NEC Changes For #14 Ampacity

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mbrooke said:
Im curious why you have an issue with engineers debating electrical theory.
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I thought you were an electrician? :lol:

I cant speak for Pete, as for myself I do not have a problem with engineers (glad you finally came clean about that:p) debating electrical theory.

As I stated earlier I was just wondering to what end?

The rules are the rules, none of us is going to be able to get anything in the ampacity tables changed. Seems like a waste of time to me.

That said, many would say the time I spend on the Internet is wasted, to each their own. :)
 
mbrooke said:
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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Wow, that seems like a huge assumption and basically an insult to the many dedicated people who have worked on CMPs for the last 100+ years.
 
iwire said:
I thought you were an electrician? :lol:

I cant speak for Pete, as for myself I do not have a problem with engineers (glad you finally came clean about that:p) debating electrical theory.

As I stated earlier I was just wondering to what end?

The rules are the rules, none of us is going to be able to get anything in the ampacity tables changed. Seems like a waste of time to me.

That said, many would say the time I spend on the Internet is wasted, to each their own. :)
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Ive never hid anything, Ive done extensive electrical work, so I am not unfamiliar with how things are in the field.

I am optimistic, you never know. Someone was able to get CSA to change after 100 years.
 
iwire said:
I thought you were an electrician? :lol:

I cant speak for Pete, as for myself I do not have a problem with engineers (glad you finally came clean about that:p) debating electrical theory.

As I stated earlier I was just wondering to what end?

The rules are the rules, none of us is going to be able to get anything in the ampacity tables changed. Seems like a waste of time to me.

That said, many would say the time I spend on the Internet is wasted, to each their own. :)
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See post #59. :p I am all for learning and debating, as anyone who reads my posts knows. But on this topic I agree that it's an unprofitable waste of time as the ampacity rules are highly unlikely to be relaxed or changed. Also, I believe discussion of foreign codes and making comparisons to ours is like comparing apples to hamburgers. It may give insight into certain things but at the end of the day I still have to abide by the NEC.
 
iwire said:
Wow, that seems like a huge assumption and basically an insult to the many dedicated people who have worked on CMPs for the last 100+ years.
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Ok now I now Im being trolled.

We both know people retire. Tell me one person who was one the CMPs 60 years ago thats still there today.
 
peter d said:
See post #59. :p I am all for learning and debating, as anyone who reads my posts knows. But on this topic I agree that it's an unprofitable waste of time as the ampacity rules are highly unlikely to be relaxed or changed. Also, I believe discussion of foreign codes and making comparisons to ours is like comparing apples to hamburgers. It may give insight into certain things but at the end of the day I still have to abide by the NEC.
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Foreign codes still follow the same laws of physics as we do.

It was my research into foreign codes that have practically confirmed AFCI will never do as claimed.
 
mbrooke said:
Foreign codes still follow the same laws of physics as we do.
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Which means nothing since we end up with electrical systems that are radically different in design and have completely different rules. In the UK, they say everything should be RCD protected. Here in the USA that concept is completely unknown.


It was my research into foreign codes that have practically confirmed AFCI will never do as claimed.
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Oh, I thought it was the laws of physics that did that? ;)
 
mbrooke said:
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.
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what listed temps? you mean the ampacity table column headers? those are just wire ratings, seems to be typical wire ratings (likely UL ratings) of various insulation types. i dont think the table is intended to suggest the ampacity # will bring wire near that temp.

and even with the wire buried under other insulation i do not believe wire temps will rise to unsafe level if you restrict it to 1.02W/ft@100ft, even if a rated OCD trips at 125% of its rating. the IEEE doc is nice, but for sake of ampacity #'s allowed by NEC, a simpler method i believe can be used, such as 1.02W/ft@100ft(max per CCC), otherwise you go to next wire size up. example, xx-2NM can generate up to 2.04W/ft@100ft(max). this inherently also bumps wire sizes when runs are greater than 100ft, thus inherently handling voltage drop issue. the 1.02W/ft@100ft basically defines many things all at the same time.
 
FionaZuppa said:
what listed temps? you mean the ampacity table column headers? those are just wire ratings, seems to be typical wire ratings (likely UL ratings) of various insulation types. i dont think the table is intended to suggest the ampacity # will bring wire near that temp.

and even with the wire buried under other insulation i do not believe wire temps will rise to unsafe level if you restrict it to 1.02W/ft@100ft, even if a rated OCD trips at 125% of its rating. the IEEE doc is nice, but for sake of ampacity #'s allowed by NEC, a simpler method i believe can be used, such as 1.02W/ft@100ft(max per CCC), otherwise you go to next wire size up. example, xx-2NM can generate up to 2.04W/ft@100ft(max). this inherently also bumps wire sizes when runs are greater than 100ft, thus inherently handling voltage drop issue. the 1.02W/ft@100ft basically defines many things all at the same time.
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Yup, the headers. They make it sound like the wire does actually get to that temperature, or at least one assumes. 60/75*C connector listing is a biggy, I can load wire to the 90*C column and the connector isnt going to reach 75*C.
 
mbrooke said:
... I can load wire to the 90*C column and the connector isnt going to reach 75*C.
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The UL489 standards limit the standard molded case circuit breaker terminal temperature rise to 50C when loaded to 100% in an open air ambient temperature of 40C.
Which seems to say that at 100% loading the conductor internal heat generation is limited to a maximum of 50C, not the ultimate rating of its insulation.
 
jim dungar said:
The UL489 standards limit the standard molded case circuit breaker terminal temperature rise to 50C when loaded to 100% in an open air ambient temperature of 40C.
Which seems to say that at 100% loading the conductor internal heat generation is limited to a maximum of 50C, not the ultimate rating of its insulation.
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you said temp rise from 40C, so it allows +50C above 40C = 90C ??
but lets just stick to NEC verbiage, NEC says the lower of wire or termination points define the temp of the end-to-end, thus it is the lower temp that wires sizes are chosen because ampacities are different based on temp ratings. so for OCD's that are rated 75C and THWN-2 that is rated 90C, we size to 75C column and apply corrections as prescribed by NEC verbiage. using this method 20A OCD for #14 is ok (conduit with two CCC's).................

but this general method doesnt work for #12 & 14 because 1) NM has been restricted to the 60C column, and 2) the outright clause NEC has that says #14 cannot have greater than 15A OCD (#12 20A) (but exceptions apply). and to boot, if my conduit is 30" below grade where temps are around 50-60F i could have an derate factor of 1.04 which bumps up ampacity, but still again not allowed because #14 cant be on a 20A OCD.

just baffles me. i understand there have been no issues with 2011NEC, but i am trying to understand the rationale behind the verbiage. i dont see much other than IEEE835-1994, but why did it take until 2011NEC for NEC to use IEEE835-1994 ???

another case scenario for allowing #14 on 20A OCD is taps. say 50ft of #12 that comes from a 20A OCD, that #12 feeds two outlets, but now you want to extend (tap) from the #12 up to a few lights. doing so with #14 is not allowed currently because of the 15A(max) OCD for #14. this should at least be one of the exceptions.
 
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FionaZuppa, I pretend to be an engineer, not an electrician. Your area equations and current density should (reasonably) accurately predict I*I*R losses, but remember that surface area varies with diameter, not diameter squared; thus a wire of double the diameter of another would have only double the surface area (for heat to be removed) but 4 times the area.

This is not offered in response to why the NFPA does what they do, but as a comment as to why results seem confusing based on equations which don't include all of the variables.

George
 
GeorgeB said:
FionaZuppa, I pretend to be an engineer, not an electrician. Your area equations and current density should (reasonably) accurately predict I*I*R losses, but remember that surface area varies with diameter, not diameter squared; thus a wire of double the diameter of another would have only double the surface area (for heat to be removed) but 4 times the area.

This is not offered in response to why the NFPA does what they do, but as a comment as to why results seem confusing based on equations which don't include all of the variables.

George
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so let me clarify. i am suggesting that the ampaciites for all wires sizes be based on 1.02W/ft@100ft. if you start here then it doesnt matter that watts has I^2 in it. i then calculated the sq.in. area of #14 (metal only) for 1ft, this is the area 1.02W has to dissipate through, etc. hence, if you up the wire size the heat density remains constant at 1.02W/ft@100ft but the area of insulation goes up, therefore the heatsink density goes down and thus the temp will go down.

in words, how much current is needed to generate 1.02W/ft of 100ft of wire. this defines the ampacity for all wires sizes up to 100ft. beyond 100ft you bump up wire size, etc. its that simple. this base # is formulated from #14 since it has the smallest insulation area (max heatsink density), etc. there are no adjustments for wire runs less than 100ft, thus you can only get max heat generation of any wire size only if you run max ampacity @ 100ft, etc. this whole model is in alignment with NEC rationale that you create the verbiage that is 100% safe, and only gets to the max thresholds in as few scenarios as possible, etc.

for NEC code as-is today, the heat #'s are not linear across wire sizes. why is that ?? wire types and "allowed use" of such wire is the same for all wire sizes, so why is one size wire allowed to generate more heat than another??

i have not read IEEE835-1994 yet, so not really sure what this std was seeking in its methodology.
 
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mbrooke said:
I will PM you a PDF that might help.
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ok, this neher & mcgrath paper describes in detail exactly what i have been saying. the "load capability" is based on pre-defined max temp rise as described in this doc. the NEC #'s as they exist today are different for different wire sizes, which is baffling to me. unless someone can point out for me, i dont see where in the N&M equations where varying the wire sizing would yield ampacity (aka "load capability") results that we not linear while keeping temp rise constant.

that said, if NEC ampacity use these N&M equations then what was the defined acceptable temp rise?? there needs to be a starting point, etc.
 
FionaZuppa said:
unless someone can point out for me, i dont see where in the N&M equations where varying the wire sizing would yield ampacity (aka "load capability") results that we not linear while keeping temp rise constant.
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I haven't look at the N&M equations, but basic physics says for a unit length of wire of diameter d and _surface_ area A

P = I^2 * R
R varies as 1/d^2
So P varies as I^2/d^2
A varies with d
So P/A varies as I^2/d^3

Temperature rise of the wire should be an increasing function of P/A (power dissipated per unit surface area of the conductor). Certainly that is true for the radiant and conductive components of heat loss, not sure about convection. So at least to a first approximation, a given maximum temperature rise will mean a fixed maximum P/A.

Thus there's nothing linear here: ampacity should vary as diameter to the 3/2, or cross-sectional area to the 3/4. How does that compare to the trend in the NEC ampacity values?

Cheers, Wayne
 
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