spray foam insulation. NM is listed for exposure to 90°C in dry environments. That's continuous, not some peak value. Understand that if you grabbed that cable at 90°C you'd be looking at 2nd if not 3rd degree burns. Knowing something about chemical reactions, if something like spray foam was exposed to much over 90°C, I'd expect runaway reactions to occur. I see recommendations for application temperatures of 110°F - 140°F depending on foam type, and absolute maximum exposure of 180°F. That's about 82°C. So, it appears that if spray foam is applied in accordance with the manufacturer's instructions, the embedded NM should not exceed its rating.

 

[letgomywago]

I think what this should teach us is that we should avoid burying our wire where it can cool in a spray foam situation. I like when they spray the roof deck and avoid the drywall bellow. It takes more but it makes the attic workable forever

 

[Fred B]

I see recommendations for application temperatures of 110°F - 140°F depending on foam type, and absolute maximum exposure of 180°F. That's about 82°C.

Most large scale foam application equipment (not a typical can) use heated elements on the mixing equipment. Those Temperatures you quote is the temperature the general equipment are preheated at least 80 deg. for the application of the foam insulation. The 180 deg recommendation is the average temp of pvc pipe heating for pressure rating of pipe. Installers are warned to do multi step application to avoid overheating of the plastic types of pipe and subsequent damage to the pipe by attempting to limit the exothermic heating to below the 180 deg mark.

**"SPRAY POLYURETHANE FOAM INSULATION – TEMPERATURES WHEN CURING AND THE RISKS TO PLASTIC PIPE & FITTING MATERIALS According to documents published by the SPFA, “The chemical reaction that takes place during the application and curing of SPF will generate exothermic temperatures above the 120°F - 130°F [49°C - 54°C] setpoint temperatures of the proportioner and hoses. Peak temperatures at the midthickness of a pass can exceed 200°F [93°C] for several minutes and reach peak temperatures 250°F - 275°F [121°C - 135°C] for a minute or two, especially for closed-cell SPF, when applied at the maximum pass thickness.” “An example of measured exothermic temperatures for a closed-cell SPF product using an HFC blowing agent applied to a heated sheet metal substrate is shown in Figure 1.” “It should be noted that newer closed-cell SPF formulations using HFO blowing agents may use different catalysts, which, in some foams, can increase the peak temperatures during curing to as high as 325°F [163°C] for short periods and can delay the time to reach the peak temperature.”

**https://plasticpipe.org/common/Uploaded files/Technical/TN-69/PPI TN-69.pdf

Will these temperatures damage NM? Don't know.

 

[gadfly56]

Most large scale foam application equipment (not a typical can) use heated elements on the mixing equipment. Those Temperatures you quote is the temperature the general equipment are preheated at least 80 deg. for the application of the foam insulation. The 180 deg recommendation is the average temp of pvc pipe heating for pressure rating of pipe. Installers are warned to do multi step application to avoid overheating of the plastic types of pipe and subsequent damage to the pipe by attempting to limit the exothermic heating to below the 180 deg mark.

**"SPRAY POLYURETHANE FOAM INSULATION – TEMPERATURES WHEN CURING AND THE RISKS TO PLASTIC PIPE & FITTING MATERIALS According to documents published by the SPFA, “The chemical reaction that takes place during the application and curing of SPF will generate exothermic temperatures above the 120°F - 130°F [49°C - 54°C] setpoint temperatures of the proportioner and hoses. Peak temperatures at the midthickness of a pass can exceed 200°F [93°C] for several minutes and reach peak temperatures 250°F - 275°F [121°C - 135°C] for a minute or two, especially for closed-cell SPF, when applied at the maximum pass thickness.” “An example of measured exothermic temperatures for a closed-cell SPF product using an HFC blowing agent applied to a heated sheet metal substrate is shown in Figure 1.” “It should be noted that newer closed-cell SPF formulations using HFO blowing agents may use different catalysts, which, in some foams, can increase the peak temperatures during curing to as high as 325°F [163°C] for short periods and can delay the time to reach the peak temperature.”

**https://plasticpipe.org/common/Uploaded files/Technical/TN-69/PPI TN-69.pdf

Will these temperatures damage NM? Don't know.

Great information, thanks for digging that up. Does the UL standard for NM make any comment on the maximum exposure temperature on a time vs temperature basis? I don't have access to the standard.

 

[don_resqcapt19]

Great information, thanks for digging that up. Does the UL standard for NM make any comment on the maximum exposure temperature on a time vs temperature basis? I don't have access to the standard.

Actually you do. UL uses the same free viewing system as the NFPA uses. Not user friendly as you can't search or copy and paste, but you can view all of the UL standards for free...just like the NFPA, you need to register with an email address.

 

[BarryO]

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

Exactly. Also, NM is massively derated: 90 degree conductors with ampacity evaluated at the 60 degree level. So it seems like 2 issues are in play: 1.) damage to conductor insulation during the installation/curing process of the insulation, in that the conductor temperature may exceed 90 C briefly while the insulation cures and then be permanently damaged. So does this preclude ever installing NM in a foam insulated wall? 2.) Should the bundling requirements be tighter for foam than for other insulation? Admittedly the bundling regs in the NEC are pretty sloppy and could use some better definition.