Why #12 restricted to 20 amp etc..

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Because of Fourier heat transfer calculations and the more complicated Neher-McGrath calculations.
 
Somthing happened in 1956. I don't remember it as I was just a twinkle in Daddy's eyes Momma didn't understand.

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Small conductor rule (T310.16) history:
The current Table 310-16 has a rather 'mobile' history, whose lineage is as follows:

In the original 1897, ampacities were listed in a table titled Rule 16, titled "Table of Carrying Capacity of Wires". 14, 12 and 10 had ampacities of 12, 17 and 24 for rubber insulation, 16, 23, and 32 for other insulations (those were your only two choices then!).

In the 1911 NEC it was called Rule 18 and was re-titled as "Allowable Carrying Capacities of Conductors."

In 1920, it became Table 1 of Rule 18.

In 1923, it was Table I to Rule 610. This was the first of three major renumberings of the NEC

In 1930 NEC, it was relocated as Table 1 of Rule 618.

In 1937, it was moved to the back of the book and became Table 1, Chapter 9. This was the second of three major renumberings of the NEC

In 1940 Table 1 was moved further back to Chapter 10.

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 at 15, 25 and 40 amps respectively.

In '59 it was relocated again and became Table 310-12. The ampacites were also changed for Type RH conductors, so this put 14, 12 and 10 at the ratings of 15, 20 and 30 amps we know today. This was the third of three major renumberings of the NEC.

In '65, the Table was renumbered 310-15.

In 1978, the 14AWG/15a, 12AWG/20a and 10AWG/30a were obelisk (?) notes at the bottom of (again renumbered) T310-16.

In 1999, the obelisk note was changed to refer to 240-3.

In 2002, the reference was changed to 240.4(D).

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What happened in 1956? I have no clue.
 
480sparky said:
Somthing happened in 1956. I don't remember it as I was just a twinkle in Daddy's eyes Momma didn't understand.

---------------------------------------------------------------

Small conductor rule (T310.16) history:
The current Table 310-16 has a rather 'mobile' history, whose lineage is as follows:

In the original 1897, ampacities were listed in a table titled Rule 16, titled "Table of Carrying Capacity of Wires". 14, 12 and 10 had ampacities of 12, 17 and 24 for rubber insulation, 16, 23, and 32 for other insulations (those were your only two choices then!).

In the 1911 NEC it was called Rule 18 and was re-titled as "Allowable Carrying Capacities of Conductors."

In 1920, it became Table 1 of Rule 18.

In 1923, it was Table I to Rule 610. This was the first of three major renumberings of the NEC

In 1930 NEC, it was relocated as Table 1 of Rule 618.

In 1937, it was moved to the back of the book and became Table 1, Chapter 9. This was the second of three major renumberings of the NEC

In 1940 Table 1 was moved further back to Chapter 10.

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 at 15, 25 and 40 amps respectively.

In '59 it was relocated again and became Table 310-12. The ampacites were also changed for Type RH conductors, so this put 14, 12 and 10 at the ratings of 15, 20 and 30 amps we know today. This was the third of three major renumberings of the NEC.

In '65, the Table was renumbered 310-15.

In 1978, the 14AWG/15a, 12AWG/20a and 10AWG/30a were obelisk (?) notes at the bottom of (again renumbered) T310-16.

In 1999, the obelisk note was changed to refer to 240-3.

In 2002, the reference was changed to 240.4(D).

---------------------------------------------------------------



What happened in 1956? I have no clue.
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That was about the time Neher-McGrath came on the scene. Not sure if it has any bearing.
 
I can get to a 81, or 84 NEC but the table changed between 78 and 87, the 60 deg col in 78 (anyway) listed the 14, 12, and 10 @ 15, 20, and 30.

That dosen't answer your question. I would not be surprised if part of the answer included the need to derate for temp and older OCPD were only rated at 60 deg C.

My tcr's only go back to 87.
 
Does changing wire sizes for voltage drop also fall under 240.4 (D)? Example: upsizing the wire to a #10 which was previously protected by a 20A breaker. Do you have to change the breaker to a 30A?

And what if you receive a mechanical cut that says 19 MCA and 25 MOCP? Do you have to go with a #10?
 
Keri_WW said:
Does changing wire sizes for voltage drop also fall under 240.4 (D)? Example: upsizing the wire to a #10 which was previously protected by a 20A breaker. Do you have to change the breaker to a 30A?

And what if you receive a mechanical cut that says 19 MCA and 25 MOCP? Do you have to go with a #10?
Click to expand...
That would defeat the purpose of derating.
 
240.4(D) answers the question: "the overcurrent protection shall not exceed the following after any correction factors for ambient temperature and number of conductors have been applied".

So you can put a 15, 20, or 30A breaker on a #10 wire, You just can't go larger unless you fit in one of the expceptions.
 
suemarkp said:
240.4(D) answers the question: "the overcurrent protection shall not exceed the following after any correction factors for ambient temperature and number of conductors have been applied".

So you can put a 15, 20, or 30A breaker on a #10 wire, You just can't go larger unless you fit in one of the expceptions.
Click to expand...

Thank you! The wording was tripping me up, as usual in the NEC. :D
 
Thanks - I didn't think he took the time to look it up that way, but rather that he had a history book of some sort that I would love to have. What dedication!
 
brother said:
I believe this was brought up before but I cant remember exactly. Does any one know why #12 was restricted to 20 amp breakers and #10 to 30 amp and # 14 to 15amp?? Im speaking of the 75 degree column of course. ;)
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My McGraw Hill NEC Handbook says it is because the next size up breaker does not provide sufficient short circuit protection.
 
mivey said:
Because of Fourier heat transfer calculations and the more complicated Neher-McGrath calculations.
Click to expand...

For those who have not delved into my other rants on this particular code section....

I did a bit of 'on-line' re-searching into the subject, and found a few interesting papers on the topic. In an earlier age I would have a book in hand I could ref, or at least grasp... But in the world of Internet information they were on real-read and scanned PDF's without text capture and could not be cut or pasted. And if I could remember where they were or even the search string that found them it would be helpful.....

ANYWAY the gist of what I read (Take my word for it - if you like - or find them yourself.) was that the Neher-McGrath calculations were originally intended for larger conductors specifically in duct banks, and shortly after became used for the 310 tables... However there was some debate by some about the feasibility of use of those calculations for smaller conductors by 'Insulated Cable Engineers Association' (ICEA) that the calculations were not suitable for use in smaller conductors in either overload or short circuit situations without damage to the conductor insulation - for those #10 and smaller... and flatly limited the use of those smaller conductors to specific amperage. Something - I DO NOT BELIEVE I AM ALONE IN THINKING.... (See page 3) However, some routinely exceed the limited values in some cases. (Normally where they believe 240.4G flatly supersedes 240.4D - not the case IMO.)

Recently Bussmann inserted code language to the NEC to allow #16 and 18 for certain applications like motor control centers - and go figure Bussmann makes fuse OCP for such things... :roll: In their RFP that made the changes in '08 - they make mention of and debate the "protection of *small* conductors". Trying to revisit the 'Insulated Cable Engineers Association' (ICEA) opinion, and Neher-McGrath calculations.

See page 21 for a look at a table by the 'Insulated Cable Engineers Association' (ICEA). Specifically the graph starts at #10, and the withstand is pretty low as the conductor is smaller. ​


Anyway I'm not going to go digging through the short circuit and AIC ratings of CB's - but I think it is a given that there is a dead zone in the case of smaller conductors for the average circuit breaker to clear faults without damage to the conductor. Especially those sized larger outside of 240.4(D) as the larger the load capacity of the CB, the CB will clear later... Although most manufactures also lump all CB's 15~30 together in AIC ratings for some reason... :roll:​

Anyway JMSO....​
 
e57 said:
However, some routinely exceed the limited values in some cases. (Normally where they believe 240.4G flatly supersedes 240.4D - not the case IMO.)
Click to expand...


are you talking about superseding without precise code rule, or something else entirely (not sure I understand )?
 
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