NM cable as feeders to HVAC equipment.

[desllc04]

Please clarify the confusion associated with using NM , (Romex) cable as a feeder to the outside unit for a HVAC system. There are many illustrations on the calculations for THWN conductors which allow for the maximum overcurrent device to be higher than the conductor rating listed in the NEC table 310.15(B)(16), but the code article for NM cable, (334.80) is very specific on the allowable ampacity and from what temperature column to use for such calculations, if I am reading this correctly.
Example: The nameplate information list the minimum circuit ampacity is 27.8 and the max overcurrent protective device, (breaker) as 40-amps. A number 10 AWG NM cable is run from the service panel, which satisfies the min requirements, to the non-fusible disconnect at the unit. Is the overcurrent device overrating the NM cable if a 40-amp breaker is used?

 

[Dsg319]

Please clarify the confusion associated with using NM , (Romex) cable as a feeder to the outside unit for a HVAC system. There are many illustrations on the calculations for THWN conductors which allow for the maximum overcurrent device to be higher than the conductor rating listed in the NEC table 310.15(B)(16), but the code article for NM cable, (334.80) is very specific on the allowable ampacity and from what temperature column to use for such calculations, if I am reading this correctly.
Example: The nameplate information list the minimum circuit ampacity is 27.8 and the max overcurrent protective device, (breaker) as 40-amps. A number 10 AWG NM cable is run from the service panel, which satisfies the min requirements, to the non-fusible disconnect at the unit. Is the overcurrent device overrating the NM cable if a 40-amp breaker is used?

No it’s perfectly fine, you follow the nameplate data for HVAC. Just like using THWN but you are stuck with 60degree ampacities with NM.

#10NM (NM has to be sized at 60degree column) is good for 30amperes which satisfies your min ampacity requirement.

Remember there is a motor in that unit which is why the breaker is oversized a bit.

Romex if ran all the way to the unit pass the disconnect being outdoors is another can of worms.

 

[infinity]

A number 10 AWG NM cable is run from the service panel, which satisfies the min requirements, to the non-fusible disconnect at the unit. Is the overcurrent device overrating the NM cable if a 40-amp breaker is used?

No the #10 conductors rated at 30 amps can be protected by a 40 amp OCPD because the OCPD is only providing ground fault and short circuit protection.

 

[kwired]

General purpose - you must protect conductors in accordance to their ampacity.

240.3 along with Table 240.3 directs you to other articles for specific applications, first item in the table is air conditioning and refrigerating equipment - overcurrent protection rules for this equipment is art 440.

This sort of equipment has two different protection methods involved, short circuit and ground fault protection and motor overload protection. Between the two the conductor is still protected sufficiently but if you just use the general rules of art 240 you may have trouble holding the branch circuit device when motors start up.

 

[don_resqcapt19]

Please clarify the confusion associated with using NM , (Romex) cable as a feeder to the outside unit for a HVAC system. There are many illustrations on the calculations for THWN conductors which allow for the maximum overcurrent device to be higher than the conductor rating listed in the NEC table 310.15(B)(16), but the code article for NM cable, (334.80) is very specific on the allowable ampacity and from what temperature column to use for such calculations, if I am reading this correctly.
Example: The nameplate information list the minimum circuit ampacity is 27.8 and the max overcurrent protective device, (breaker) as 40-amps. A number 10 AWG NM cable is run from the service panel, which satisfies the min requirements, to the non-fusible disconnect at the unit. Is the overcurrent device overrating the NM cable if a 40-amp breaker is used?

NM can't be installed outside.

 

[kwired]

NM can't be installed outside.

True.
Generally speaking it will be acceptable to run NM to the disconnect that is typically mounted on the wall of the structure and then change over to a different wiring method to go to the HVAC unit.

That NM cable ran inside is still limited to 60C ampacity, but overcurrent protection can possibly be more than 30 amps for this sort of equipment

 

[suemarkp]

Should have quoted the message as it appears to be have been deleted. Was responding to ToryRutter message above (which is now gone) not kwired's response.

Yes it can if used for one of the items listed in 240.4(G). HVAC is one of those items. The conductor must be rated for the MCA of the HVAC unit. The breaker can be rated higher than that but no higher than the max breaker value listed on the HVAC nameplate. Overload is handled within the compressor, so having only short circuit and ground fault protection is sufficient.

 

[Dennis Alwon]

No, a 40 amp OCPD cannot protect #10 conductors rated at 30 amps because it only offers ground fault and short circuit protection.

I think you mean it can be used for the reason you stated. The overload protection in the motor will handle the over current.

 

[tom baker]

I was going to post a Mike Holt graphic but that incorrect post is now gone.