NEC Changes For #14 Ampacity

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iwire said:
Or the other facts, like loading etc.
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I counted 8 14/2's there- a lot but even if they were all loaded to the allowable 15 max, imo, still not enough heat would have been generated to cause that degree of burning. What might have happened is that the zip tie guy was a little overzealous with tie on the left and pinched some conductors under those plastic jackets (it is possible to do this even with plastic tie wraps and too, the modern jacket on nm isn't quite as hardy as the older stuff). I'm thinking this because of the burn pattern- the tie on the right doesn't allow much breathing room between the nms either, yet there is nowhere near the degree of discolored insulation directly underneath the tie.
 
pinched the pvc on the inside?? i dont think a plastic zip around that diameter would have enough force to damage the pvc on the inside. cant know for sure unless there was some forensics done on that bundle. but heck, its one of this scenarios we want to avoid, but can be addressed outside of ampacity, possibly through derating, or just blatantly not allowed (perhaps a starfish holder would be allowed, holds many nm's together whiles keeping a space between them). and seems odd that only that small section was bundled, perhaps that was a test setup?

a NEMA article, just not sure when it says "temps were never objectionable" if that is applied to testing installations that were derated, or, if the bundled wiring was tested at full ampacity w/o derating.
https://www.nema.org/Technical/Docu...Spray-Foram Insulation Used in Residences.pdf
 
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FionaZuppa said:
pinched the pvc on the inside?? i dont think a plastic zip around that diameter would have enough force to damage the pvc on the inside. cant know for sure unless there was some forensics done on that bundle. but heck, its one of this scenarios we want to avoid, but can be addressed outside of ampacity, possibly through derating, or just blatantly not allowed (perhaps a starfish holder would be allowed, holds many nm's together whiles keeping a space between them). and seems odd that only that small section was bundled, perhaps that was a test setup?

a NEMA article, just not sure when it says "temps were never objectionable" if that is applied to testing installations that were derated, or, if the bundled wiring was tested at full ampacity w/o derating.
https://www.nema.org/Technical/Docu...Spray-Foram Insulation Used in Residences.pdf
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The zip tie scenario is certainly possible if it is tightened enough-the insulation covering the conductors in those outer nms can be compressed enough to cause damage , compromising it. I just don't think heat would just build up under that one particular tie, yet it wouldn't to the same degree a just a few inches away under virtually identical spacing under that other tie.- An awful lot of scorch there confined to that one spot and we have millions of installs with lots of nm packing pvc in insulated walls that isn't burned up (and its probably a good bet to assume that the vast majority of it isn't derated either).

Never say never with things even as harmless as those zips- common sense should prevail that you would only be trying to route the cables in a uniform fashion, not squeeze them, but if there is a way to damage something, it will be done.
 
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FionaZuppa said:
...i am want to see the evidence as to why NEC changed it...... and why NEC has the 15A OCD restriction.
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iwire said:
I have been doing this work for more than 30 years now and I have never once wondered or concerned myself about the NECs choice of ampacities.
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peter d said:
It's not just you. Debating and discussing ampacity is an exercise for engineers and people with a lot of free time on their hands. I've said it many times and will say it again - we have a good system here that has worked for a long time now.
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jim dungar said:
Most likely the NEC did not perform any of these analysis for their Small Conductors.
Their 'restrictions' are likely not much more than feel good values that address cable construction, conductor insulation and installation considerations over the past 100 years of electrical installations. As such the report values bear no relationship to real world physics.

Your deep dive analysis may be more germane to equipment standards, like those that allow #18AWG in fixtures.
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mbrooke said:
Its built into the numbers and codes themselves.
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The OP is certainly a valid inquiry. On the other hand, as my favorite code instructor once said "This book is written in blood" I think often that is a good enough answer. On the other hand, I too question the rationale for certain things in the same way ya'll are - I think it is good to do so and can be very educational. On the other hand, I think one could go in circles for a long time and arrive at the same basic model we have now.

FionaZuppa said:
so my point is, if its insulation damage from heat that NEC is trying to protect against, then use a std based on watts/area, or watts/ft of wire to define ampacity and OCD #'s.
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FionaZuppa said:
but ok, thats for the older insulation types, what about wire rated at 90C but has terminations rated 75C, why still restricted to 15A OCD ?? seems less to do with failing insualtion and more to do with heat generation. the Gexol insulation can run real hot w/o issue, so when will NEC add a 110C column? are ampacities based on insulation damage by temp, or just generation of heat? seems to be the latter given insulation like Gexol is out there. sure, terminations are still not common in 90C, but you get my point.
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jim dungar said:
The UL489 standards limit the standard molded case circuit breaker terminal temperature rise to 50C when loaded to 100% in an open air ambient temperature of 40C.
Which seems to say that at 100% loading the conductor internal heat generation is limited to a maximum of 50C, not the ultimate rating of its insulation.
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IMHO and based on experience insulation damage due to heat is probably at the bottom of the list of concerns. I would think that heat buildup at connections, devices, and panelboards is probably more of a factor.

Voltage drop was touched upon in this discussion - VD under continuous load is one thing, but consider VD under fault current and time-current curves for OCPD's. Maybe this consideration (whether intentional/documented or just based on gut feel and experience) it is somewhat inherent in ampacity charts and has bearing on why #14, #12, #10 are non-linear with others.

For example last spring I uncovered a ground fault in a 4" sq metal box, everything wired to code, where the ungrounded conductor burned a hole through the back of the box and charred the wood behind it. It was a 277V lighting circuit with #10 THHN running about 300ft fed by a Siemens model BQD 20A breaker. (CCC's were oversized due to voltage drop.) Complaint was that breaker would trip occasionally. Upon inspection I found that it took about 3 seconds on a dead-short to trip the breaker, no doubt due to voltage drop limiting the fault current. If careful, one could weld with it and not trip the breaker. Definitely not a safe scenario.

I can only imagine what would happen if this were a 14-2 NM running a long distance and a non-metallic box. Perhaps an EE could calculate fault current on 500ft distance and apply that to time-current curve of different OCPD sizes, and see how safe you really feel about NEC ampacity tables or changes to it.
 
AJElectric said:
For example last spring I uncovered a ground fault in a 4" sq metal box, everything wired to code, where the ungrounded conductor burned a hole through the back of the box and charred the wood behind it. It was a 277V lighting circuit with #10 THHN running about 300ft fed by a Siemens model BQD 20A breaker. (CCC's were oversized due to voltage drop.) Complaint was that breaker would trip occasionally. Upon inspection I found that it took about 3 seconds on a dead-short to trip the breaker, no doubt due to voltage drop limiting the fault current. If careful, one could weld with it and not trip the breaker. Definitely not a safe scenario.

I can only imagine what would happen if this were a 14-2 NM running a long distance and a non-metallic box. Perhaps an EE could calculate fault current on 500ft distance and apply that to time-current curve of different OCPD sizes, and see how safe you really feel about NEC ampacity tables or changes to it.
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this can be addressed via verbiage on wiring and not by ampacity. there is nothing verbiage can do to protect against a hazard if the wiring is not to code and/or faulty, etc. i suspect you mean the additional ohms of the long run and not "voltage drop" since all of our OCD items are current based.
 
FionaZuppa said:
this can be addressed via verbiage on wiring and not by ampacity. there is nothing verbiage can do to protect against a hazard if the wiring is not to code and/or faulty, etc.
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Just semantics. I suspect the initial failure was due to poor connection & heat buildup melting insulation, hard to tell when everything is fried, when I say "according to code" I'm referring to sizing and grounding and methods - in fact the #10 wire was sized above code req for the 8A lighting load. True the actual failure may have been due to a code violation - but that is non sequitur the failure could have just as well been a rodent or physical damage.

Understood, clearing a fault is different from continuous load ampacity that you're addressing. I believe this is germane to the conversation because it could play into the NEC sizing req for the smaller sizes, all I'm saying is it's something worth considering.

FionaZuppa said:
i suspect you mean the additional ohms of the long run and not "voltage drop" since all of our OCD items are current based.
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Correct it isn't really voltage drop but the high impedance resulting in lower fault current resulting in more time elapsed before OCD clears - which is really the bottom line - open circuit before starting fire. The amount of energy at point of fault is still enough melt wires and steel, so given enough time (seconds) has high probability of starting a fire vs. near-instantaneous trip with shorter runs. When it comes to sizing conductor vs. OCD I think this is an important factor that should be considered but isn't really addressed in the code (or is it????) Maybe I'm crazy and this shouldn't be a concern?

#14 at 2.6 Ohms per 1000ft on a 500ft run = 2.6 ohms total. 120V/2.6 = 46 amps. A standard GE THQL1115 breaker, 15A, would take 5-30 seconds to clear. That's a long time of arky-sparky.

#14 on a 20A breaker would be 12-90 seconds, quite a bit longer.

#12 at 1.6 Ohms on a 15A would clear in 1-9 seconds, still too long but much better.

#10 would clear in less than 3 seconds.

All I'm saying is with no voltage drop sizing rules to prevent extended fault-clear times, NEC would really be going out on a limb to increase ampacities of smaller sizes vs. OCD size.
 
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AJElectric said:
Understood, clearing a fault is different from continuous load ampacity that you're addressing. I believe this is germane to the conversation because it could play into the NEC sizing req for the smaller sizes, all I'm saying is it's something worth considering.
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Ill go on a limb and say the NEC has never been concerned about fault clearing to that level in the past. I do not believe breaker disconnect times have ever been a concern. Breakers are not required to have magnetic trip in UL testing, in fact we can legally have #14 on a 40 amp breaker feeding a motor.

Though thats not to say fault theory ought not to be discussed or investigated further.

Correct it isn't really voltage drop but the high impedance resulting in lower fault current resulting in more time elapsed before OCD clears - which is really the bottom line - open circuit before starting fire.
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As long as the OCPD clears before the wire reaches dangerous temps I don't see a fire taking place no matter the fault current magnitude.



The amount of energy at point of fault is still enough melt wires and steel, so given enough time (seconds) has high probability of starting a fire vs. near-instantaneous trip with shorter runs. When it comes to sizing conductor vs. OCD I think this is an important factor that should be considered but isn't really addressed in the code (or is it????)
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How high is that probability? This is a really interesting question, and I am not saying your wrong, but where in reality would a short circuit taking its time to clear a fault start a fire? What would that scenario look like? What materials would be involved?

In theory the NEC does address it (depending on how you look at it), but not in the way most expect. ;)


Maybe I'm crazy and this shouldn't be a concern?

#14 at 2.6 Ohms per 1000ft on a 500ft run = 2.6 ohms total. 120V/2.6 = 46 amps. A standard GE THQL1115 breaker, 15A, would take 5-30 seconds to clear. That's a long time of arky-sparky.

#14 on a 20A breaker would be 12-90 seconds, quite a bit longer.

#12 at 1.6 Ohms on a 15A would clear in 1-9 seconds, still too long but much better.

#10 would clear in less than 3 seconds.

All I'm saying is with no voltage drop sizing rules to prevent extended fault-clear times, NEC would really be going out on a limb to increase ampacities of smaller sizes vs. OCD size.
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You know your not crazy, not in the least bit. In fact Id say you are very observant. :) You just touched on something that has been a concern for many, many folks. In fact it was this very "weakness" in the NEC that helped push AFCIs into the code:


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

http://newscience.ul.com/wp-content...lity_to_Mitigate_Parallel_Arcing_Faults_1.pdf

http://newscience.ul.com/wp-content...lity_to_Mitigate_Parallel_Arcing_Faults_2.pdf

http://library.ul.com/wp-content/uploads/sites/40/2015/02/BreakerMitigationofArcFaults.pdf



IEC 60364, more pricscely IEC 60364-4-41 Protection for Safery-- Protection Against Electric Shock requires that all circuits 32 amps and under have their Hot and ground (earth) conductors be sized in such a mannner that a bolted fault at the furthest point of a circuit trips the breaker in 0.4 to 0.2 seconds with a voltage of 230 volts L to ground. (see pic)

The theory is that when a fault occurs there will be voltage drop across the EGC, on average 1/2 the line to ground voltage. At 230 volts this would be 115 volts. Therefore as the breaker is taking its time to trip, about 115 volts will appear on exposed conductove metal relative to other gorunded objects. This voltage can harm a person, so it must be removed as soon as possible.

Here in the US our line voltage being 120 volts (thus about 60 volts during a fault), therefore one has more substance argue disconnect times in relation to electric shock are less of a concern. So in that regard your concern is not only valid, it is the norm in 230 volt countries. :D:thumbsup::D Smart guy :cool:

Now, where does this all play in the role of fire mitigation? This is where it gets clear as mud.

The IEC has hinted, and even makes FPN like notes that required disconnect times can mitigate fire.

What holds this claim up is ironically UL. UL did many tests where on black and white it was documneted that: "breakers can be effective at mitigating arcing faults provided the available fault current can be guaranteed to exceed the magnetic trip level of the circuit breaker by a factor of 1.25."

And in theory it makes sense. Faults relying on thermal trip persist much longer, espcially when a fault is observed under an oscilliscope. The current is not a linear sine wave, but much like we observe a fault with the naked eye: sporadic choppy spurts of current draw as the fault itself sputters. This sputtering has a much lower RMS current relative to peak current, thus tripping the breaker thermally takes even more time if it were a solid short circuit.

In fact, magnetic trip was even evaluated by UL as the possiblity of useing a breaker with a known mag trip at the panel and then having AFCI protection at the outlets for cords. Any arc fault in the NM itself would trip the breaker.

Here is the ROP starting at around 70-129 (139 in the viewer)


https://www.nfpa.org/Assets/files/AboutTheCodes/70/70-A2013-ROP.pdf


What makes this a contrversial mystery is that it has never been proven whether or not faults taking there time to trip a breaker result in fire out in the real world, or if such a phenomnon ought to be called an arc fault itself since a substationally carbonized path is needed to sustain a continous arc at 120 volts.
 
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A most enlightening thread, bravo gents!

I'm rather curious now as to the relevance of mag trip settings vs. a fault .

As you've shown, impedance equates to time.

But this is assuming a total fault , what about a partial or high R fault?

~RJ~
 
mbrooke said:
And oh, something to compliment that :D


http://www.resolvematters.ca/09/images/file/Cable_Ampacity.pdf
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interesting article. i would still have some Q's around their conclusion that the insulation allowed the temps to rise so high that thermal damage was done. almost seems like the original wire may have been faulty, some arc'ing occured, and then finally full failure.

this leads me to ask another Q. how many electricians are derating for wire that runs in attics where the attic has no roof line insulation? i ask because most attics like this get well above 30C in summer. as example, my own home panel is outside, all wires run up the garage wall and into garage area attic (nothing is insulated in this area. summer time the attic there gets fairly hot and there is no venting). from what i can see there was no derating done.

and, did i miss it. NEC OCD sizing is not applied after wire derating ??
 
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I wonder.....

I wonder.....

T310.15(B)(2)(a) addresses thermal differences FZ, admittedly it's imposed as much as it could be.....

That said, so what? The thermal characteristics of an OCPD should be sized 80 % to begin with , and it smoked because the distal thermal event wasn't on the OCPD radar.

This evidences all thermal events , other than which heats the entire conductor back to and inclusive of said OCPD , making any incendiary event possible long before the P in OCPD busts a move.

I guess the only 'P' left in the standard OCPD would be the mag trip levels .

So the Q follows..... if we (and by we, i mean those chained to the NEC) could choose our mag trip settings, would any of this work out differently?

~RJ~
 
FionaZuppa said:
interesting article. i would still have some Q's around their conclusion that the insulation allowed the temps to rise so high that thermal damage was done. almost seems like the original wire may have been faulty, some arc'ing occured, and then finally full failure.
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You know, you are on the right track here. That is a sound possibility. It could be the cable was already damaged.



this leads me to ask another Q. how many electricians are derating for wire that runs in attics where the attic has no roof line insulation? i ask because most attics like this get well above 30C in summer. as example, my own home panel is outside, all wires run up the garage wall and into garage area attic (nothing is insulated in this area. summer time the attic there gets fairly hot and there is no venting). from what i can see there was no derating done.

and, did i miss it. NEC OCD sizing is not applied after wire derating ??
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Honestly many dont, but Id argue insulation can be worse then a hot attic. The foam insulation PDF posted prior is rather eye opening. In any case you still de-rate for temperature, not doing so is a code violation.
 
Curious nerd said:
Thank you for the kind words, especially given the high standards on this forum.
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Kind words that I truly mean :D

That is very eye opening. It basically says more then 1 run of 2 wire NM must be de-rated in foam insulation. I will argue the test is somewhat flawed being the NM can draw heat into its neighboring test run, but it still makes a valid point.

I also like how they openly admit a GFCI does what an AFCI does :lol:!
 
One problem I had with that publication is that they kept talking specifically about foam insulation. The whole point of having R values is that they allow comparing apples to oranges by measuring total resistance to heat flow. At any given R value, a priori it shouldn't matter whether the heat is building up behind foam, behind cellulose, or behind recycled shredded blue jeans. The temperature rise should be the same in all cases.
 
Curious nerd said:
One problem I had with that publication is that they kept talking specifically about foam insulation. The whole point of having R values is that they allow comparing apples to oranges by measuring total resistance to heat flow. At any given R value, a priori it shouldn't matter whether the heat is building up behind foam, behind cellulose, or behind recycled shredded blue jeans. The temperature rise should be the same in all cases.
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One small problem is that the R value describes how an insulation system handles the flow of heat from one side to the other.
It is not necessarily an accurate guide to what happens when you immerse a heat source in the middle of the insulation.
For example, if part of the R value for fiberglass batt insulation is a radiant barrier and air infiltration barrier on one side, a heat source right in the middle will see an effective R value of less than half of the system R value (think of unequal value resistors in parallel).
 
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