Are over current fires a myth?

[mbrooke]

I repeatedly hear fire safety experts and fire fighters claim that a 30amp fuse will burn down a home, but is that true or just a myth? Is a dangerous over current condition even possible in practical terms?

 

[Smart $]

It's a myth until it happens. It can happen.

Beyond that, it's all a matter of statistics and compliance.

 

[mbrooke]

It's a myth until it happens. It can happen.

Beyond that, it's all a matter of statistics and compliance.

But what mechanism triggers that fire?

 

[mjf]

Do you mean a 30 amp fuse with properly sized conductors or are you referring to instances where folks replaced a properly sized 15 or 20 amp fuse with a 30 amp fuse?

 

[mbrooke]

Do you mean a 30 amp fuse with properly sized conductors or are you referring to instances where folks replaced a properly sized 15 or 20 amp fuse with a 30 amp fuse?

14 gauge wire protected by a 30 amp fuse.

 

[Smart $]

But what mechanism triggers that fire?

Heat and/or arc flash in the vicinity of combustibles. May not be at/near the fuse itself. Could be elsewhere in the circuit.

 

[mbrooke]

Heat and/or arc flash in the vicinity of combustibles. May not be at/near the fuse itself. Could be elsewhere in the circuit.

But how does over loading that circuit cause excessive heat or relaxing of combustible materials?

 

[Smart $]

But how does over loading that circuit cause excessive heat or relaxing of combustible materials?

Wire temperature increases exponentially with increase in current. Insulation breaks down under the higher temperature, conductors arc around already hot combustibles. POOF! Fire ensues.

 

[don_resqcapt19]

Without some other problem, it would be my opinion, that a 30 amp load on #14 would be unlikely to raise the temperature of the wire itself to the point where it could ignite ordinary building construction or furnishing materials, at least not over a short time. Pyrolysis may over time lower the ignition point of the material enough that the #14 carrying 30 amps could ignite the materials. #14 carrying 34 amps will not operate at a temperature that exceeds 302?F.


However this high conductor operating temperature will cause insulation damage and may result in an arcing condition that could start a fire.

 

[electrofelon]

14 gauge wire protected by a 30 amp fuse.

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.

 

[Jraef]

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 think this is a good point.

Also, we tend to think of the fixed wiring only but remember, that fixed wiring goes to an outlet, and the devices plugged into that outlet are designed around expecting that the circuit protection for the RATING of that outlet has not been exceeded either. So yes, maybe the 14ga wire will not ignite, but what about the 18ga cord on the toaster that is over heating because the toast curled inside, jamming the toaster release mechanism and the toaster mfr was expecting a 15A fuse or breaker to clear that condition before the wire was damaged? The 18ga cord catches on fire, it spreads to the oven mitts on a peg near by, then to the cabinets and the whole house burns down. The 14ga wire survived though... Or like the surgeon joke goes; "The operation was a success. We lost the patient however."

Read this: http://www.interfire.org/features/electric_wiring_faults.asp

 

[mbrooke]

Without some other problem, it would be my opinion, that a 30 amp load on #14 would be unlikely to raise the temperature of the wire itself to the point where it could ignite ordinary building construction or furnishing materials, at least not over a short time. Pyrolysis may over time lower the ignition point of the material enough that the #14 carrying 30 amps could ignite the materials. #14 carrying 34 amps will not operate at a temperature that exceeds 302?F.


However this high conductor operating temperature will cause insulation damage and may result in an arcing condition that could start a fire.

That's what makes me wonder, and I breaks down into 3 questions IMO:

1. At what point will NM-B exceed 90*C in,


a. a wooden framed wall with no insulation

b. a wooden framed home with R-14 fiber glass bat (for example) on an outside wall.


2. At what point do temperatures have to be high enough (or long enough) to lower the ignition temperature of wood.

3. At what point will current heat NM-B to the point a fire will start on fresh wood factoring 1(a) and 2(b)



Of course my thinking isn't perfect, more an educated guess.

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.

You know, that makes a lot of sense now that I think about it.

 

[electrofelon]

If you didnt read the paper posted by Jraef:

When PVC is exposed to temperatures of 200 ? 300?C, it chars and the char is a semiconductor. Not surprisingly, this can lead to leakage currents and arcing. But Nagata and Yokoi [ [13] ] found that if virgin PVC was heated to the rather low temperature of 160?C, impressing 100 V across 1 mm of insulator thickness was sufficient to cause ignition of the insulation. Furthermore, if the insulation had previously been preheated to 200 ? 300?C, then ignitions occurred when the preheated insulation was raised to only very mild temperatures during the voltage test?from room temperature to 40?C?were found sufficient (Figure 3).

This is interesting and partially addresses the OP, but it appears we still need something to get the temp up to twice the 90 degree rating of the conductor.....

 

[mbrooke]

If you didnt read the paper posted by Jraef:



This is interesting and partially addresses the OP, but it appears we still need something to get the temp up to twice the 90 degree rating of the conductor.....

But is this based on IEC rated PVC or our THHN conductors?

 

[kwired]

Another thing I have observed over the years is when you have say an old home with old fuse box still in use is there is often multiple conductors landed on each fuse holder. The fuseholder has a 30 amp fuse installed mostly because they can't find anything with a higher setting that will fit the fuseholder though it only has 15 or 20 amp conductors supplied from it. You may at times have overloaded conductors, but if there are multiple conductors load is often divided enough that there is no significant overloading that ever causes a problem. But you also don't really know when that one problem in a thousand chances is going to pop up. And even when it does it maybe just burns out a connection and the circuit stops working beyond that point, but then a small percentage of those do possibly ignite combustible materials somehow and then you have a structure fire. Of course there is also 16 and 18 AWG cords like Jraef mentions that maybe the appliance mfgr was counting on 15 or 20 amp overcurrent protection when designing the appliance.

 

[mbrooke]

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.

Correct, but how is that determined? Since the NEC charts take all considerations in account, that 90*C was probably taken as a worst case scenario, 25 amps with the conductor in exceptionally thick insulation, something like R-100 (foam insulated home that several feet thick).


In a regular wall Id imagine more current would be needed to exceed 90*C.

 

[mbrooke]

Another thing I have observed over the years is when you have say an old home with old fuse box still in use is there is often multiple conductors landed on each fuse holder. The fuseholder has a 30 amp fuse installed mostly because they can't find anything with a higher setting that will fit the fuseholder though it only has 15 or 20 amp conductors supplied from it. You may at times have overloaded conductors, but if there are multiple conductors load is often divided enough that there is no significant overloading that ever causes a problem. But you also don't really know when that one problem in a thousand chances is going to pop up. And even when it does it maybe just burns out a connection and the circuit stops working beyond that point, but then a small percentage of those do possibly ignite combustible materials somehow and then you have a structure fire. Of course there is also 16 and 18 AWG cords like Jraef mentions that maybe the appliance mfgr was counting on 15 or 20 amp overcurrent protection when designing the appliance.

That is in part what got me wondering. I see loads of homes with fuse boxes where the 15 amp circuits were changed over to 30 amp fuses. Each time I see no insulation damage to the ore itself, yet I constantly hear Firefighters say the high current will cause the wire to catch fire.



In terms of appliance cords I think one has to consider IR2 heating. Breakers have magnetic trip in them, so if a fault occurred in a toaster, the current magnitude may get into the MT tripping the breaker faster reducing IR2 heating during the fault. In theory with a thermal only breaker the cord might get very hot waiting to heat the bimeal in the breaker. However, its not all bad IMO. That appliance cord is often rated for a higher temperature. Of course I wonder how a lamp cord would pass.


If I am correct there is a minimum size a cord can be such as to a 4 watt alarm clock?

 

[electrofelon]

Correct, but how is that determined? Since the NEC charts take all considerations in account, that 90*C was probably taken as a worst case scenario, 25 amps with the conductor in exceptionally thick insulation, something like R-100 (foam insulated home that several feet thick).


In a regular wall Id imagine more current would be needed to exceed 90*C.

I fully agree and also have wondered what the standards or test conditions was for the 310.16 ampacities. clearly there is a wide range of installation scenarios that use the same table.

 

[mbrooke]

I fully agree and also have wondered what the standards or test conditions was for the 310.16 ampacities. clearly there is a wide range of installation scenarios that use the same table.

In order for the table to work without engineering (thermodynamic equations) or installation charts (insulation vs no insulation) it would have to assume absolute worst case 1in billion scenario.


This gives insight I guess. From Don19, see proposal 6-99:

http://iaei-western.org/Files/Code/2011/2011-ROP.pdf

The ampacities for both copper and aluminum conductors with a conductor
temperature rating of 75?C are identical between the NEC and CEC.
The ampacities for conductors with temperature ratings of 60?C and 90?C
were calculated from the 75?C values using the equation shown on the pages
provided as supporting material that also contain supporting tables. The
equation is defined in the following standards:
IEEE Std 835-1994, IEEE Standard Power Cable Ampacity Tables,
Section 3.4.2 Adjustment for change in maximum conductor temperature or
temperature due to dielectric loss.
AIEE-IPCEA Power Cable Ampacities, 1962 (AIEE Pub. No. S-135-1 /
IPCEA Pub. No. P-46-426) Volume 1 ? Copper Conductors, Adjustment for
Change in Parameters, page III.
60?C copper conductor temperature ? There are only two differences (5 amps)
in ampacities between the NEC and the CEC and the CEC matches the rounded
calculated values on these two conductor sizes. There are three values where
the NEC and CEC agree but all three differ from the rounded calculated values
by 5 amps. A proposal is also being submitted to correct the CEC on the five
minor deviations.
90?C copper conductor temperature ? There are only two differences (5 amps)
in ampacities between the NEC and the rounded calculated ampacities. The
CEC is lower on all values and a proposal is being submitted to correct the
CEC to agree with the rounded calculated values and the revised NEC. The
CEC agrees with the values in the 1978 NEC.

 

[don_resqcapt19]

That's what makes me wonder, and I breaks down into 3 questions IMO:...

2. At what point do temperatures have to be high enough (or long enough) to lower the ignition temperature of wood. ...

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

... Around 1900, fires started being reported with steam or hot-water heating pipes where the pipes had been passed
through wood members. Ignitions were being observed typically 3 months to 15 years after installation4,5. The
installations typically involved hot-water or low-pressure steam, where temperatures should be not much over
100?C (212?F). A small fraction of the incidents evidently involved some boiler malfunctions, and notably high
temperatures would be expected. But for the majority, it is clear that these ignitions are not be categorized as
external ignitions. It might be noted that investigators making the determinations of the cause in these long-term,
low-temperature wood ignitions included Prof. Ira Woolson6 (the first US professor of fire science) and Voitto
Virtala7 (the ?father? of fire science in Finland).

The above quote is from this document