NEC Changes For #14 Ampacity

[don_resqcapt19]

..."short" 25ft out on the 50ft run, and that "short" is a 25A short, well, the ocd laughs at it, the #14 melts some time later. ...

Why would you think that small amount of current would melt a #14 copper conductor? It will carry that current forever without getting hot enough to boil water.

 

[FionaZuppa]

Please explain to me how a 'short circuit' is only gong to allow just 19 amps of current to pass in a correctly wired circuit.

you pull some 14 in buried conduit (insulated egc). when done the hot and gnd insulation was damaged and the metal is exposed (just a small amount). the space between the nicks is 12ft. 1yr later the conduit fills with enough water to cover the nicks. the dirty water is 6ohms between the two nicks. thats 2400 watts across 12ft, thats 200w/ft, the water in the pipe along with the metal conduit thermal conductivity to earth keep temps in check in that area, but the #14 outside that zone will have issue with 20A + any additional amps that the real load may put on the bc.

with a 15A ocpd we would expect that "short" to be broken at some point. if the ocpd is 40A and a 15A motor starts up with 25A inrush, well, things would seem normal.

thats one example.

 

[iwire]

you pull some 14 in buried conduit (insulated egc). when done the hot and gnd insulation was damaged and the metal is exposed (just a small amount). the space between the nicks is 12ft. 1yr later the conduit fills with enough water to cover the nicks. the dirty water is 6ohms between the two nicks. thats 2400 watts across 12ft, thats 200w/ft, the water in the pipe along with the metal conduit thermal conductivity to earth keep temps in check in that area, but the #14 outside that zone will have issue with 20A + any additional amps that the real load may put on the bc.

And you think that 12 ft of 14 AWG will dissipate 2400 watts for hours on end? You think 14 AWG is gong to survive long at all pushing out 200 watts per foot?

In no time at all the damage will progress into a line to line or line to ground short circuit and open the breaker.

Again has been pointed out what we have now works, you seem determined to fix something that is not broken.

 

[don_resqcapt19]

And you think that 12 ft of 14 AWG will dissipate 2400 watts for hours on end? You think 14 AWG is gong to survive long at all pushing out 200 watts per foot? ...

Actually I think #14 will carry 20 amps forever without damage.

Of course if the 20 amps is flowing because of damage to the conductor insulation, it will soon fail at the point of damage.

 

[FionaZuppa]

missing my point folks. the NEC defines ampacity table the way it does for many reasons. then comes along some exceptions that seem to shift it from the realm of very cautious (using strict verbiage, like 15A(max) ocpd for #14) to a realm of "hmmm, that could be dangerous". under expected use with proper install, a non-continuous load single motor bc that needs 13A while running and starts with 28A inrush where the bc is 30A (yes, the ocpd defines the bc, thus has a 30A ocpd), has potential to be more hazardous than most other situations, one of which i noted by example.

given that NEC says the ocpd defines the bc, then the bc (whatever it needs to be) should be using a wire sized appropriately from the ampacity table regardless of the load type. a 30A ocpd should have wire size chosen from ampacity table, perhaps the exception (for motor) is that you can choose it using the 90C column.

 

[FionaZuppa]

Why would you think that small amount of current would melt a #14 copper conductor? It will carry that current forever without getting hot enough to boil water.

i didnt say it would melt in the location that is in the water. perhaps some other dry location that has poor heatsink ability, etc. would be a real pita to figure out easily if the #14NM-B keeps burning up in a interior wall (the NM-B transitions to pull wire via a jbox, etc). lots of reasons i suppose, just giving one here.

 

[mbrooke]

missing my point folks. the NEC defines ampacity table the way it does for many reasons. then comes along some exceptions that seem to shift it from the realm of very cautious (using strict verbiage, like 15A(max) ocpd for #14) to a realm of "hmmm, that could be dangerous". under expected use with proper install, a non-continuous load single motor bc that needs 13A while running and starts with 28A inrush where the bc is 30A (yes, the ocpd defines the bc, thus has a 30A ocpd), has potential to be more hazardous than most other situations, one of which i noted by example.

28 amps inrush would not trip a 15 amp breaker unless it lasts more then several seconds which is not the cas3 with motors.. The concern with inrush is for the first few cycles when the motor first starts up pulling 100amp or more.

given that NEC says the ocpd defines the bc, then the bc (whatever it needs to be) should be using a wire sized appropriately from the ampacity table regardless of the load type. a 30A ocpd should have wire size chosen from ampacity table, perhaps the exception (for motor) is that you can choose it using the 90C column.

True, but with motors the theory is the wire will not see a sustained 30amps. In your description of a short I agree with Iwire. A fault dissipating 4000 watts will generate such intense heat that will either burn clear or short circuit to the point fault current becomes hundreds of amps.

 

[don_resqcapt19]

i didnt say it would melt in the location that is in the water. perhaps some other dry location that has poor heatsink ability, etc. would be a real pita to figure out easily if the #14NM-B keeps burning up in a interior wall (the NM-B transitions to pull wire via a jbox, etc). lots of reasons i suppose, just giving one here.

It won't melt anywhere at 25 amps unless something else is going on.

The 90°C ampacity of #14 copper is 25 amps.

The temperature of the conductor will not exceed 90°C with a current of 25 amps, under the conditions in the ampacity table...that is not more than 3 conductors in a raceway or cable, ambient not exceeding 30°C or something else that requires derating, like being buried in thermal insulation.

 

[GoldDigger]

Also the temperature at which the insulation starts to (slowly at first) degrade is far below the temperature at which the copper melts.
That said, we have two concerns in this thread:
1. The current which will damage the insulation.
2. The current and power dissipation which will prevent the non-bolted resistive fault which might produce that overload can remain stable and neither burn our nor become a low enough resistance to trip the OCPD.

 

[mbrooke]

i didnt say it would melt in the location that is in the water. perhaps some other dry location that has poor heatsink ability, etc. would be a real pita to figure out easily if the #14NM-B keeps burning up in a interior wall (the NM-B transitions to pull wire via a jbox, etc). lots of reasons i suppose, just giving one here.

Is this what you are referring to in terms of a a conductor leaking into water?


https://www.youtube.com/watch?v=EViyccc2t9w

 

[FionaZuppa]

28 amps inrush would not trip a 15 amp breaker unless it lasts more then several seconds which is not the cas3 with motors.. The concern with inrush is for the first few cycles when the motor first starts up pulling 100amp or more.

True, but with motors the theory is the wire will not see a sustained 30amps. In your description of a short I agree with Iwire. A fault dissipating 4000 watts will generate such intense heat that will either burn clear or short circuit to the point fault current becomes hundreds of amps.

nameplates usually have #'s that are a tad bigger than real. how many folks clamp meter motors and then downsize the breaker accordingly? the motors exception allows ocpd's bigger than what the std ampacity table says for the given wire size, and scoots around the verbiage for max ocpd size for #14 and #12, etc. make it a 20A ocpd on #14 for a hvac motor that has inrush of 28A and runs at 13A, the motor is not the issue, its the potential to overload the #14 because the ocpd could allow such conditions.

 

[mbrooke]

nameplates usually have #'s that are a tad bigger than real. how many folks clamp meter motors and then downsize the breaker accordingly? the motors exception allows ocpd's bigger than what the std ampacity table says for the given wire size, and scoots around the verbiage for max ocpd size for #14 and #12, etc. make it a 20A ocpd on #14 for a hvac motor that has inrush of 28A and runs at 13A, the motor is not the issue, its the potential to overload the #14 because the ocpd could allow such conditions.

In theory yes, it does allow overload, but the motor's overload stops that. The only thing thats left is short circuits and grounds faults which most of the time pull more then 40amps.

I think the question (imho) is can a short circuit or ground fault ever occur which pulls less then 100amps but more then 20?

 

[don_resqcapt19]

nameplates usually have #'s that are a tad bigger than real.

And the wire sizing in the code is based on 125% of an even higher current (in most cases) the current shown in the tables at the end of Article 430.

how many folks clamp meter motors and then downsize the breaker accordingly?

I would expect that no one does that as there no good reason to do that.

the motors exception allows ocpd's bigger than what the std ampacity table says for the given wire size, and scoots around the verbiage for max ocpd size for #14 and #12, etc. make it a 20A ocpd on #14 for a hvac motor that has inrush of 28A and runs at 13A, the motor is not the issue, its the potential to overload the #14 because the ocpd could allow such conditions.

I think you can find cases where 30 or even 40 amp breakers would be permitted for the short circuit and ground fault protection of a motor branch circuit using #14 wire.

This has been the code for many code cycles...show us real world examples of problems caused by the code rules. Actually if you have such examples you should submit them as PIs to change the 2020 code.