NEC Changes For #14 Ampacity

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mbrooke

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why are we testing the breakdown voltage?

The same setup with a variac and isoaltion transformer could in theory also test break down voltage.


But in any case, like all scientists you should test your hypothesis.

Get a variac and high current transformer with some themro-couples. Amp clamp different sized cables under different thermal conditions (open air, insulation, ect) and test how hot the wire gets. This will yield the best results.

A simar test was done here in a real life application to prove (disprove) a code requirement:

http://iaeimagazine.org/magazine/20...ng-temperature-field-installation-experiment/
 

FionaZuppa

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The same setup with a variac and isoaltion transformer could in theory also test break down voltage.


But in any case, like all scientists you should test your hypothesis.

Get a variac and high current transformer with some themro-couples. Amp clamp different sized cables under different thermal conditions (open air, insulation, ect) and test how hot the wire gets. This will yield the best results.

A simar test was done here in a real life application to prove (disprove) a code requirement:

http://iaeimagazine.org/magazine/20...ng-temperature-field-installation-experiment/

an interesting real world test. my next Q would be though, how did the original nec proposal get drafted w/o real-world data/testing ??? for the nec contributors to say "well, the wire gets hotter with more amps, so we will restrict the amps as this keeps the wire cooler" is not a good method imho, i mean zero amps is also good, keeps the wire very cool.
 

FionaZuppa

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this real world vegas test, installed by licensed electricians, from the write-up:

The first setup had five 12 AWG conductors with THHN/THWN-2 insulation. These were installed in 10 feet of 1/2″ EMT and the return run in 3/4″ EMT. A thermocouple was installed in each of these runs. Also, a thermocouple was installed inside the disconnect of the evaporative cooler these conductors fed, and then one thermocouple was installed to measure outside ambient temperature at approximately 48″ above the roof surface, near the top of the cooler.

what was the actual wiring per conduit? five #12's in 1/2" emt doesnt make sense.
 

mbrooke

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this real world vegas test, installed by licensed electricians, from the write-up:



what was the actual wiring per conduit? five #12's in 1/2" emt doesnt make sense.

I will read the study again, but I think they were looking for a situation where the conductors already needed de-rating. The code claims additional de-rating is needed, however the article theoretically proves not to be so.
 

mbrooke

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240.4 (B) May offer some incite behind 240.4(D) and the reduction in ampacity:


(B) Overcurrent Devices Rated 800 Amperes or Less.
The next higher standard overcurrent device rating (above
the ampacity of the conductors being protected) shall be
permitted to be used, provided all of the following conditions
are met:

(1) The conductors being protected are not part of a branch
circuit supplying more than one receptacle for cordand-
plug-connected portable loads.

(2) The ampacity of the conductors does not correspond
with the standard ampere rating of a fuse or a circuit
breaker without overload trip adjustments above its rating
(but that shall be permitted to have other trip or
rating adjustments).
(3) The next higher standard rating selected does not exceed
800 amperes.


If NM is rated at 17 amps 60*C, then a 20amp device would technically be violating the intent of this rule even though it applies to larger conductors.

NM would do fine at 75*C, however I think its reasons like this that put it at the 60*C column:

http://www.inspectorsjournal.com/forum/uploads/ahis/20091221154745_047.JPG

http://images1.cableorganizer.com/a...bleway-support-system/images/10_install-4.jpg

http://s15.photobucket.com/user/TonyPE/media/P1010987_edited-1.jpg.html

http://www.doityourself.com/forum/a...led-too-tight-through-joist-subfloor-back.jpg


I frequently see NM abused like this.


......................................

But I still ask a similar question. Canada does not restrict NM to 60*C (in fact you could use 90*C if terminals let you), and 240.4D type rules do not apply to some fixed loads. Perhaps NM is bundled as much up North?
 

FionaZuppa

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240.4 (B) May offer some incite behind 240.4(D) and the reduction in ampacity:

If NM is rated at 17 amps 60*C, then a 20amp device would technically be violating the intent of this rule even though it applies to larger conductors.

NM would do fine at 75*C, however I think its reasons like this that put it at the 60*C column:

http://www.inspectorsjournal.com/forum/uploads/ahis/20091221154745_047.JPG

http://images1.cableorganizer.com/a...bleway-support-system/images/10_install-4.jpg

http://s15.photobucket.com/user/TonyPE/media/P1010987_edited-1.jpg.html

http://www.doityourself.com/forum/a...led-too-tight-through-joist-subfloor-back.jpg


I frequently see NM abused like this.


......................................

But I still ask a similar question. Canada does not restrict NM to 60*C (in fact you could use 90*C if terminals let you), and 240.4D type rules do not apply to some fixed loads. Perhaps NM is bundled as much up North?

well, i have two replies:
1) if the wiring method/scheme is an issue then address the wiring method scheme.
2) the ohms per say " the bundled length" looks to be very small in those pics. have these so-called abused romex installations been tested in a way like the Vegas testing ??

the Vegas write-up, the five #12's to me sounds like two bc's with a common egc, maybe two in the 1/2" and three in the 3/4". i think the intent of this test is not bundling but more just about addressing the proposed derating just because its exposed to the sun.
 

mbrooke

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well, i have two replies:
1) if the wiring method/scheme is an issue then address the wiring method scheme.

I agree here, and I think it goes to the fact many mis-use small conductors hence the extra conservatives. If small conductors weren't abused as much there would be a lot less need for 240.4 (D).

2) the ohms per say " the bundled length" looks to be very small in those pics. have these so-called abused romex installations been tested in a way like the Vegas testing ??

They haven't been tested but it goes like this: when multiple conductors are energized those inside heat up a lot more because they cant dissipate heat as well. On the contrary, they are surrounded by wires that generate heat themselves so heat dissipation is downright awful. This is why the code requires de-rating when more then 3 current carrying conductors are inside the same raceway.

Also, what do you mean by ohm value?


the Vegas write-up, the five #12's to me sounds like two bc's with a common egc, maybe two in the 1/2" and three in the 3/4". i think the intent of this test is not bundling but more just about addressing the proposed derating just because its exposed to the sun.

It is about the exposed sun, but I brought up de-rating because many feel that the standard de-rating used in the past (when needed) is more then enough. Solar exposer isn't an issue in any case according to the study and what electricians have observed in the real world.
 

FionaZuppa

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Also, what do you mean by ohm value?
as in the dissipated energy per inch (I^2*R), etc.

the close proximity of multiple wires say for a distance of 1" where just beyond those end points the wires splay apart from one another, the heat density in that 1" area should pose no hazard even if #14 was allowed on a 20A ocd (or in cases where some or all of the wires were installed per NEC exceptions).
 

mbrooke

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as in the dissipated energy per inch (I^2*R), etc.

the close proximity of multiple wires say for a distance of 1" where just beyond those end points the wires splay apart from one another, the heat density in that 1" area should pose no hazard even if #14 was allowed on a 20A ocd (or in cases where some or all of the wires were installed per NEC exceptions).

True, but I am wondering this: what if the wire is embedded in thermal insulation? How does the heat dissipation effect the cable's rise in temperature?
 

FionaZuppa

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True, but I am wondering this: what if the wire is embedded in thermal insulation? How does the heat dissipation effect the cable's rise in temperature?

if its thermal insulation then i would expect the heat to stay inside, thus the temp of the metal would rise with a temp gradient across the insulation, the gradient would be higher because the thermal R is higher for thermal insulation, etc. hotter wire does not mean the outside of the cable/insulation is hotter, so i dont see the hazard. the issue with temp is if and only if the temp (heat energy) is 1) affecting the insulation or metal itself, or 2) is escaping and affecting nearby objects. if we keep heat inside under the insulation so that the temp outside is lower, is that a bad thing ??
 

mbrooke

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if its thermal insulation then i would expect the heat to stay inside, thus the temp of the metal would rise with a temp gradient across the insulation, the gradient would be higher because the thermal R is higher for thermal insulation, etc. hotter wire does not mean the outside of the cable/insulation is hotter, so i dont see the hazard. the issue with temp is if and only if the temp (heat energy) is 1) affecting the insulation or metal itself, or 2) is escaping and affecting nearby objects. if we keep heat inside under the insulation so that the temp outside is lower, is that a bad thing ??


Maybe Im wrong, but wouldnt the copper and THHN insulation be hotter then say in open air? How would insulation prevent heat from the copper migrating to the THHN?


No matter what the THHN itself will be hotter.
 

FionaZuppa

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Maybe Im wrong, but wouldnt the copper and THHN insulation be hotter then say in open air? How would insulation prevent heat from the copper migrating to the THHN?


No matter what the THHN itself will be hotter.

not exactly. thermal insulation blocks the heat thus the temp gradient across the zone is higher (lower if based on distance, higher if based on temp diff, etc). as example, think of basic physics experiments where the experiment is bound inside a thick walled styrofoam container, this keeps the system isolated from the outside environment. a thermal insulation has high R (relatively speaking). if the insulation has high R value then the heat generated from the wire would be trapped under the insulation thus the metal would simply get hotter and the insulation cooler on the outside. the contact zone between wire and ID of the insulation would be at same temp, but as you pass through the insulation from ID to OD the temp would decrease down to ambient on the OD of the insulation. only an infinitely thick insulation could show ambient temp at the OD layer, but the higher the R value the lower the real-world OD temp would be.

as example www.youtube.com/watch?v=5sw1tNeJ0Rw
and
https://www.youtube.com/watch?v=Pp9Yax8UNoM
 
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mbrooke

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not exactly. thermal insulation blocks the heat thus the temp gradient across the zone is higher (lower if based on distance, higher if based on temp diff, etc). as example, think of basic physics experiments where the experiment is bound inside a thick walled styrofoam container, this keeps the system isolated from the outside environment. a thermal insulation has high R (relatively speaking). if the insulation has high R value then the heat generated from the wire would be trapped under the insulation thus the metal would simply get hotter and the insulation cooler on the outside. the contact zone between wire and ID of the insulation would be at same temp, but as you pass through the insulation from ID to OD the temp would decrease down to ambient on the OD of the insulation. only an infinitely thick insulation could show ambient temp at the OD layer, but the higher the R value the lower the real-world OD temp would be.

as example www.youtube.com/watch?v=5sw1tNeJ0Rw
and
https://www.youtube.com/watch?v=Pp9Yax8UNoM



True, but the conductor covering (PVC) would also get hotter with the metal. This the conductor covering will become hotter inside insulation then in free air.
 

FionaZuppa

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True, but the conductor covering (PVC) would also get hotter with the metal. This the conductor covering will become hotter inside insulation then in free air.

this is all "in free air". an insulator with R value creates a temp gradient from ID to OD of the insulation. the contact boundary between ID of insulation and OD of the metal will be at same temp, which may not mean much w/o other data, such as UL testing of insulating type (look at Goxel for example). so in my example, lets say it is Goxel insulation and this type has a very high thermal R value, a test of the wire temp when wire is under various loads tells us what, ........ hmmm, not sure, depends on how the test is done, are we reading temp right at the ID of the insulation, or is it OD temp that NEC is worried about. see, when we start looking deeper into how things really work it kinda baffles me as to why romex is bound to 60C column. lets pose another example, if the metal is at say 125C but the OD of insulation is near ambient, and, the insulation type is UL rated for 140C, is this an issue if bundled into "romex"?

this argument seems to flip-flop between "oh, its temp of insulation we need to look at" and "its temp of surrounding areas because of the way romex is commonly used". i suspect its both but the NEC verbiage does not seem to represent that well, and, the wire types today dont match up to the NEC verbiage, like why romex is bound to the 60C column.

your thoughts?
 

mbrooke

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this is all "in free air". an insulator with R value creates a temp gradient from ID to OD of the insulation. the contact boundary between ID of insulation and OD of the metal will be at same temp, which may not mean much w/o other data, such as UL testing of insulating type (look at Goxel for example).

Keep in mind that come real world you are dealing with places where NM will be in contact with insulation. Think a foam insulated house.


so in my example, lets say it is Goxel insulation and this type has a very high thermal R value, a test of the wire temp when wire is under various loads tells us what, ........ hmmm, not sure, depends on how the test is done, are we reading temp right at the ID of the insulation, or is it OD temp that NEC is worried about. see, when we start looking deeper into how things really work it kinda baffles me as to why romex is bound to 60C column. lets pose another example, if the metal is at say 125C but the OD of insulation is near ambient, and, the insulation type is UL rated for 140C, is this an issue if bundled into "romex"?

Im sure the thermal insulation will be fine as well as everything surrounding it, however the dielectric insulation (the PVC copper covering) when subjected to excessive temperatures will begin to deteriorate. Even if all else is fine, the copper covering must last the life of the house or building. Expecting 60 years out of NM would not be unreasonable. If the copper covering starts to crack and fall off after 20 years that would be both an inconvenience and hazard.



this argument seems to flip-flop between "oh, its temp of insulation we need to look at" and "its temp of surrounding areas because of the way romex is commonly used". i suspect its both but the NEC verbiage does not seem to represent that well, and, the wire types today dont match up to the NEC verbiage,

your thoughts?


In theory its both. You are correct though, the NEC isnt to explicit about what the intent is behind said rules. High temperatures can degrade both the copper covering and also the surrounding material. As an example, NM stapled to a wood stud frequently reaching an actual real temperature of 210*F. Two things will happen: 1. Over time the insulation will become weak, brittle, crack and start to fall off. 2. Those high temps can turn the wood into charcoal lowering its ignition point.


The available practical guidance—i.e., the fires that have been documented to have occurred when
wood members were exposed to heating sources at 77ºC (170ºF) or higher—forms a reliable, scientific basis for
concluding that an ignition hazard exists if a heat source at 77ºC or higher is applied to a wood member for a
protracted period of time.


http://www.doctorfire.com/low_temp_wood1.pdf







like why romex is bound to the 60C column.

I will have to find it, but if I am correct the Massachusetts Electrical Code lets you run NM at 75*C provided that its not in contact with thermal insulation.
 

mbrooke

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Page 11:


http://www.mass.gov/eopss/docs/dfs/osfm/cmr/cmr-secured/527012.pdf

334.80. Delete the second paragraph and revise the first paragraph to read as follows:
334.80 Ampacity. Type NM, NMC, and NMS cable shall have conductors rated at 90°C
(194°F). Where installed in thermal insulation, the ampacity of conductors shall be that of 60°C
(140°F) conductors. The ampacity of Types NM, NMC, and NMS cable installed in cable tray
shall be determined in accordance with 392.11.

Let me double check if its still holds true...
 

FionaZuppa

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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
In terms of safe design and safe practices for the installation of heat-producing devices adjacent to wood surfaces, it
should not be a new or surprising piece of information that 250ºC would represent an extremely hazardous condition
and that 77ºC, in fact, must not be exceeded if the heating is prolonged

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
 
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