I would like your thoughts on proper wiring for a split system air conditioner. We have installed several this summer with very clear instructions and diagrams. However the last one purchased was from a different manufacturer and I want to be sure I'm doing the right thing. The unit pulls 25 FLA and manufacturer call for min C.B. of 40A and max fuse of 40A. Per their installation manual we are to use a 40A breaker with #10 wire. I would not normally use #10 with a 40A breaker. I have visted with some of my local resources (1 elec engineer and 2 master elec/contractors) and rcvd 3 different opinions as to wire size/ circuit breaker size. I would appreciate you input on this. Any code references would be helpful.
Split system A/C
- Thread starter nollkeith
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LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Check out Art. 440.
Wire is sized per the MCA rating and the breaker is sized between the min and max breaker/fuse specified. If you have an FLA but no MCA on the nameplate, then you need to calculate that which is basically FLA of the compressor + FLA of outside fan motor + 25% of the compressor FLA. There is another rule in 440 that tells you how large the breaker can be (similar to the motor rules of NEC 430).
Article 440 tells you all that, but not very plainly. It does help to know that just about every split AC has a built in thermal protection circuit. This is what prevents the wire from melting of there is an overload in the compressor and why the breaker can be sized more than the ampacity of the wire. Just about every manufacturer does these calcs for your now days with the MCA and breaker size labels on the nameplate.
If the MCA turns out to be 25A, you could even use #12 copper if the instructions didn't say otherwise. This is because of 240.4(D) and the reference of NEC 440 in table 240.4(G).
Article 440 tells you all that, but not very plainly. It does help to know that just about every split AC has a built in thermal protection circuit. This is what prevents the wire from melting of there is an overload in the compressor and why the breaker can be sized more than the ampacity of the wire. Just about every manufacturer does these calcs for your now days with the MCA and breaker size labels on the nameplate.
If the MCA turns out to be 25A, you could even use #12 copper if the instructions didn't say otherwise. This is because of 240.4(D) and the reference of NEC 440 in table 240.4(G).
- Location
- New Jersey
- Occupation
- Journeyman Electrician (retired)
All the information needed to install this unit should be on the nameplate. You are required by Article 440 to use this information and not a calculation. The MCA (minimum circuit ampacity) is used to size the conductors. As Mark mentioned the requirements of 240.4(D) do not apply to AC units under Article 440. You would size the conductors using table 310.16. If you terminations are all 75 degree rated (very likely) than you can use the 75 degree column of the table. So for a MCA of 20 amps #14 THHN is permitted, for an MCA of 25 amps #12 THHN is permitted and for an MCA of 35 amps #10 THHN is permitted. The maximum CB or fuse size does not have to be sized according to the size of the conductors since it is only providing ground fault and short circuit protection. So if you MCA is 25 amps, #12 THHN with a 40 amp CB or fuse is permitted. The maximum fuse or CB size can be any size between the MCA and the maximum OCPD value listed on the unit. One last thing, if you're using NM cable than you must use the 60 degree column for conductor sizing. For #14 and #12 conductors this does not change the ampacities listed above.
Jim W in Tampa
Senior Member
- Location
- Tampa Florida
Do not overlook voltage drop if this run is very long
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