NEC Changes For #14 Ampacity

[mbrooke]

ok, but in the study of forceful destruction i can see why letting it cook for days or weeks is a good educational exercise, but how does this play into the ampacity #'s as we know them today (nec2012)?

It might, it might not in the sense that if the wire does reach an actual temp of 90*C, what happens to it? I did find this, which is food for thought from the AS/NZ code:

The normal operating temperature of thermoplastic cables, including flexible cords installed as installation wiring, are based on a conductor temperature of 75°C. This is due to the risk of thermal deformation of insulation if the cables are clipped, fixed or otherwise installed in a manner which exposes the cable to
severe mechanical pressure at higher temperatures. V-90 and V-90HT insulated cables may be operated up to the maximum permissible temperatures 90°C and 105°C provided that the cable is installed in a manner that is not subject to, or is protected against, severe mechanical pressure at temperatures higher than 75°C. Such applications also allow for cables to be used in—

(a) locations where the ambient temperature exceeds the normal 30°C, e.g. equipment wiring in
luminaires and heating appliances, or in roof spaces affected by high summer temperatures; and

(b) locations affected by bulk thermal insulation that restricts the dissipation of heat from the cable.

It is possible (just a wild guess) that at higher temperatures stapling may play a role with our NM cable being kept out of the 90*C column despite having 90*C insulation. The last parts (a) & (b) might also give hint why 90*C is used in NM despite the CCC restricted to 60*C.

i mean, i can watch romex turn to dust just by leaving it exposed to sunlight during the summer, the UV will destroy it in no time, but this is not very beneficial in developing ampacity #'s. the NEC is about rules and #'s that make sense for proper installation and use with some safety tolerance for faults. i suspect once a wire gets itself into that zone outside of proper installation and use, perhaps by a fault for whatever reason, the ampacity table likely no longer has relevance because the fault, it it were to do damage or create a fire, would do that regardless of ampacity #'s.

i kinda went on tanget with this free air and baking test. those are odd conditions that are likely to not be prevented by way of ampacity #. my sandwich test is to see if a "nec worse case scenario"can produce any crazy temps of the wire, and from there we can evaluate if those temps will be harmful to the surroundings.

I dont think it was wasted effort imo, its always a good idea to know what temperature wire can handle before deteriorating. If anything you are verifying the 90*C claim. Once known you test specimens can survive 90*C, another variable if error is eliminated from the testing.

somewhere out there was a post about a store ceiling catching fire, it was stated that the 240v "romex" line had migrated into the insulation bay in the ceiling and basically covered itself with the blown-in insulation, and thats where the visual damage was, and some had concluded that due to the coverage of the insulation the temps rose too high causing the fire. but oddly, no forensics to see if the wire itself may have been faulty at that location, i could not see why the temps on the wire, even under the loose insulation, would climb so high that it would cause fire. ocpd was not faulty. and sure, the temps may have gone up to start the fire, but perhaps not because it was under loose insulation.

Sounds like this:

http://www.resolvematters.ca/09/images/file/Cable_Ampacity.pdf

Now you are thinking Several electricians who viewed this article in another forum said the same thing: Its possible that the wire overheated from damage rather then insulation suffocation.

so, it is quite impossible to account for all the odd situations that may cause an issue, its just not possible, but what we can do (nec) is write rules and #'s that make sense given proper installation and use. 20A ocpd on #14 romex (either change ampacity #'s or allow the 75C column), seems ok to me (maybe apply voltage drop rule to limit run size, whatever), but we'll see what the sandwich test show for temps, i am not expecting a "oh geez" moment.

Personally, the NEC should develop various ampacity tables based on how the wire is installed. Nearly all electrical codes outside of North America let you do this, and it does pay off for them without incident. I believe the NEC should restrict #14 to 15 amps only where in dense thermal insulation or foam insulation. Where in open air or in a wall without insulation there is absolutely zero risk over the course of 60 years placing #14 on a 20amp OCPD. PVC cables smaller then 2.08mm2 #14 (like 1.5mm2 T&E) are run at 16 and 20amps for decades under such conditions without a single sign of overheating.

 

[mbrooke]

an interesting article. i read the whole thing. the degradation of breakdown voltage due to aging is interesting. but nothing in here that i can see that would be a good metric for the existing ampacity #'s. it gave me a better understanding as to how NM wire degrades over time with temp as a factor. in summary, the colder the NM can stay the slower the degradation rate.

Exactly. :thumbsup: While the article itself is trying to make another point, its does eloquently demonstarte that temperature plays a direct role in the life expectancy of NM cable.

I am not trying to sway your opinion to one side of another in all these posts, but rather giving away information which can be used in critical thinking to either support or go against your hypothesis after tests result yield raw data.

 

[peter d]

Personally, the NEC should develop various ampacity tables based on how the wire is installed. Nearly all electrical codes outside of North America let you do this, and it does pay off for them without incident. I believe the NEC should restrict #14 to 15 amps only where in dense thermal insulation or foam insulation. Where in open air or in a wall without insulation there is absolutely zero risk over the course of 60 years placing #14 on a 20amp OCPD. PVC cables smaller then 2.08mm2 #14 (like 1.5mm2 T&E) are run at 16 and 20amps for decades under such conditions without a single sign of overheating.

:roll::roll:

The NEC cannot account for all the "what if" scenarios and mis-use, therefore I think a conservative rule is good in this case. As for T&E "without a single sign of overheating", you have no evidence of that whatsoever. It's pure speculation on your part just to put European wiring methods in a favorable light.

 

[FionaZuppa]

:roll::roll:

The NEC cannot account for all the "what if" scenarios and mis-use, therefore I think a conservative rule is good in this case. As for T&E "without a single sign of overheating", you have no evidence of that whatsoever. It's pure speculation on your part just to put European wiring methods in a favorable light.

post #458 has test data that shows relationship between temp and degradation. wire will degrade at 30C, just very slowly. i am moving towards a strong conclusion that using a 20A ocpd for #14 is still extremely conservative, but i still need to get more data to have good conclusion about use of romex (NM) wire.

 

[mbrooke]

:roll::roll:
The NEC cannot account for all the "what if" scenarios and mis-use,

Not the point I was trying to make. Where cable is not covered with insulation the NEC should allow higher loading. .

therefore I think a conservative rule is good in this case.

If going by a one size fits all, of course.

As for T&E "without a single sign of overheating", you have no evidence of that whatsoever. It's pure speculation on your part just to put European wiring methods in a favorable light.

Pure assumption. You are welcome to ask Tony, Besoker, Adrin Witt or register for an international forum. T&E where applied correctly has not demonstrated overheating.

 

[romex jockey]

Why some foreign wiring configuration(s) allow wiggle room for CCC ampacity , where the nec simply provides a blanket doctrine should be your query Pete


~RJ~

 

[Curious nerd]

<snip>
Sounds like this:

http://www.resolvematters.ca/09/images/file/Cable_Ampacity.pdf

Now you are thinking Several electricians who viewed this article in another forum said the same thing: Its possible that the wire overheated from damage rather then insulation suffocation.
<snip>

There are other references to relatively heavy conductors (e.g #2) overheating in thermal insulation. If it were on topic I might invest time to dig them up. The results won't apply to 14 gauge of course.

 

[Curious nerd]

I am skeptical of the tests at http://goodsonengineering.com/wp-co...yurethane-Foam-Systems-on-Wiring-Ampacity.pdf, because the numbers they list for free air sound so unlikely. 24 amps through #12, they claim, was heating the cable to anywhere from 114 to 146 Celsius.

 

[mbrooke]

I am skeptical of the tests at http://goodsonengineering.com/wp-co...yurethane-Foam-Systems-on-Wiring-Ampacity.pdf, because the numbers they list for free air sound so unlikely. 24 amps through #12, they claim, was heating the cable to anywhere from 114 to 146 Celsius.

I agree, difficult for me to judge whats do it, but those free air numbers are unrealistic.

 

[FionaZuppa]

ok, got the romex into the foam board. you cant see it, but its held down with 2" wide clear packing tape. i noted where the TC's will go. very carefully removed the insulation on one CCC to expose the copper, a TC will touch it there and a TC will touch the sheath right in the center. the romex is exactly 48" long (96" RT). the sadnwich will be weighted with some brick pavers to minimize/eliminate air gap, and once in the sandwich i will plug the ends where romex exits to eliminate that airflow path.

I am skeptical of the tests at http://goodsonengineering.com/wp-co...yurethane-Foam-Systems-on-Wiring-Ampacity.pdf, because the numbers they list for free air sound so unlikely. 24 amps through #12, they claim, was heating the cable to anywhere from 114 to 146 Celsius.

those #'s seem off to me.

 

[FionaZuppa]

i decided to use my CM660 meter to be the temp reader, it is more accurate than my PID controllers, but i need to order two RTD connectors from Omega on monday. by the time i get the connectors the whole thing will be ready for testing.

 

[mbrooke]

i decided to use my CM660 meter to be the temp reader, it is more accurate than my PID controllers, but i need to order two RTD connectors from Omega on monday. by the time i get the connectors the whole thing will be ready for testing.

Awesome!

Any other tests in the meantime? :angel:

 

[FionaZuppa]

Awesome!

Any other tests in the meantime? :angel:

no other tests as of now. is there something you want to see me measure?

 

[mbrooke]

no other tests as of now. is there something you want to see me measure?

Yup!


A test in Fiberglass, being what most homes have as thermal insulation. The fiberglass test will show if the most common use of NM is conservative and if so to what degree.

 

[Curious nerd]

As long as you've got high current and thermocouples you could test the claim I've seen that Wago 222's should only be used for low current because they don't have enough conductor contact area.

 

[FionaZuppa]

Yup!


A test in Fiberglass, being what most homes have as thermal insulation. The fiberglass test will show if the most common use of NM is conservative and if so to what degree.

we went over this in the thread, rigid foam sandwich (24x24x4", R10min in either direction) was agreed to be a "NEC worse case scenario". why would we want to know what the wire does in a proper installation

As long as you've got high current and thermocouples you could test the claim I've seen that Wago 222's should only be used for low current because they don't have enough conductor contact area.

i'll get some for a later test, i can temp test it in free air and bury it in the rigid foam too. in the meantime, can you start a new thread for this.

 

[mbrooke]

we went over this in the thread, rigid foam sandwich (24x24x4", R10min in either direction) was agreed to be a "NEC worse case scenario". why would we want to know what the wire does in a proper installation

As a relative reference to worse case scenario. We have an open air value, and we will have a worse case, imo knowing where average lies is a good idea. If the average lies close to worse case scenario, then there was good motive to keeping #14 as close to 15 amps as possible instead of a rule along the lines 'where NM is encased within dense spray foam insulation, the final ampacity shall not exceed that in the 60*C column'.

 

[FionaZuppa]

As a relative reference to worse case scenario. We have an open air value, and we will have a worse case, imo knowing where average lies is a good idea. If the average lies close to worse case scenario, then there was good motive to keeping #14 as close to 15 amps as possible instead of a rule along the lines 'where NM is encased within dense spray foam insulation, the final ampacity shall not exceed that in the 60*C column'.

i can run a glass test later, but i already have expectations for that, should run much cooler due to other factors that woven glass has. what exactly is a common install of NM and glass? NM stapled to a stud and then the cavity filled with a glass batt? or, is it common for NM to be sandwiched between two layer of R13 glass? what would be the test scenario for "common"?

 

[mbrooke]

i can run a glass test later, but i already have expectations for that, should run much cooler due to other factors that woven glass has. what exactly is a common install of NM and glass? NM stapled to a stud and then the cavity filled with a glass batt? or, is it common for NM to be sandwiched between two layer of R13 glass? what would be the test scenario for "common"?

Probably the prior, but Id also test the 2xR13.

 

[FionaZuppa]

Probably the prior, but Id also test the 2xR13.

if its NM on wood in a bay with glass, where does the TC go? just touching the side of the sheath? under between wood & sheath ??