What temperature does Romex insulation melt?

[68Malibu383]

I inspected a house yesterday and some of the Neutral wires in the panel had gotten very hot as evidenced by melted insulation. During the inspection, neutral bus bar was 170ﾟ. I'm sure the temperature would have needed to be much, much higher than 170ﾟ for the insulation to melt and just curious if anyone knows.

 

[James L]

Finding an exact answer has proven to be an impossible task for me in the past. I remember seeing once, that the insulation will start breaking down if the ambient temperature is above 100 degrees Celsius, which would be about 212 degrees Fahrenheit

 

[texie]

In addition to finding the cause of the over heated neutrals the OP needs to fix the doubled up neutral connections.

 

[Hv&Lv]

Just looking at it it appears the top one is the culprit.

 

[WasGSOHM]

Romex insulation = "polyvinyl chloride"
and the melting point = 185F.
Maybe some hybrid of this melts at a higher temp.
I guess I'd search on the varieties of PVC.

 

[MD Automation]

My first 2 thoughts are improperly torqued connection on that top neutral (I agree w/ Hv&Lv, that lug is more discolored than the rest) - and possibly a mis-wired MWBC - with both hots accidentally on the same leg? This assumes that the breaker for that circuit(s) functions properly and is not oversized.

 

[James L]

Just looking at it it appears the top one is the culprit.

For sure. The baking is worse at top, and less farther down. Prolly loose screw

 

[mopowr steve]

Well isn’t it what the temperature limitations that we’re all familiar with?
If it’s newer romex it should have 90*C rated conductors in other words 194*F
What you may have in that panel may be the pre-1990’s wire which was THN or 75*C conductors or 167*F rated.

 

[K8MHZ]

Romex insulation = "polyvinyl chloride"
and the melting point = 185F.
Maybe some hybrid of this melts at a higher temp.
I guess I'd search on the varieties of PVC.

Are you sure that's not 185C ? This from Wiki

Melting point: 100 °C (212 °F) to 260 °C (500 °F)

 

[WasGSOHM]

Are you sure that's not 185C ? This from Wiki

Melting point: 100 °C (212 °F) to 260 °C (500 °F)

Let's go with your numbers.
I should have checked several sources to eliminate typos when I have no idea what is reasonable.

To allow for a safety factor and a 50 year service lifetime, when should 90C Romex melt?

 

[retirede]

The temperature rating of conductor insulation is to limit long-term degradation. “Thermal aging characteristics of the polymer” to quote Anixter. It has nothing to do with the melting point.

 

[don_resqcapt19]

Romex insulation = "polyvinyl chloride"
and the melting point = 185F.
Maybe some hybrid of this melts at a higher temp.
I guess I'd search on the varieties of PVC.

Not very likely given the temperature rating of the insulation on the conductors of NM cable is rated at 90°C (194°F)

 

[Fred B]

Not very likely given the temperature rating of the insulation on the conductors of NM cable is rated at 90°C (194°F)

The older NM was only rated to 60^o C, have a lot of that here on older homes. So it is more likely to suffer the type of damage seen in OP. Even seen it under lighting fixtures.

 

[ptonsparky]

I would think the temperature of the glowing connection would be more of an issue.

 

[kwired]

Also remember 90C insulation is supposed to be able to withstand that temp with no damage.

Then there is the temp at which PVC starts to soften and becomes more flexible vs what temp it may start to become liquid.

 

[garbo]

I inspected a house yesterday and some of the Neutral wires in the panel had gotten very hot as evidenced by melted insulation. During the inspection, neutral bus bar was 170ﾟ. I'm sure the temperature would have needed to be much, much higher than 170ﾟ for the insulation to melt and just curious if anyone knows.

A lot depends if its the old style NM cable or the newest tyle NMB that came out in early in 1980's. The NMB insulation always appeared to be type THWN which in dry locations good for think 105 degrees C. Old style NM cable always appeared to be type TW insulation. If the white wire that you are talking about is feeding 2 circuits and if bith hot wires on the same phase the white wire would draw too much current. On a shared nuetral wired correctly the white wire would only draw the imbalance current. Ex = first hot wire drawing 10 amps and othet hot wire drawing 12 amps white wire would inly draw 2 amps. A qualified petson must go over why the nuetral buss is running so hot. Should never be but a few degrees warmer then surrounding area.