NEC Changes For #14 Ampacity

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R value alone doesnt tell you everything about the insulation. it may take 24" for R20 for XYZ but only 8" for R20 for ABC (physical construction matters). The material itself also has other properties. as example, a material may have high R value with very high heat capacity, which changes the overall behavior of the insulation. a fiberglass batting that is say 30" thick and very loosley packed may be R30, yet rockwool that is R30 at 12" and more densely packed will have different impact on the heat coming out of the wires.
 
FionaZuppa said:
R value alone doesnt tell you everything about the insulation. it may take 24" for R20 for XYZ but only 8" for R20 for ABC (physical construction matters). The material itself also has other properties. as example, a material may have high R value with very high heat capacity, which changes the overall behavior of the insulation. a fiberglass batting that is say 30" thick and very loosley packed may be R30, yet rockwool that is R30 at 12" and more densely packed will have different impact on the heat coming out of the wires.
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That is true. Therefore it would make sense the NEC bases the tables to assume worse case scenario.
 
mbrooke said:
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... in fact we can legally have #14 on a 40 amp breaker feeding a motor.

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.

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...
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The issue would no doubt be a rare occurrence, probably why it isn't directly addressed in the code (and IMO doesn't need to be). How common is an extremely long run, where conductors are _not_ oversized for VD? Surely there are many installations like this (but a very very very small percentage of all branch circuits in the country) - then how many of those installations have been subject to a fault - then how many of those resulted in damage warranting investigation - then in how many of those was the investigation carried out with time-to-clear in mind (or remaining evidence after fire even able to invoke the idea of too-long time-to-clear)??? Indeed, it would be hard to find a scenario where extended time-to-clear could be proven as the cause.

mbrooke said:
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?
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Ever done any MIG or TIG welding? I can assure you, starting a fire with prolonged arcing is easy. As worst-case scenario, picture a cord-and-plug connected pendant-mounted HID fixture above dry animal bedding, cord damaged by age/rodent, sparks fly, big hot fire.

mbrooke said:
...In theory the NEC does address it (depending on how you look at it), but not in the way most expect.
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This is sort of my point, once you start playing around with NEC ampacity tables you must consider more factors than just cable temp ratings or thermal buildup thresholds - among others, I think that fault current/time-to-clear factor is an important one and especially with the smaller conductors.
Noting that the NEC is "written in blood" I don't think time-to-clear on long runs really needs to be addressed, ampacity tables have a proven history, but if ampacities are increased under certain conditions then all factors must be considered. It's a huge can of worms IMHO. I don't see the value in increasing ampacities (or decreasing, for that matter) introduces a new world of risks.
 
AJElectric said:
The issue would no doubt be a rare occurrence, probably why it isn't directly addressed in the code (and IMO doesn't need to be). How common is an extremely long run, where conductors are _not_ oversized for VD? Surely there are many installations like this (but a very very very small percentage of all branch circuits in the country) - then how many of those installations have been subject to a fault - then how many of those resulted in damage warranting investigation - then in how many of those was the investigation carried out with time-to-clear in mind (or remaining evidence after fire even able to invoke the idea of too-long time-to-clear)??? Indeed, it would be hard to find a scenario where extended time-to-clear could be proven as the cause.
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Of course. However keep in mind that Id say roughly about 1/3 to 2/3 of all branch circuits in North America will not trip a breaker magnetically at the furthest point.


A dozen factors play a role. Available fault current from the POCO, wire size, length and circuit breaker itself. Many older single pole resi breakers would not trip magnetically until 20x or even 32x current was reached. Some like FPE, Bulldog, ect do not even have magnetic trip.


Ever done any MIG or TIG welding? I can assure you, starting a fire with prolonged arcing is easy.
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The question is, how long is the arcing?


As worst-case scenario, picture a cord-and-plug connected pendant-mounted HID fixture above dry animal bedding, cord damaged by age/rodent, sparks fly, big hot fire.
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In theory a single short circuit tripped quick can do it by sending down some sparks.


This is sort of my point, once you start playing around with NEC ampacity tables you must consider more factors than just cable temp ratings or thermal buildup thresholds - among others, I think that fault current/time-to-clear factor is an important one and especially with the smaller conductors.
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Granted time-to-clear is debated in terms of importance, however I disagree that the NEC has any concern. If the NEC was concerned about time-to-trip they would have disconnect time requirements like the IEC. As is under NEMA and UL standard circuit breakers are not even required to have magnetic trip.



Noting that the NEC is "written in blood" I don't think time-to-clear on long runs really needs to be addressed, ampacity tables have a proven history, but if ampacities are increased under certain conditions then all factors must be considered. It's a huge can of worms IMHO. I don't see the value in increasing ampacities (or decreasing, for that matter) introduces a new world of risks.
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Even in short runs such as 60 feet I can have a scenario where the breaker does not trip magnetically. In fact in most homes some circuits having a typical 100 foot length will not immediately. If the NEC was concerned they would either have made it a requirement or require grossly over sized conductor for each circuit coupled with a magnetic trip threshold for each breaker.
 
mbrooke said:
However keep in mind that Id say roughly about 1/3 to 2/3 of all branch circuits in North America will not trip a breaker magnetically at the furthest point.
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:lol:

What a load of bull.


And something you pulled out of thin air other than defective breakers or breakers without instantaneous trip units that is far from true.



Even in short runs such as 60 feet I can have a scenario where the breaker does not trip magnetically. In fact in most homes some circuits having a typical 100 foot length will not immediately.
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I am going to test this theory of yours and post the results. :D
 
iwire said:
:lol:

What a load of bull.
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Coming from someone who I take it never calculated fault current? :roll:


And something you pulled out of thin air other than defective breakers or breakers without instantaneous trip units that is far from true.
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Then show me the numbers to back up your claim.



I am going to test this theory of yours and post the results. :D
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One of these will spare you the grief of having to short circuit every outlet :thumbsup:


http://www.fluke.com/fluke/m3en/ins...ltifunction-installation-tester.htm?pid=72323
 
mbrooke said:
Coming from someone who I take it never calculated fault current? :roll:
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Correct

Then show me the numbers to back up your claim.

No, forget calculations, time for real world results.
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I will share those results with you regardless of the outcome.


One of these will spare you the grief of having to short circuit every outlet :thumbsup:
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There is no grief in a real world test.
 
iwire said:
Correct
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Yet you can confidently claim breakers (almost) always magnetically trip without knowing the fault current? :blink:please explain.



I will share those results with you regardless of the outcome.
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I think thats the whole point, raw data disproves or confirms a theoretical hypothesis. My money is on 50/50 on average.


There is no grief in a real world test.
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In one of those its as easy as pie. Gives you the numbers in 2 seconds :thumbsup:
 
iwire said:
Hell no, :D I can't confidently say that anymore so than you can claim more than 50% will not.
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I can claim a rough guesstimate. Both us know in a typical 2,000 sqft dwelling the average home run length is about 125 feet from the panel to the furthest point. Assuming worse case scenario by ignoring AC impedance (reactance) and assuming an infinite buss the restive (ohm) impedance of 1000 feet 14 awg is 2.575 ohm at 77*F. 1000/4=250, which divided by two is 125 feet (125 feet to the fault and 125 feet back to the panel). 2.575/4=0.64375. Using ohms law that gives us 186 amps of short circuit current. In reality that current will be lower as their is no such thing as an infinite buss in the real world since the service drop and utility transformer impedance also play a role.

ohm per 1000 feet: http://www.interfacebus.com/AWG-table-of-different-wire-gauge-resistance.html


With that said most typical residential breakers trip around 13x. 13 x 15= 195 amps. This falls short of a worst case fault current of 186 amps. Granted some breaker trip lower then 13x, however it is still a gamble due to manufacturing differences and available fault current from the utility. Thus a rough 50/50 guesstimate is not far off when all is factored.


Anyway, to back up my breaker claims UL tested several different makes in this paper around page 16:

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