Why is 12 Gage THHN 20 amps

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tom baker

tom baker

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The ampacity of 12 gage THHN is 30 amps, per 310.16, but

240.4(D) Small Conductors. Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied.
(3) 14 AWG Copper. 15 amperes
(5) 12 AWG Copper. 20 amperes
(7) 10 AWG Copper. 30 amperes

Most electricans then state the ampacity of 12 AWG THHN is 20 amperes, when its actually 30.

This provision has been in the NEC for a long time, its only for 14,12 and 10 AWG, 18 and 16 were added in the 2008 NEC.

Why does the NEC restrict the overcurrent protection on these small conductors?
 
Tom is right. The ampacity is 30. That is the amount of current the wire could take, without risking damage to its insulation. The fact that we have to protect it at 20 amps does not alter the fact that the ampacity is 30. Those are two separate concepts. The fact that we have to limit its use to a value driven by the terminal provisions also does not alter the fact that the ampacity is 30. That is a third, still separate, concept.

Now, back to Tom's question. I don't know. (How often do you get a complete and honest answer from an engineer? :D )
 
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 1923, it was Table I to Rule 610.

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.

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.

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).




Now the question becomes: What was different about RHH?
 
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tom baker said:
Sparky - where is your information from?
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Codebooks.

Feb2009small.jpg
 
steve66 said:
That's a pretty impressive collection!!

Are you missing anything???
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An original 1897 (mine is a reprint), 1899, 1903, 1904, 1905, 1907, 1909, 1913, 1915, 1917, 1942, 1943, 1945 (war years..... paper rationing and all), 1954 and 1958.

Got any you want to get rid of?
leer.gif
 
RICK NAPIER said:
That is an impressive library. How long did it take to acquire?
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Well, let's see.... I started with the 1896 right after Willy McKinley got elected.......... Utah had just been granted statehood, the Post Office began rural delivery, and flickers had just got started.... you young-uns call 'em movies.

Seriously..... Everything from the 1987 on is one I bought for work.... the rest I've slowly acquired in the past 4 years or so.
 
tom baker said:
The ampacity of 12 gage THHN is 30 amps, per 310.16, but

240.4(D) Small Conductors. Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied.
(3) 14 AWG Copper. 15 amperes
(5) 12 AWG Copper. 20 amperes
(7) 10 AWG Copper. 30 amperes

Most electricans then state the ampacity of 12 AWG THHN is 20 amperes, when its actually 30.

This provision has been in the NEC for a long time, its only for 14,12 and 10 AWG, 18 and 16 were added in the 2008 NEC.

Why does the NEC restrict the overcurrent protection on these small conductors?
Click to expand...




The 3 different size conductors referenced in 240.4(D) are the 3 sizes of conductors that are most installed. Due to that, there is some cushion built into the safety margine for those conductors.
 
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Pierre C Belarge said:
The 3 different size conductors referenced in 240.4(D) are the 3 sizes of conductors that are most installed. Due to that, there is some cushion built into the safety margine for those conductors.
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That would be my guess too. Old houses where you can't even read the insulation type may end up being incorrectly protected by an OCPD too large. By keeping it simple there would be no confusion and less danger.
 
I think they gave thought to how much it could go to before tripping a breaker and came up with the numbers. Back in 56 i tried telling them they were wrong but being i was only 8 at the time they didn't listen to me.
 
realolman said:
480 sparky... you are da man.:smile:
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Yep, Sparky is my hero on this forum, He's knowledgable, doesnt lunge out at others too much, does good work, and he seems to be a real electrician, he's more about the craft than the beans............SaaLuTe
 
I've alwys been taught the reasoning goes back to 110.14 and 310.15(B)(2).
Most 20 amp devices are rated for 60? thus the 25 amp on #12.
Add to that, in many commercial applications you 4 or more cc condcutors in a conduit giving you a derated amapcity of 24 amps.
With these two situations taken into consideration thre CMP decided to limit the overcurrent device to 20 amps.
I have no old ROPs to support that, but that explaination was given me by some CMP members.
 
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