Are over current fires a myth?

[mbrooke]

I have read fire reports where pyrolysis of wood occurred from contact with hot water or steam heating pipes.

The above quote is from this document

That is interesting, I thank you for posting that.

So in theory chronic overloads which push wire insulation over 90*C could dry out wood to the point of lowering ignition temperatures setting fire. This of course Id imagine would take years, but perhaps months if well over 100*C.

 

[templdl]

In open air the wire is able to dicipate heat where s wire that is not able to the heat escalates because to wire is not able to use the open air around it to radiate heat. 90degC rated wire when encased within Romex will end up getting hotter then that same wire in open air. The run that Romex through walls and in attics laying within insulated increased the wires inability to dicipate heat which increases the risk of a fire.

 

[steve66]

Quite unlikely as the insulation of nearly all 14 gauge conductors can handle 25 amps from table 310.16. I imagine most fires are from loose connections that arc and/or build up heat and that one in a thousand spark that lands in the perfect spot and smolders and eventually catches something on fire rather than going out.

Since we are talking about fuses, I think we are talking about mostly older homes, with older types of wiring.

I doubt the old cloth covered wire would handle a very high temp.

 

[petersonra]

Branch circuits typically seen in residences are very conservatively sized. My guess would be that while it would get warm, a 30A fuse on a #14 wire is unlikely to cause the wire insulation to burst into flames with modern wires.

However, I am less convinced of that for some of the ancient wiring that was in use at one time that still might be found in some residences, especially those that have fuses.

 

[kwired]

This long term exposure to even somewhat low level heat is why rules for recessed luminaires requires 1/2 inch spacing from wood framing members.

 

[ron]

An ongoing joke in my house is that when they hear a fire being reported on the news, right away one of them tells me it is an electrical fire. It is surprising how many fires get associated with an electrical problem. I wonder how many of them really are. :happysad:

 

[kwired]

An ongoing joke in my house is that when they hear a fire being reported on the news, right away one of them tells me it is an electrical fire. It is surprising how many fires get associated with an electrical problem. I wonder how many of them really are. :happysad:

Or that all electrical fires are the fault of the electrician that installed premises wiring. Often user equipment that is not handled properly is the cause of the fire, or an improper repair by untrained people.

Electric heater starts nearby combustibles on fire - many just hear "electric" heater and "fire" and assume it is an "elecrical fire", when in reality the user is at fault for allowing the combustibles too close to the heater.

 

[K8MHZ]

I have seen a fire caused by a homeowner connecting an air conditioner with 14AWG ran through the attic. There was a 20 on that circuit when we got there. The NMS was in contact with and buried by a bunch of clothes.

I also saw some 12AWG connected to a 30 burned through.

Granted, that's only 2 times out of many, but it does happen.

 

[K8MHZ]

I think electric space heaters pose a much larger risk than a breaker one size off.

You should see the damage they do to old, incorrectly installed wiring that was installed 75 years ago.

I will try to get some pics for you.

In Michigan, space heaters cause many fires.

 

[kwired]

I have seen a fire caused by a homeowner connecting an air conditioner with 14AWG ran through the attic. There was a 20 on that circuit when we got there. The NMS was in contact with and buried by a bunch of clothes.

I also saw some 12AWG connected to a 30 burned through.

Granted, that's only 2 times out of many, but it does happen.

Was this cable coiled up or just passing through those combustible materials?

 

[K8MHZ]

Was this cable coiled up or just passing through those combustible materials?

It was just dragged through the attic, then covered with piles of clothing. It was feeding an old air conditioner.

My EC friend got called on a real hot summer day about the air conditioning not working. HO told EC that husband of HO ran the cable through the attic. When my EC friend opened the door to the attic, it was full of smoke and he could see a glow.

The fire department came and soaked down the fire. No damage to the structure. I got the wonderful job of correcting the wiring in the stinky, wet attic.

 

[steve66]

I have seen a fire caused by a homeowner connecting an air conditioner with 14AWG ran through the attic. There was a 20 on that circuit when we got there. The NMS was in contact with and buried by a bunch of clothes.

I also saw some 12AWG connected to a 30 burned through.

Granted, that's only 2 times out of many, but it does happen.

Good point. I believe Window AC units connected through an undersized extension cord can cause a fire faster than a Zippo. I know at least one person this has happened to.

 

[kwired]

Good point. I believe Window AC units connected through an undersized extension cord can cause a fire faster than a Zippo. I know at least one person this has happened to.

But I don't see 14 AWG on a 20 amp circuit doing that very easily. Have seen coiled up extension cords supplying heavy loads get hot though, but stretch them out and they are fine. Need to watch for this with retractable cord reels also.

 

[gadfly56]

But I don't see 14 AWG on a 20 amp circuit doing that very easily. Have seen coiled up extension cords supplying heavy loads get hot though, but stretch them out and they are fine. Need to watch for this with retractable cord reels also.

Good advices. I took a quick peek, and Bayco has a 30' reel with 16/3, but it's listed for only 10 amps. A Bayco "professional" product, 50' with 12/3 is listed for only 15 amps.

 

[don_resqcapt19]

I have seen a fire caused by a homeowner connecting an air conditioner with 14AWG ran through the attic. There was a 20 on that circuit when we got there. The NMS was in contact with and buried by a bunch of clothes.

I also saw some 12AWG connected to a 30 burned through.

Granted, that's only 2 times out of many, but it does happen.

The clothing is almost the equivalent of insulation. Testing done by the copper development association showed excessive temperature rise for NM cable in insulation with loads below the rated ampacity.
As I recall some of that information was in the ROP and ROC from when they added the words shown in bold below to 334.80.

334.80 ...Where more than two NM cables containing two or more current-carrying conductors are installed, without maintaining spacing between the cables, through the same opening in wood framing that is to be sealed with thermal insulation, caulk, or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(3)(a) and the provisions of 310.15(A)(2), Exception, shall not apply. ...

 

[kwired]

The clothing is almost the equivalent of insulation. Testing done by the copper development association showed excessive temperature rise for NM cable in insulation with loads below the rated ampacity.
As I recall some of that information was in the ROP and ROC from when they added the words shown in bold below to 334.80.

Still Mark had a single cable that was encased, and though he didn't specify exactly what the load was, if it were an AC unit like he said and on a 20 amp breaker it probably didn't draw much more then 13 amps or it likely trips breaker when starting.

 

[mbrooke]

#14 is good for 100amps

#14 is good for 100amps

That's right!

According to this:



http://paceforensic.com/pdfs/Circuit_Breakers_The_Myth_of_Safety.pdf

 

[K8MHZ]

Still Mark had a single cable that was encased, and though he didn't specify exactly what the load was, if it were an AC unit like he said and on a 20 amp breaker it probably didn't draw much more then 13 amps or it likely trips breaker when starting.

IIRC, there was something wrong with the air conditioner. I remember the EC telling the HO not to use it any more and to get a new one if she needed A/C.

 

[Barbqranch]

Still Mark had a single cable that was encased, and though he didn't specify exactly what the load was, if it were an AC unit like he said and on a 20 amp breaker it probably didn't draw much more then 13 amps or it likely trips breaker when starting.

It could draw a lot more than 20 amps and not trip if it had Zinsco breakers.

 

[cowboyjwc]

Quite unlikely as the insulation of nearly all 14 gauge conductors can handle 25 amps from table 310.16. I imagine most fires are from loose connections that arc and/or build up heat and that one in a thousand spark that lands in the perfect spot and smolders and eventually catches something on fire rather than going out.

I had to go to court one time on some outlets that were sparking every time something was plugged in or unplugged (there's way more to the story), anyway the lawyer asked me if it was a fire hazard and I told the judge that he wasn't going to like my answer, he told me to give it and he would decide, I said yes and no. He said you're right I don't like it, explain please. I told him those things could spark from now until the end of time and never cause a problem or they could spark tomorrow land on some paper sitting on the counter and burn the place to the ground. It's hard to tell when something is an actual fire hazard.