i need help understanding a correction

[Treg the electrician]

this i a correction i am dealing with .
we have a heat pump with a MCA 21 max OCP 25 .

I ran #12 THHN and put it on a 25 amp breaker. The following is what he stated when i questioned the correction. if im wrong could somebody please educate me here.







While Table 310.16’s ampacities are pretty much as you stated, the limitations we must comply with are the foot notes to the table identified by the asterisk (*) that appears next to the conductor sizes on the left. Here we read that #10 is to be protected at 30 amps, #12 is to be protected at 20 amps and #14 is to be protected at 15 Amps



Since your min ckt ampacity is (if I recall correctly) around 21 amps and your MOCP is 25 Amps as listed on the unit’s label you sized the OCP correctly. However, this places your #12 conductors over the 20 Amp OCP stated on the table footnote. Your will need to install #10 Cu to replace the #12.

 

[wwhitney]

The correction is incorrect.

The footnote to 310.16 says to see 240.4(D). 240.4(D) has the 20A limit for #12 conductors, however, the text at the beginning of 240.4(D) reads "Unless specifically permitted in 240.4(E) or (G), . . ." So if 240.4(G) specifically permits it, you can exceed 20A on #12 conductors. And 240.4(G) includes the reference to "Air-conditioning and refrigeration equipment circuit conductors" which need to obey Article 440, Parts III and IV. So since you have an Article 440 application, 240.4(D) does not apply to your circuit conductors.

Cheers, Wayne

 

[infinity]

If the #12 conductors are in a raceway they are rated in this application for 25 amps so you're code complaint. If you use NM cable then you'll need #10 AWG conductors.

 

[augie47]

While I agree with the other posts you might wait on input from some of our Washington members as they have their own Code which at times differs from the NEC

 

[Treg the electrician]

ok thank you for the info im going to fight it . i will keep you posted....

 

[Treg the electrician]

i will wait until tomorrow thanks again

 

[infinity]

ok thank you for the info im going to fight it .

You should. #12 THHN has an ampacity of 25 amps @ 75°C. With a MCA of 21 amps your conductors are large enough. Wayne has given you the applicable code sections which allow you to ignore exactly what the correction said that you've violated.

 

[Joe Villani]

Double check the labeling on the liquidtight or carflex to the unit.

Most have a temp rating of 60 degrees C when used in wet locations

 

[infinity]

Double check the labeling on the liquidtight or carflex to the unit.

Most have a temp rating of 60 degrees C when used in wet locations

I don't think that was even considered because the correction notice makes no mention of the raceway.

 

[Joe.B]

 

[Treg the electrician]

i responded to the inspector with the info you guys gave me and this was his response , keep in mind i have 12 THHN on a 25 amp breaker going to a heat pump......

General consensus and commentary differ from your interpretation:

• Small Conductor Rule: NEC Section 240.4(D)(5) explicitly limits the overcurrent protection for #12 AWG copper conductors to 20 amperes.
• Ampacity vs. Protection: While #12 THHN wire itself has a 90°C ampacity rating of 30 amps, it can only be loaded to 25 amps at 75°C, according to Nassau National Cable. NEC restricts the overcurrent protection of these conductors to 20 amps in most situations.
• Heat Pump Application: Although heat pump circuits are considered continuous loads and require specific sizing calculations (minimum circuit ampacity - MCA), the basic rule of protecting #12 copper wire at 20 amps still applies unless specifically permitted by exceptions in NEC 240.4(E) or (G). These exceptions typically relate to specific conductor applications or tap conductors, and generally do not override the small conductor rule for branch circuits.

 

[Treg the electrician]

i normally dont argue with inspectors very much , i get corrections like everybody else but this just seems stupid to me . ive done literally thousands of these circuits over the years and ive never seen this correction. its a bid deal to do this one over again


please advise

 

[infinity]

i normally dont argue with inspectors very much , i get corrections like everybody else but this just seems stupid to me . ive done literally thousands of these circuits over the years and ive never seen this correction. its a bid deal to do this one over again


please advise

The code is clear on this one and there is no margin for interpretation on the inspectors part.

 

[don_resqcapt19]

i responded to the inspector with the info you guys gave me and this was his response , keep in mind i have 12 THHN on a 25 amp breaker going to a heat pump......

General consensus and commentary differ from your interpretation:

• Small Conductor Rule: NEC Section 240.4(D)(5) explicitly limits the overcurrent protection for #12 AWG copper conductors to 20 amperes.
• Ampacity vs. Protection: While #12 THHN wire itself has a 90°C ampacity rating of 30 amps, it can only be loaded to 25 amps at 75°C, according to Nassau National Cable. NEC restricts the overcurrent protection of these conductors to 20 amps in most situations.
• Heat Pump Application: Although heat pump circuits are considered continuous loads and require specific sizing calculations (minimum circuit ampacity - MCA), the basic rule of protecting #12 copper wire at 20 amps still applies unless specifically permitted by exceptions in NEC 240.4(E) or (G). These exceptions typically relate to specific conductor applications or tap conductors, and generally do not override the small conductor rule for branch circuits.

That is not correct. 240.4(G) specifically refers to Article 440, Parts III, IV, and V. Article 440 permits you to size the conductor to the minimum circuit ampacity shown on the nameplate and to provide protection at the maximum overcurrent protection also shown on the nameplate.
440.35 in Part IV states that the minimum ampacity of the conductors is the minimum circuit ampacity as shown on the nameplate.
440.22(C) in Part III specifies that the branch circuit short circuit and ground fault protection shall not exceed the maximum overcurrent protective device shown on the nameplate.
These two clearly permit a 12 AWG rated at 75°C to be protected by a 25 amp overcurrent protective device based on the minimum circuit ampacity of 21 amps and the maximum OCPD of 25 amps shown on the nameplate.

 

[Treg the electrician]

you people are awsome! thank you for your guidance

 

[Joe.B]

Go to his boss.

 

[infinity]

It's pretty simple if you break it down:

Table 310.16, #12 THHN=25 amps@75° C (the temperature rating of the terminals)
240.4(D)(5) #12 limited to 20 amp unless covered under 240.4(G)
Heat Pumps fall under 240.4(G) so you can ignore the 20 amp limitation in 240.4(D) and use the ampacity from Table 310.16.

240.4(G) Overcurrent Protection for Specific Conductor Applications.
Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G).

 

[letgomywago]

Even though you're right everything I've done with air conditioners and mini splits I do in 10awg for future proofing. Lots of guys do this even though is overkill and he might just be used to it.