NEC Changes For #14 Ampacity

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mbrooke

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post #138, wow, that pdf seems to suggest that the NEC exceptions for #14 & #12 could possibly allow a hazardous condition


any wire run that is compliant in the 75C column has potential to char wood as described in the PDF.

however, the PDF does not talk anything about amps vs wire temps. check out this Nasa pdf (wire temps vs amps for different wiring scenarios).
http://snebulos.mit.edu/projects/reference/International-Space-Station/TM102179.pdf

If the wire goes to 77*C, hence why you don't find anything over 75*C in theory when applied to NM in theory. In reality the wire rarely if ever gets to the actual temperature when applied correctly. Ie. #10 doesn't automatically reach 140*F at 30amps and 167*F at 35 amps with 194*F at 40 amps. These are more often worse case 1 in a hundred million scenarios airing on the conservative side.

One thing many take for granted is that 310.15 B (16) is based on worse case scenarios. Code assumes the wire will be completely embedded in dense thermal insulation at an ambient temperature of 86*F. Thus when many model 310.15 (B) 16 using open air or conduit against a wall the numbers come much higher then the table making it look conservative.
 

FionaZuppa

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If the wire goes to 77*C, hence why you don't find anything over 75*C in theory when applied to NM in theory. In reality the wire rarely if ever gets to the actual temperature when applied correctly. Ie. #10 doesn't automatically reach 140*F at 30amps and 167*F at 35 amps with 194*F at 40 amps. These are more often worse case 1 in a hundred million scenarios airing on the conservative side.

One thing many take for granted is that 310.15 B (16) is based on worse case scenarios. Code assumes the wire will be completely embedded in dense thermal insulation at an ambient temperature of 86*F. Thus when many model 310.15 (B) 16 using open air or conduit against a wall the numbers come much higher then the table making it look conservative.

well, this has sparked enough interest by me, i have enough test gear to run some amps vs temp testing. i'll post back my setup later.
 

mbrooke

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well, this has sparked enough interest by me, i have enough test gear to run some amps vs temp testing. i'll post back my setup later.

Very good! :D:cool:

Write up a hypothesis and see how it turns out. Try several scenarios like Fiberglass bats and bundling. Those will yield surprising results. BTW, dont forget to make sure more then a few feet is under such conditions since the exposed wire on the outside can conduct heat away.
 

FionaZuppa

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Very good! :D:cool:

Write up a hypothesis and see how it turns out. Try several scenarios like Fiberglass bats and bundling. Those will yield surprising results. BTW, dont forget to make sure more then a few feet is under such conditions since the exposed wire on the outside can conduct heat away.

my initial testing will be std #14 Romex in a rigid foam sandwich (1.5" each side) with some thermocouples and a current controlled power supply. the foam will be carved on each side to accept 1/2 the thickness of the wire so that there are no gaps when sandwiched. this setup just to see how hot the wire gets under such restrictions, etc.
 

mbrooke

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my initial testing will be std #14 Romex in a rigid foam sandwich (1.5" each side) with some thermocouples and a current controlled power supply. the foam will be carved on each side to accept 1/2 the thickness of the wire so that there are no gaps when sandwiched. this setup just to see how hot the wire gets under such restrictions, etc.

Sounds good :thumbsup: FWIW Id use more then 1.5 feet just to have good heat entrapment.
 

FionaZuppa

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Part Time Electrician (semi retired, old) - EE retired.
If the wire goes to 77*C, hence why you don't find anything over 75*C in theory when applied to NM in theory. In reality the wire rarely if ever gets to the actual temperature when applied correctly. Ie. #10 doesn't automatically reach 140*F at 30amps and 167*F at 35 amps with 194*F at 40 amps. These are more often worse case 1 in a hundred million scenarios airing on the conservative side.

One thing many take for granted is that 310.15 B (16) is based on worse case scenarios. Code assumes the wire will be completely embedded in dense thermal insulation at an ambient temperature of 86*F. Thus when many model 310.15 (B) 16 using open air or conduit against a wall the numbers come much higher then the table making it look conservative.

right, those temps are just the rated insulation temps which are grouped by types. we rarely see the termination terminals (device) rated above 75C. however, the ampacity #'s chosen seem to be rooted around wire temps in a select few application scenarios such as when romex runs in a wall filled with R19, or when 20 nm's squeeze together through a chase nipple (hole in top plate, etc etc) into a service panel, these are some of those worst case scenarios, etc. although this discussion is around NM-B we can carry the argument over to % fill too as derating is based on temps of wires. it all boils down to heat that transfers out. we in fact do not want the wire to get near the rated insulation temp, which i think NEC is trying to control, just not fully understanding (yet) the safety factor in the ampacity #'s.

if wire co applies add a thin layer of an aerogel before applying the PVC/Nylon jackets, the insulating jacket (all 3 materials) will be rated much higher temp, and, under any load the OD temps will be lower vs non-gel coated wire. would this prompt a new ampacity column with different allowed wiring applications?

why has the CEC made a move to up ampacity #'s ?
 

mbrooke

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right, those temps are just the rated insulation temps which are grouped by types. we rarely see the termination terminals (device) rated above 75C. however, the ampacity #'s chosen seem to be rooted around wire temps in a select few application scenarios such as when romex runs in a wall filled with R19, or when 20 nm's squeeze together through a chase nipple (hole in top plate, etc etc) into a service panel, these are some of those worst case scenarios, etc. although this discussion is around NM-B we can carry the argument over to % fill too as derating is based on temps of wires.


Bongo! I agree. While in most cases the ampacity tables are very conservative, the NEC must take into account all possible real world scenarios. Even if a one in a million chance exists that a code complaint installation could overheat the NEC must factor in a consideration and in turn adjustment for that.


FWIW in cases where 20 current carrying conductors are bundled a de-rating of 50% via Table 310.15(B)(3)(a) is required.



it all boils down to heat that transfers out. we in fact do not want the wire to get near the rated insulation temp, which i think NEC is trying to control, just not fully understanding (yet) the safety factor in the ampacity #'s.

True. THHN could probably survive 167*F (75*C) over some time, however in any case having the insulation remain cool as possible is best since the wire is intended to last for decades.


if wire co applies add a thin layer of an aerogel before applying the PVC/Nylon jackets, the insulating jacket (all 3 materials) will be rated much higher temp, and, under any load the OD temps will be lower vs non-gel coated wire. would this prompt a new ampacity column with different allowed wiring applications?

We already have something like that with high temp wires. They have their own ampacity tables in the NEC. But it still would not factor in temps at the terminals or splices. We could install 250*C wire and have it run at 95*C, but that would require high temp terminals and wire nuts. Circuit breakers would have a very difficult time with the heat being drawn into their internals.

why has the CEC made a move to up ampacity #'s ?
d


Million dollar question. One theory that I have: THHN is far more tolerant of high temps then old rubber insulation. When one looks at old ampacity tables from the days of rubber to now not much has changed. Second since exceeding 20amps is unlikely with a fixed load 240.4 D rules can be omitted.
 

FionaZuppa

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Bongo! I agree. While in most cases the ampacity tables are very conservative, the NEC must take into account all possible real world scenarios. Even if a one in a million chance exists that a code complaint installation could overheat the NEC must factor in a consideration and in turn adjustment for that.
well, i am not sure they can..... where's the list of all possible scenarios? NEC has some restrictions and allowances in the code, but i dont see a list of "all possible scenarios". who's to say a compliant romex installation (a bc bound to 60C column) in scenario #3,456 doesnt pose a hazard? see, i dont think every scenario can be accounted for. this is a limit model which brings things back down to zero, hence, zero current is the safest for all known and unknown wiring scenarios. but, we cant live in a world of zero, we have to accept some risk, etc.

a local street crossing where i used to live was always somewhat dangerous, no traffic light there for 30+ years. then one day a person was hit by a car and then suddenly a light went up, however, this did not prompt the locale to put up lights at every crossing similar to this one.
 

mbrooke

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well, i am not sure they can..... where's the list of all possible scenarios?

Its built into the numbers and codes themselves.



NEC has some restrictions and allowances in the code, but i dont see a list of "all possible scenarios".

Look at it like this. Wire can be installed almost anywhere under any conditions even when taking other code rules into account. Table 310.15(B)(16) doesn't get to specific other then ampacity for a given wire size and insulation type while article 310 doesn't go much past saying you adjust for ambient and de-rate for bundling. Therefore, they must assume wore case scenario. 3 current carrying conductors in a raceway in an 86*F environment can be anything from conduit in moist earth, to conduit attached to a concrete wall, to conduit in direct sunlight, to NM in very dense thermal insulation for 200 feet. All of those 4 scenarios will have radically different current capacities from the ability (or inability) to dissipate heat. Thus 310.15 (B) (16) must take the worse case scenario, that being cable or conduit in dense thermal insulation. The table must also consider an adverse conditions the code protects against, such as the wire under full load in thermal insulation when a short circuit occurs. The brief (yet large) short circuit current along with the time it takes to open the breaker will add additional heat. The wire must be able to withstand this worse case scenario without melting or setting something on fire.


Just to give you an idea regarding heat dissipation IEC based codes let you select ampacity based on condition of use. 2.5 mm2 (a bit larger then our 2.08mm2 #14) twin and earth cable (The euro version of Romex) has different ampacities based on where or how it will be used. If the wire is not covered in thermal insulation it is rated 27amps. In a wall touching thermal insulation 20 amps. If embedded in dense thermal insulation then it goes down to 13.5 amps.

Such a table could be developed for the NEC (and I believe it should be evaluated), however any CMP member can always argue conditions today will change tomorrow. An example would be a basement with open rafters and beams. In theory we could easily load #14 to 25 amps. I wouldn't loose any sleep. It will dissipate the heat well and not reach questionable temperatures. However if someone buys the home and wants to finish the basement adding thermal insulation, things change. The electrician has to either come back re-pulling all new wire or the cable simply gets covered over when the HO does not know any better. Suddenly you end up with a much larger waste of copper or the owner now lives with a potential fire hazard. Therefore a good argument exists that for ease and safety 310.15 should simply assume wore case scenario right off the bat.



who's to say a compliant romex installation (a bc bound to 60C column) in scenario #3,456 doesnt pose a hazard?

It could, but when going by the worse case scenario the probability sharply goes down.



see, i dont think every scenario can be accounted for. this is a limit model which brings things back down to zero, hence, zero current is the safest for all known and unknown wiring scenarios. but, we cant live in a world of zero, we have to accept some risk, etc.

You are correct, but we can get very close to zero without actually going to zero. In the real world fires from fixed building wiring itself is rare. In fact nearly every electrician working in the field will tell you about the only time they see melted or damaged dielectric insulation is with code violations, loose connections or a short circuit where the OCPD failed to trip. Damaged wire insulation from overheating in code complaint installs is virtually unheard of.




a local street crossing where i used to live was always somewhat dangerous, no traffic light there for 30+ years. then one day a person was hit by a car and then suddenly a light went up, however, this did not prompt the locale to put up lights at every crossing similar to this one.


We can not control people and how they drive, however we can set common sense rules greatly reducing those hazards especially when its building practices. Picture something along the lines of this. Bridges often have a very high degree of safety built into them along with more safety factors added on top of that. Same goes for most buildings. The theory is its best to spend a bit more upfront and have confidence under all scenarios (and even some of those that might be unforseen) then to have a bridge fall down or in the case of the NEC a home burn down.
 

FionaZuppa

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I guess bundling really does make a difference.

surely does have an affect, but this is a wiring scheme issue and not a ampacity issue. where's the bundling derating section in NEC ?? would conduit CCC's rule fix this bundling issue?
 
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