Ampacity Rules for 12 gauge THHN Wire

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JonathanEngr

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North Carolina, USA
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Professional Engineer
Hi, everyone. I have a circuit that was installed a few years ago, and the wire is 12 gauge THHN wire in conduit with two conductors, a neutral and a ground. The wires are connected to a 30A breaker, and I've been told that's acceptable since there's no reduction due to there being less than 3 conductors in the conduit. At the same time I've been told that the 30A rating in the NEC manual is for derating purposes only, and that 12 gauge wire can only have a 25A breaker max. I was also told by others that under no circumstances can 12 GA wire be used with more than a 20A breaker.

Can someone de-mystify the NEC code and allowable current in the wire? Thanks!
 
Refer to 240.4(D) for overcurrent protection limitations. It's complicated "It depends on what the meaning of the word IS is".
 
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Art 240 restricts #12 to a 20 amp over current device EXCEPT when installed on equipment covered in 240(G) such as motors or HVAC equipment
 
There are two of these (two sets of conduit with two conductors, one neutral and one ground as stated above). One is connected to a 3HP rotary valve and the other is connected to a 1/3 HP cleaning hood.
 
JonathanEngr said:
There are two of these (two sets of conduit with two conductors, one neutral and one ground as stated above). One is connected to a 3HP rotary valve and the other is connected to a 1/3 HP cleaning hood.
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There is no way that the circuit to the 1/3 hp cleaning hood is complaint.
Would need to know the voltage and if the 3 hp motor is 3 phase or single phase to know if that circuit is compliant.
 
JonathanEngr said:
There are two of these (two sets of conduit with two conductors, one neutral and one ground as stated above). One is connected to a 3HP rotary valve and the other is connected to a 1/3 HP cleaning hood.
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The 3 hp motor would depend on the motor itself. If they have overload protection built in, then the overcurrent protective device is only there for ground fault or short circuit, not for overcurrent protection.
 
don_resqcapt19 said:
There is no way that the circuit to the 1/3 hp cleaning hood is complaint.
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If it's 120V single phase, the Table 430.428 FLC is 7.2A, 250% of that is 18A, which does not correspond to a standard size, so 430.52(C)(1) Exception 1 allows going to the next standard size up, or 20A. Yes?

Cheers, Wayne
 
JonathanEngr said:
There are two of these (two sets of conduit with two conductors, one neutral and one ground as stated above). One is connected to a 3HP rotary valve and the other is connected to a 1/3 HP cleaning hood.
Click to expand...
So because these are motor loads:

(a) the 20A OCPD limit on #12 in 240.4(D) does not apply (motor loads are listed in 240.4(G))
(b) the ampacity of the #12 conductors will be 20A if either the breaker or the load has 60C terminations, or (more likely) 25A if they both have 75C terminations
(c) if the motor has separate overload protection, the breaker is just providing short circuit and ground fault protection and may be sized higher than the ampacity of the conductor.

Cheers, Wayne
 
wwhitney said:
If it's 120V single phase, the Table 430.428 FLC is 7.2A, 250% of that is 18A, which does not correspond to a standard size, so 430.52(C)(1) Exception 1 allows going to the next standard size up, or 20A. Yes?

Cheers, Wayne
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But the OP says the conductors are on 30 amp breakers.
 
No one can give you the correct answer without the correct information.

1. So both motors are single phase.?
2. What is the voltage on each motor?
3. do either of the motors have overload protection (thermal overload) built in?
 
JonathanEngr said:
So I just need to change the breakers to 20A and see if the breaker is tripped? And if so, just have new 10 GA wire pulled? And the circuits are all single phase.
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We don't have enough information about the 3 hp motor to answer this. It is possible, that 12 AWG on a 30 amp breaker is permitted by the code.
 
Ah! So it is a lot more complicated than I thought :unsure: Let me get the information on the components and get back to you. It will be early next week since I'm currently out of town. I really appreciate all of the time everyone has taken at this point to answer my question.
 
JonathanEngr said:
Ah! So it is a lot more complicated than I thought :unsure: Let me get the information on the components and get back to you. It will be early next week since I'm currently out of town. I really appreciate all of the time everyone has taken at this point to answer my question.
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No problem. With the NEC often the devil is in the details.
 
wwhitney said:
If it's 120V single phase, the Table 430.428 FLC is 7.2A, 250% of that is 18A, which does not correspond to a standard size, so 430.52(C)(1) Exception 1 allows going to the next standard size up, or 20A. Yes?

Cheers, Wayne
Click to expand...
Yes, but conductor size could actually be 14 AWG with that scenario and still be on the 20 amp breaker.
 
Don't forget what are the temperature rating of terminals? A #12 THHN is max rating of 30A but if the terminals are only rated to 75o then the same #12 is only good to 25A. and If they are 60o Then 20A.
 
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