NEC Changes For #14 Ampacity

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FionaZuppa said:
i can generate the whole table based on total wire diameter (wire + insulation). the previous calcs where just using wire (metal) diameter, so new #'s will be slightly less than the previous ones, even better for NEC, etc. but heck, using just wire (metal) diameter is even better as it removes any differences a wire maker may have in insulation thickness, etc. there will certainly be heatsink variances based on insulation thickness, type of insulation, etc etc, so there has to be some generalization done, otherwise there will need to be sets of calculations based on the numerous types of insulation and how it is applied to wire.

i will hunt down the two docs referenced.
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Those docs would be very interesting to see :)
 
don_resqcapt19 said:
Per the substantiation for proposal 6-99 the ampacities are based on the following technical documents.
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well, "IEEE Std 835-1994" the 1994 seems to suggest its a doc from the year 1994. NEC2002 lists #14 ampacity as 20/20/25, so why then in NEC2008 does #14 ampacity change to 15/20/25 ?? did the NEC just not use the IEEE Std when writing NEC2002 ??

so if wire makers start using Gexol insulation the NEC will create a 110C column? #14 with Gexol is rated 41A @110C, apparently this ampacity is based on IEEE 835-1994.

w/o reading 835-1994 yet, NEC seems to be rooted in wire heat and not so much about actual IEEE calc. i say this because i dont see NEC approving Gexol coating #14 on a 30A OCD, do you? it would generate too much heat (although not damage the insulation).

does NEC make us derate ampacities if a raceway has mixed insulation types? the arguments seem to be rooted in heat generation with heat from one wire affecting insulation on other wires.
 
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This thread might interest you:


http://www.diynot.com/diy/threads/cross-sectional-area-current-capacity.436680/#post-3404852


It comes to about 16 to 17 amps. Which would make sense that the NEC values are being rounded.



Yes, as I said, it's a modelling of (Method C) of Table 4D5 - so, yes, 70°C conductor temperature. If I had the time, I could try modelling Method A. However, looking at Table 4D5, the difference between Method C and Method A is always a factor of between 1.35 and 1.49 (1.38 for 1.5mm² and 1.35 for 2.5mm²), so if you divided the Method C figure by 1.49 (to be safe/conservative) or by 1.38 (based on the 1.5/2.5mm² figures), you wouldn't be far off - so, for 2.08mm² Method A (70°C), that would be about 16.5A (or 17.8A).
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FionaZuppa said:
look at the link in this page, it brings you to his power curve. #14 is 2.08mm^2, which lands above 20A on his chart. using that CCC equation it takes #14 to 24.61A
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Thats correct, but that is for wire in open air (stapled direct to wood). However when the same reduction factor (method A, enclosed in an insulated wall) is applied the ampacity comes in under 20amps for #14.


Yes, as I said, it's a modelling of (Method C) of Table 4D5 - so, yes, 70°C conductor temperature. If I had the time, I could try modelling Method A. However, looking at Table 4D5, the difference between Method C and Method A is always a factor of between 1.35 and 1.49 (1.38 for 1.5mm² and 1.35 for 2.5mm²), so if you divided the Method C figure by 1.49 (to be safe/conservative) or by 1.38 (based on the 1.5/2.5mm² figures), you wouldn't be far off - so, for 2.08mm² Method A (70°C), that would be about 16.5A (or 17.8A).
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My theory is that when all is considered the ampacity of #14 is probably around 17-18 amps at 60*C, hence why it keeps switching back and forth in the code cycles. IMO its just rounding that number. Keep in mind that 240.4(D) limits you to 15 amps, and motor circuits like AC units are always 125% of the max load so #14 doesnt actually see 20amps. In such a case the code making panels do not have much concern if rounding up or down as the result will always be the same.
 
mbrooke said:
Thats correct, but that is for wire in open air (stapled direct to wood). However when the same reduction factor (method A, enclosed in an insulated wall) is applied the ampacity comes in under 20amps for #14.





My theory is that when all is considered the ampacity of #14 is probably around 17-18 amps at 60*C, hence why it keeps switching back and forth in the code cycles. IMO its just rounding that number. Keep in mind that 240.4(D) limits you to 15 amps, and motor circuits like AC units are always 125% of the max load so #14 doesnt actually see 20amps. In such a case the code making panels do not have much concern if rounding up or down as the result will always be the same.
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ok, i am following, so did NEC finally take a read of IEEE-835-1994 in 2007 ?? NEC2002 has 20/20/25

and ok, the 125% for max load on motors, still allows 20A OCD. regardless of if load actually gets to 20A or not, fact is the #14 still sits on a 20A OCD, motor alone may not be the only cause of amps on the wire, etc. 1.25x18 = 22.5, so rounding down to 20 is ok, is below the IEEE calc by 2.5amps (open air).
 
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I am not trying to give anyone a hard time herd but I am truly curious where this discussion is leading?

I have been doing this work for more than 30 years now and I have never once wondered or concerned myself about the NECs choice of ampacities.

Today on this forum we have one EE telling us how the NEC minimums are not enough for good design and we have others arguing the ratings are too conservative.

Is it just me that has no problem just living with what we have?:)
 
iwire said:
I am not trying to give anyone a hard time herd but I am truly curious where this discussion is leading?

I have been doing this work for more than 30 years now and I have never once wondered or concerned myself about the NECs choice of ampacities.

Today on this forum we have one EE telling us how the NEC minimums are not enough for good design and we have others arguing the ratings are too conservative.

Is it just me that has no problem just living with what we have?:)
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just wondering why NEC chooses the #'s they do.

and just for clarity, its only Romex that is deeded to the 60C column even though NM-B is rated 90C, so outside of romex wire if i have non-romex wire and the terminations are rated 75C, why is the #14 still bound to a 15A OCD ?

and perhaps i goofed, it looks like the ampacity change was done on the 2011 NEC, where 2008 NEC had 20/20/25. i thought i had pulled up a 2008 table showing 15/20/25.
 
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480sparky said:
The 'small conductor rule' dates back to the 1956 NEC In 1956, the following was added to the bottom of the table:
"The current-carrying capacities for Type RHH conductors for sizes 14, 12 and 10 shall be the same as designated for Type RH conductors in this Table." This put 14, 12 and 10 RHH at 15, 20 and 30 amps respectively.

I have yet to find anything that says why. None of my handbooks from that era say one peep about it. Most likely, anyone involved in the NEC back then is gone, and perhaps the real reason will forever be lost to the ages.
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It makes me wonder why they didn't just put the "small conductor" ampacities in Table 310.15(B)(16) in the first place, instead of beating around the bush with this fine print note.

It's not like these are uncommon conductors that are rarely ever used. These are probably the most common sizes sold in terms of total linear feet. Probably even in terms of total copper mass.
 
Carultch said:
It makes me wonder why they didn't just put the "small conductor" ampacities in Table 310.15(B)(16) in the first place, instead of beating around the bush with this fine print note.

It's not like these are uncommon conductors that are rarely ever used. These are probably the most common sizes sold in terms of total linear feet. Probably even in terms of total copper mass.
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but ok, thats for the older insulation types, what about wire rated at 90C but has terminations rated 75C, why still restricted to 15A OCD ?? seems less to do with failing insualtion and more to do with heat generation. the Gexol insulation can run real hot w/o issue, so when will NEC add a 110C column? are ampacities based on insulation damage by temp, or just generation of heat? seems to be the latter given insulation like Gexol is out there. sure, terminations are still not common in 90C, but you get my point.
 
FionaZuppa said:
ok, i am following, so did NEC finally take a read of IEEE-835-1994 in 2007 ?? NEC2002 has 20/20/25
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I think they did based on what the harmonization task group wanted changed. If you can get IEEE-835-1994 I will see what the values come out to be, but imo I think its more of a rounding issue.


and ok, the 125% for max load on motors, still allows 20A OCD.
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True, however motors are a tad different from general use circuits. The breaker only provides short circuit and ground fault protection while the wire and motor is protected by overload an device. If the motor begins to pull more then its rated current the overload device opens protecting the motor in turn wire.



regardless of if load actually gets to 20A or not, fact is the #14 still sits on a 20A OCD, motor alone may not be the only cause of amps on the wire, etc. 1.25x18 = 22.5, so rounding down to 20 is ok, is below the IEEE calc by 2.5amps (open air).
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Believe it when serving motor loads you can legally put the #14 on a 40amp breaker.

The 125% is after the max current draw is known. So if I have an AC unit with an MCA of 20 amps, the unit will not pull more then 16amps continuously. If it pulled say 17 amps the MCA would be 21.25, thus the installer would have to choose #12.
 
Carultch said:
It makes me wonder why they didn't just put the "small conductor" ampacities in Table 310.15(B)(16) in the first place, instead of beating around the bush with this fine print note.

It's not like these are uncommon conductors that are rarely ever used. These are probably the most common sizes sold in terms of total linear feet. Probably even in terms of total copper mass.
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Its interesting, at one point they did (see pic from the 1965 NEC), where only the 90*C column was left as it is today for de-rating purposes:
 
i also question application of ambient temperature. if my conduit is 24" in the ground, do i use the ground temp at 24" below grade as ambient? this might lead me to apply x1.04 correction factor for #14 rated @90C to get 26A. yet i still cant use a 20A OCD on that?

and in a nec2008 revision looks like where NM was booted down to 60C column (see http://forums.mikeholt.com/showthread.php?t=122166).
 
mbrooke said:
True, however motors are a tad different from general use circuits. The breaker only provides short circuit and ground fault protection while the wire and motor is protected by overload an device. If the motor begins to pull more then its rated current the overload device opens protecting the motor in turn wire.

The 125% is after the max current draw is known. So if I have an AC unit with an MCA of 20 amps, the unit will not pull more then 16amps continuously. If it pulled say 17 amps the MCA would be 21.25, thus the installer would have to choose #12.
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interesting, this seems to suggest the NEC relies on nameplate ratings and believes in the device ocd/otd to do its job. why's that, i have a 1.5HP hvac motor where the motor wires look to be #18 in size, inrush is above 30A and hi speed winding runs about 14.2 amps. heck, my bro-inlaw had a pool pump motor running so hot you could smell it burning from 10ft away, the internal over temp was not working (simply failed from being old and going through a zillion heat cycles), but was still running under circuit OCD amps, certainly a scenario that exceeds the allowed exceptions for motors. NEC should be protecting the wire under all wiring cases w/o exceptions, etc. if #14 is bound to 15a OCD then #14 should not be used on motor with MCA of 20, etc.
 
FionaZuppa said:
interesting, this seems to suggest the NEC relies on nameplate ratings and believes in the device ocd/otd to do its job. why's that, i have a 1.5HP hvac motor where the motor wires look to be #18 in size, inrush is above 30A and hi speed winding runs about 14.2 amps. heck, my bro-inlaw had a pool pump motor running so hot you could smell it burning from 10ft away, the internal over temp was not working (simply failed from being old and going through a zillion heat cycles), but was still running under circuit OCD amps, certainly a scenario that exceeds the allowed exceptions for motors. NEC should be protecting the wire under all wiring cases w/o exceptions, etc.
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#18 wire in open air can handle more current, and the temperature rating of the insulation could be higher. Open up a range or look at the whip of a cooktop/oven, the cord coming in is #8 or #6 on a 40/50amp circuit but the wire coming off the terminal block is either #14 or #12 with ultra high temperature insulation (at least 250*C if I remember correctly)

In terms of motors the NEC believes that the overload will do its job. FWIW in the CEC its assumed electric heat will never pull more then the data plate, and over seas many fixed loads are assumed not to pull more then the wire its rated for.

Of note, the amapcity tables in the NEC and IEC (really all indoor building wire) are very conservative. Some codes are more loose while some more strict but in actual operation wire never reaches the claimed temperature in any ampacity table. The reason is from built in safety factor.

#14 NM might need say 70 amps continuous in open air to reach 194*F, a safety factor is then added bring it down to 25 amps. Because the wire might be in thermal insulation rather then open air another adjustment is added to 20amps. Being that wire might end up in dense foam insulation, installer error, DIY screw up, ect, ect the code adds another safety factor bring it down to 15 amps.

To what degree safety factors are applied and where is up to those making the code.


Id be curious to know what current is needed to bring NM to the actual temperatures listed, but I will bet you its a lot more then claimed.
 
iwire said:
I am not trying to give anyone a hard time herd but I am truly curious where this discussion is leading?

I have been doing this work for more than 30 years now and I have never once wondered or concerned myself about the NECs choice of ampacities.

Today on this forum we have one EE telling us how the NEC minimums are not enough for good design and we have others arguing the ratings are too conservative.

Is it just me that has no problem just living with what we have?:)
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It's not just you. Debating and discussing ampacity is an exercise for engineers and people with a lot of free time on their hands. I've said it many times and will say it again - we have a good system here that has worked for a long time now.
 
peter d said:
It's not just you. Debating and discussing ampacity is an exercise for engineers and people with a lot of free time on their hands. I've said it many times and will say it again - we have a good system here that has worked for a long time now.
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Im curious why you have an issue with engineers debating electrical theory.


The believe the OP is asking all the right questions. He is learning and we all learn by asking questions.
 
peter d said:
I couldn't care less if you debate about electrical theory. You can debate this out the next 100 pages but it's highly unlikely you're going to change anything.
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Even then, Id rather know for myself. Keep in mind those on the CMP and in our trade arent going to be at their job forever. A new generation will have to take their place.
 
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