Why is 12 Gage THHN 20 amps

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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?
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So some of the Idiots out there in the field dont kill everyone????.
 
quogueelectric said:
So some of the Idiots out there in the field dont kill everyone????.
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Isn't that the purpose of the entire NEC?

I think the OPs question is why are three specific wire gauges limited to what's referred to as the small-conductor rule. Why aren't all the others treated the same way?
 
As electrical professionals, we are obliged to exert a little extra effort to protect the weakest amongst us (meaning the weakest equipment we work with). Given the name of the article, "small conductors," I think the answer lies somewhere along these lines.
 
We may never know why, perhaps Charlie is right on "small conductors".
I believe that a lot of rules in the NEC on fill adjustment, temp derating, 25 ohms, 6 ft, were determined a long time ago and that information is generally lost to us. It may exist in the IEEE archives.
 
brother said:
maybe because , 20 amp breakers are nicer looking than 30 amp breakers?
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What's worse is when you breaker up a panel, and the breakers you've got were made in two different plants. I hate having 30 breakers in a panel and 26 of the are black, and 4 are very dark gray.;)
 
iaov said:
Cool collection Ken. Whats realy impressive though is the fact that you can tell us the history of all this. Wow!
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Too bad I can't remember any of it.
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I did that bit of research a long time ago, and keep it on the computer for just such an instance.
 
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...

I haven't read all the posts in the thread, I hope I'm not repeating something. But I think it's because it's pretty much expected by the CMP's that 12 and 14 AWG circuits (I don't know how much my theory applies to 10 AWG) are rather frequently overloaded despite OCPD's, a lot of which are old or Fedral Pacific or whatever else. And the old fuses are goofed with a lot too.

It would be fun to put a 20, a 30 and a 40 amp load on 100', 200' and 300' of THHN and NM and measure the tempratures.
 
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