ampacity of # 12 THHN copper AWG.

[reyamkram]

Per table 310.15 (B) (16)----under the 75 deg column #12 can carry 25 amps, But, it has 2 asterisk symbols, and refers you to 240.4 (D) (5)--12 AWG copper 20 amp.
I understand that #12 THHN copper at 75 deg, can only handle 20 amps. per the NEC. Question, I am interpreting that correctly, and anything higher like 25 or 30
amp breaker would be in violation of the NEC, and pose a probable electrical hazard???.

Thank you, for any and all formation.

 

[electrofelon]

Per table 310.15 (B) (16)----under the 75 deg column #12 can carry 25 amps, But, it has 2 asterisk symbols, and refers you to 240.4 (D) (5)--12 AWG copper 20 amp.
I understand that #12 THHN copper at 75 deg, can only handle 20 amps. per the NEC. Question, I am interpreting that correctly, and anything higher like 25 or 30
amp breaker would be in violation of the NEC, and pose a probable electrical hazard???.

Thank you, for any and all formation.

Yes that "small conductor" provision limits the OCPD to 20 amps for general uses, however that would not apply for motor/hvac loads

 

[roger]

See the notes below Table 310.15(B)16 specifically 240.4(D)

Roger

 

[infinity]

You are permitted to use the 25 amp rating of the ##12 conductors for certain installations so IMO it is not unsafe to do so even if the conductor ampacity is limited to 20 amps by 240.4.

 

[winnie]

Another way to think of it: The _ampacity_ of a conductor is only one of the factors you must consider. You have to separately consider the allowed overcurrent protection.

For #12 THHN the ampacity is 25A at 75C. However for general purpose circuits you are only permitted to protect #12 conductors with a 20A breaker.

In other circumstances, you might be permitted to protect the #12 with a 25A or even higher rated breaker (especially for motor circuits) but the ampacity remains 25A with 75C temperature limits.

For things like residential services you might encounter the opposite condition, for example #4 Cu MC cable has a 75C ampacity of 85A, but you can protect it with a 100A breaker.

-Jon

 

[LarryFine]

Conductor temperature and current ratings are based on protection from insulation damage, not conductor damage.