Are over current fires a myth?

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mbrooke

Batteries Included
Location
United States
Occupation
Technician
Am I correct to assume that at 40amps the voltage drop would be so severe on a typical branch circuit that it is likely someone would say something was wrong? (HO calls electrician)
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
A house burned across the street from a friend of mine. The fire investigator told him that a bathroom fan had a bad connection that developed into a resistive fault that caught fire. A fault like this can start a fire without the current exceeding the rating of the breaker.
 

readydave8

re member
Location
Clarkesville, Georgia
Occupation
electrician
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:
I asked prospective tenant why she was moving, she said house she had been living in burned. I asked what caused the fire, she said it was an electrical fire.

Turned out that grease spilled on (electric) stove burner and caught fire, therefore it was an electrical fire.:happysad::blink:
 

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
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.

a 1A current on 14ga wire could cause wire to get very hot, all depends on how heat can be removed.

14ga copper in STP w/ no insulation will melt around 1980F, and will fuse at just about 166A, which is well beyond what a 30A OCD would allow.

could a 30A fuse on 14ga wire cause a fire? sure, but under some rare conditions. as mentioned already, wiring is not a homerun from A to B, there's pigtails in there, connections to devices like lights switches and outlets.

here's an example of hazard. last year i redid my kitchen backsplash with some nice tile, had to pull out some outlets (still connected to wires), did the tile, pushed outlets back, then i heard a bzzzzzz, bzzzzzz, sound, it was faint, but after investigating more closely i found that all five outlets has loose wire screws and one outlet so loose that there was small arc from screw to wire which was making that notorious 60Hz bzzzzz sound.

2nd example of hazard. a few months back i pressed the test buttons on four afci breakers in my panel, the 3rd one simply went "hummmmmmmmm" and smoke started to come out of the breaker, i quickly turned off main. the breaker was faulty and the lever didnt even move after i pressed test button. yanked that breaker out and it was already fairly warm (all that within ~5sec timeframe). this is why you should test these items per manufacturer recommendation, but since the breakers are "hidden" in panel they go forgotten, not seen like smoke detectors, etc.
 
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kwired

Electron manager
Location
NE Nebraska
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.
I find myself quite often having to explain why nothing is wrong when they have sparks when plugging something in or unplugging it while under load, and even then I'd say at least half still don't understand. Warm dimmer switches is also something they never understand.
 

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
cowboyjwc said:
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.


I find myself quite often having to explain why nothing is wrong when they have sparks when plugging something in or unplugging it while under load, and even then I'd say at least half still don't understand. Warm dimmer switches is also something they never understand.

the sparking outlet is however a good candidate to describe the fire hazard by probability. the probability is likely low that a spark from outlet causes a catastrophic fire. "yes & no" answer summarizes all probability, but you can likely quantify "yes" and "no" with probability of their own, like 99.9% no and 0.1% yes. you would have to back those #'s with statistics and lab tests (not sure if the folks at UL would have such data, etc). but, you get my point.
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
Pardon me...

But could it be possible that code table 310.15 B 16 is based around the terminal than the wire itself? Code always talks about 60*C vs 75*C and outside of cable jackets terminals are always dictating that.
 

kwired

Electron manager
Location
NE Nebraska
Pardon me...

But could it be possible that code table 310.15 B 16 is based around the terminal than the wire itself? Code always talks about 60*C vs 75*C and outside of cable jackets terminals are always dictating that.

I believe it is about conductor temperature, which also will effect termination temp as well as insulation temp.

90 deg C is far from too high for the conductor itself, but likely is high for most 600 volt or less terminals but most conductor insulation used today can take 90 deg C. So we have to find the right balance that works for all three areas in any application.
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
I believe it is about conductor temperature, which also will effect termination temp as well as insulation temp.

90 deg C is far from too high for the conductor itself, but likely is high for most 600 volt or less terminals but most conductor insulation used today can take 90 deg C. So we have to find the right balance that works for all three areas in any application.


Id say so. In fact when conductors are over loaded I see the most insulation damage around terminals.
 
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