NEC Changes For #14 Ampacity

[mbrooke]

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 ??

Id say both, one on one with one on the other.

 

[FionaZuppa]

Id say both, one on one with one on the other.

ok, a test for later. easily doable.

 

[mbrooke]

ok, a test for later. easily doable.

 

[FionaZuppa]

today i got the TC's mounted into the foam board. i'd say the tip of the TC's have about 1-3grams of force up against the wire. the bare copper has a small slice of mica between it and the TC with thermal paste on both sides, and some thermal paste on the TC that is on the sheath.

now i just need two rtd connectors from Omega for the TC's and we'll be ready to cook.

 

[FionaZuppa]

have a read. i suspect my #'s will be close to these, and maybe this info is being used by CEC to change their tables??

The University of Toronto study indicates that the conductors will not be subjected to
objectionable temperatures even under very severe conditions.

http://gfretwell.com/electrical/nema_romex_in_foam_study.pdf

 

[FionaZuppa]

and this doc which basically describes what i talked about in the start of this thread around the heat flux density of wire vs amps. but the free air temps listed are not near where mine were. my amps and TC (temp) seem to be accurate (CM660 accurate), so why the big diffs???

http://loesshillsengineering.com/wp-content/uploads/2015/08/thermal_model_for_wire.pdf

 

[mbrooke]

have a read. i suspect my #'s will be close to these, and maybe this info is being used by CEC to change their tables??

The University of Toronto study indicates that the conductors will not be subjected to
objectionable temperatures even under very severe conditions.

http://gfretwell.com/electrical/nema_romex_in_foam_study.pdf

The results of the study could very well have been used in the decision process changing the CEC to legally allow #14 to carry 20 amps. The only question which I have is what was the highest current used? 15, 20 or 25?

and this doc which basically describes what i talked about in the start of this thread around the heat flux density of wire vs amps. but the free air temps listed are not near where mine were. my amps and TC (temp) seem to be accurate (CM660 accurate), so why the big diffs???

http://loesshillsengineering.com/wp-content/uploads/2015/08/thermal_model_for_wire.pdf

I think 3 current carrying conductors might be playing a role. The wire you tested was only a single conductor rather then 3 inside a cable. Further, I think the equations (and assumptions) in the paper do not reflect the real world imo. Fiberglass is assumed as a cylinder around the cable, but in the real world when fiberglass is placed against cable it does not make perfect contuer as the fiberglass is compressed which in itself changes the thermal properties. When fiberglass is compressed, its thermal insulation properties actually go down rather then staying them same.

 

[mbrooke]

In all the calculations that I do, and most of the papers that I come across, show that #14 is actually slightly oversized (or that number 12 is slightly undersized). #12 appears to get a bit hotter at 20 amps then #14 at 15 amps continuous current. It could be that #14 has a bit of extra safety factor, or just the way the sizes came out when the AWG system was developed leaving #12 a bit small. For #12 to have equal CCC ratios to #14 it would need to be roughly around 3.48mm2 instead of 3.31mm2 as it is now.

 

[FionaZuppa]

In all the calculations that I do, and most of the papers that I come across, show that #14 is actually slightly oversized (or that number 12 is slightly undersized). #12 appears to get a bit hotter at 20 amps then #14 at 15 amps continuous current. It could be that #14 has a bit of extra safety factor, or just the way the sizes came out when the AWG system was developed leaving #12 a bit small. For #12 to have equal CCC ratios to #14 it would need to be roughly around 3.48mm2 instead of 3.31mm2 as it is now.

so now you are talking about normalizing heat density across the wire sizes. this is something i talked about in this thread. even if you took #14 up to #12 heat density (for its existing ampacity) i dont think #14 temps would climb in ut-oh fashion.

thus far the only "interesting" data seems to be the UL doc that explains (in a formal way) degradation in the dielectric due to physical installation and high temps. the only thing that doc didnt show was insulation degradation of under the UL listed temp vs time vs degradation. it says they accelerated aging but do not define what that aging graph looks like, so not very clear how their aging process maps back to say a #14 NM that runs at 12A on continuous load for say 12 months.

my test will simply show the temps. what has to follow, either by using existing data or obtaining new data, is how these temps pose a hazard (if any) over time.

 

[FionaZuppa]

a couple of pics before i get on the horn with Omega

the TC installation is rather straightforward, they are in a shallow channel and the tips contact the wire. the other pic simply shows how i terminated the end of the NM, basically using eyelet lugs that are bolted together and then the whole thing soldered, to reduce any ohms there thus reducing heat generation at that junction point.

 

[mbrooke]

so now you are talking about normalizing heat density across the wire sizes. this is something i talked about in this thread. even if you took #14 up to #12 heat density (for its existing ampacity) i dont think #14 temps would climb in ut-oh fashion.

Not necessarily. But in the real world there may be more hesitation to hold #12 back from 25 or 30 amps then 14 from 20 or 25 amps. Its more food for thought really.

But I will say this, out of everyone Ive spoken with so far regarding wire ampacity, you seem to have this down better then everyone else (even me). :happyyes:

thus far the only "interesting" data seems to be the UL doc that explains (in a formal way) degradation in the dielectric due to physical installation and high temps. the only thing that doc didnt show was insulation degradation of under the UL listed temp vs time vs degradation. it says they accelerated aging but do not define what that aging graph looks like, so not very clear how their aging process maps back to say a #14 NM that runs at 12A on continuous load for say 12 months.

They did not, but they did prove wire will age faster at higher temps.

my test will simply show the temps. what has to follow, either by using existing data or obtaining new data, is how these temps pose a hazard (if any) over time.

Temps alone are all we need. Once we know the temps we can make a final conclusion with all the information given thus far.

 

[mbrooke]

a couple of pics before i get on the horn with Omega

the TC installation is rather straightforward, they are in a shallow channel and the tips contact the wire. the other pic simply shows how i terminated the end of the NM, basically using eyelet lugs that are bolted together and then the whole thing soldered, to reduce any ohms there thus reducing heat generation at that junction point.

Perfect! :thumbsup:

 

[FionaZuppa]

ok, Omega TC connectors are on their way ($1.50ea, $8 to ship, go figure), and i ordered a bag (50) of Wago 222-412's, i can cook these later


and just me re-visiting what i said a few posts back about free air temps, i may be wrong, those other tests might have been NM with two CCC's in free air, not a single wire like i had tested. so, its all good data but not all in same context, etc. i think we can say the amps ability of just a single wire in THHN/THWN (free air) is well above any ampacity #'s NEC has while staying below wire rated temp, but this does not represent any proper use of NM in real world, etc. i'll cook a piece of NM in free air now.

the sandwich test will reveal better data.

 

[mbrooke]

ok, Omega TC connectors are on their way ($1.50ea, $8 to ship, go figure), and i ordered a bag (50) of Wago 222-412's, i can cook these later


and just me re-visiting what i said a few posts back about free air temps, i may be wrong, those other tests might have been NM with two CCC's in free air, not a single wire like i had tested. so, its all good data but not all in same context, etc. i think we can say the amps ability of just a single wire in THHN/THWN (free air) is well above any ampacity #'s NEC has while staying below wire rated temp, but this does not represent any proper use of NM in real world, etc. i'll cook a piece of NM in free air now.

the sandwich test will reveal better data.

And, one last request, NM stapled to wood without anything. This will give 4 real world examples of heat dissipation.

The wago is for dessert.

 

[FionaZuppa]

just so we have another reference for data, this is 24" (48" RT) of the same NM wire, cooking at 15A now, it has to cook for about 1hr before i'll record the temp. the TC is on the wire in a way that would be NM laying flat-horizontal, so we can see what the temp might be from two CCC's giving up heat where that energy heats the top wire more, etc. ambient today is 60F.

And, one last request, NM stapled to wood without anything. This will give 4 real world examples of heat dissipation.

The wago is for dessert.

i can only do one test of NM in a bay w/ glass and covered like a real wall, and one test of it just on wood (i assume you mean free air). this is an important factor to note (the "one" test) because installation characteristics play a role. as example, i may get the NM to not lay perfectly flat against the wood and this will skew the #'s slighlty because the heat dissipation will be different vs another install where NM is laying perfectly flat on the wood. this type of test, as was done in that UL test, needs a repeatability component to obtain a set of data per test, which is something i cant do currently.

so as of now, the tests are/were:

all #14 btw
1) a piece of bare copper wire from NM tested in free air - done
2) a piece of insulated copper wire from NM tested in free air - done
3) a piece of full NM in free air - TBD, cooking now
4) a piece of full NM in rigid foam sandwich - TBD, waiting on Omega connectors
5) a piece of NM in a 2x R13 glass sandwich - TBD, need to get glass
6) a piece of NM in a bay on wood w/ glass and bay finished like a typical ext wall of a home - TBD, need to build it
7) a piece of NM on a piece of wood (or maybe this is #6 but w/o the glass?) - TBD, need to build it
8) wago 222 tests - TBD, waiting on 222's to arrive

 

[mbrooke]

just so we have another reference for data, this is 24" (48" RT) of the same NM wire, cooking at 15A now, it has to cook for about 1hr before i'll record the temp. the TC is on the wire in a way that would be NM laying flat-horizontal, so we can see what the temp might be from two CCC's giving up heat where that energy heats the top wire more, etc. ambient today is 60F.

Very cool

Technically the NEC would let you go higher being 60*F, but lets see what this amounts to first.

 

[FionaZuppa]

Very cool

Technically the NEC would let you go higher being 60*F, but lets see what this amounts to first.

from what i saw, it was 10F for 10F, thats +10F ambient = +10F wire temp. we can add 30F to this specific test. i have not run the math for this 10-for-10 but its what i saw between 60 & 70F ambients. if anyone can confirm the math that would help.

where does NEC get 86F(30C) anyways? having a set # seems odd given all the allowed uses of NM where ambient can way higher and lower. how would i know what to pick for ambient when attempting to derate a wire? today my outside temp is 70F, the NM in my 6" non-insulated wall might be 85F because sun hitting that wall today is heating the bay, but in dead summer the temp in that bay might be 130F, dunno, are there std's based on zones and sun exposures? IRBC in some sections breaks up criteria based on zones, and in some cases for wind calcs have coefficients for surroundings (nearby buildings, trees, etc etc). if we derated NM for my home, especially where all of the service panel BC's exit the panel into the bay i am talking about, #14 would likely be derated so low a 15A ocpd would be too big, and #12's would all have to be on 15A ocpd's.

 

[mbrooke]

i can only do one test of NM in a bay w/ glass and covered like a real wall, and one test of it just on wood (i assume you mean free air)

.

Yup, wood in free air. One side of NM will be resting again the wood, the other in air.

this is an important factor to note (the "one" test) because installation characteristics play a role. as example, i may get the NM to not lay perfectly flat against the wood and this will skew the #'s slighlty because the heat dissipation will be different vs another install where NM is laying perfectly flat on the wood. this type of test, as was done in that UL test, needs a repeatability component to obtain a set of data per test, which is something i cant do currently.

True but imo with small effort it will be good enough. If the values are well under none desirable it would be safe to assume temps would not be all that higher laying perfectly flat.

so as of now, the tests are/were:

all #14 btw
1) a piece of bare copper wire from NM tested in free air - done
2) a piece of insulated copper wire from NM tested in free air - done
3) a piece of full NM in free air - TBD, cooking now
4) a piece of full NM in rigid foam sandwich - TBD, waiting on Omega connectors
5) a piece of NM in a 2x R13 glass sandwich - TBD, need to get glass
6) a piece of NM in a bay on wood w/ glass and bay finished like a typical ext wall of a home - TBD, need to build it
7) a piece of NM on a piece of wood (or maybe this is #6 but w/o the glass?) - TBD, need to build it
8) wago 222 tests - TBD, waiting on 222's to arrive

Looks good to me. If anything changes or I get an idea I will let you know.

 

[FionaZuppa]

after 30min cooking NM at 14.45A the temp is at 74.7F. wow, this is very close to the free air test done of the one insulated wire in free air. remember, cooking NM is two CCC's, so thats 2x the heat at any given point.

i suspect the NM is itself acting as a good heatsink, there's much more surface area to dump the heat coming from the wires inside. this might lead me to make a statement about some of the other free air NM tests that have been documented, and that is, measuring the temp of the NM on the sheath may not be an accurate description of actual wire temps inside.

that said, having NM in a rigid foam sandwich should allow all of the components to reach same equilibrium temp, and as such, i am expecting both TC's to have same readings. this was of some debate back in this thread, where exactly do you measure temps in a multi-layered cable like NM? the most important area has to be the copper itself, from the OD of the copper outwards there has to be a temp gradient due to the R values of the things between, etc, the gradient is very small when in a foam sandwich = "NEC worse case scenario".

 

[mbrooke]

from what i saw, it was 10F for 10F, thats +10F ambient = +10F wire temp. we can add 30F to this specific test. i have not run the math for this 10-for-10 but its what i saw between 60 & 70F ambients. if anyone can confirm the math that would help.

Im not sure its linear, I think with colder temps their is accelerated heat dissipation?

where does NEC get 86F(30C) anyways?

Your guess is as good as mine, but I think common worse case temps in a building.

having a set # seems odd given all the allowed uses of NM where ambient can way higher and lower. how would i know what to pick for ambient when attempting to derate a wire?

You de-rate based on the 90*C column. So at 60*C it would take (in theory) 27 amps to get that wire to 90*C.

today my outside temp is 70F, the NM in my 6" non-insulated wall might be 85F because sun hitting that wall today is heating the bay, but in dead summer the temp in that bay might be 130F, dunno, are there std's based on zones and sun exposures?

None, other then a roof top adder added to the NEC which looks like it will be pulled from the 2017 NEC. I however will post a CCC table that has the sun factored in. Stand by, this table is very interesting (eye opening ) to say the least.

IRBC in some sections breaks up criteria based on zones, and in some cases for wind calcs have coefficients for surroundings (nearby buildings, trees, etc etc). if we derated NM for my home, especially where all of the service panel BC's exit the panel into the bay i am talking about, #14 would likely be derated so low a 15A ocpd would be too big, and #12's would all have to be on 15A ocpd's.

This is where ambient and conductor bundling come in. De-rating is based on 90*C, so at 130*F ambient #14 will be rated 17.75amps, #12 at 21.3amps. In dwellings you need to de-rate per code, but there are many bundles of NM going into panels without issue, and in majority I would say this stems from the transient load of dwelling units. One, maybe two conductors tops will see full load, and even then it is usually brief. In fact, if one takes an amp clamp of the main feeder, omitting HVAC, the load usually is around a few amps with peaks up to 20 or 30.