MCA & MOCP Breaker and Wire Size Calculator

MCA is Minimum Circuit Ampacity, the manufacturer's computed minimum conductor ampacity for the unit under NEC 440.4(B) and the UL 1995/UL 60335-2-40 listing standards: 125 percent of the largest motor's rated load amps plus the sum of all other concurrent loads. Its only job is to size the wire. Pick a conductor from NEC Table 310.16 whose ampacity meets or exceeds the MCA, using the column that matches the termination temperature rating, 75 C on most modern equipment per NEC 110.14(C). MCA is not the running current and not the breaker size.

MOCP is Maximum Overcurrent Protection, the largest fuse or breaker the manufacturer permits for the unit, listed on the nameplate per NEC 440.4(B) and limited by NEC 440.22. Install the largest standard rating from NEC 240.6(A) that does not exceed the MOCP; going one size over it is a code violation and voids the listing per NEC 110.3(B). A smaller breaker is legal as long as the unit can start and run without nuisance trips, and the practical floor is the smallest standard size at or above the MCA.

Because on a hermetic refrigerant motor-compressor circuit the breaker only provides short-circuit and ground-fault protection. Running overload protection comes from the compressor's internal or external overload devices per NEC 440.52, and the manufacturer verified during listing that the MOCP-rated device still protects an MCA-sized conductor under fault conditions. NEC 240.4(G) explicitly exempts Article 440 circuits from the small-conductor breaker limits in 240.4(D), which is why a 14 or 12 AWG conductor on a 35 A breaker is legal here and nowhere else in a house.

No. This is the most common field mistake with nameplate sizing. The 125 percent factor on the largest motor is already built into the MCA by the manufacturer per NEC 440.4(B) and 440.33. Applying NEC 210.20(A) continuous-load math on top of MCA double-counts the factor and lands you one or two wire sizes heavy. The conductor simply needs ampacity greater than or equal to the printed MCA.

HACR stands for Heating, Air Conditioning, and Refrigeration, a UL listing category confirming the breaker can handle the group locked-rotor and cycling profile of HVAC equipment. Where the equipment listing calls for HACR protection, NEC 110.3(B) makes it mandatory. In practice virtually every modern molded-case residential and commercial breaker sold since around 1990 carries the HACR marking, so this is a checkbox, not a special purchase; just confirm the marking on older panels.

Not unless the manufacturer says so. If the nameplate specifies fuses without mentioning a circuit breaker or HACR breaker, the listing was established with fuses and NEC 110.3(B) requires following it, so install a fused disconnect at the unit. Most nameplates from the last few decades read Max Fuse/Ckt Bkr or Max Fuse or HACR Breaker, which permits either device. When in doubt, the installation instructions for the model number control.

Indoors, yes, but with two catches. First, NEC 334.80 limits NM cable ampacity to the 60 C column of Table 310.16 no matter what the conductor insulation is rated, so an MCA of 19.9 A needs 12 AWG copper NM (20 A at 60 C) even though 14 AWG THHN in conduit would satisfy the 75 C column. Use this calculator's 60 C termination toggle for NM. Second, NM cannot be used outdoors or in wet locations (NEC 334.12), so the run typically transitions to THWN in a liquidtight whip or UF cable for the final outdoor stretch to the disconnect and unit.

First choice: look up the manufacturer spec sheet by model number, because the published MOCP governs. If the model tag is also gone, NEC 440.22(A) is the fallback the manufacturer used: the device is sized at up to 175 percent of the compressor rated load amps (plus other loads per 440.22(B)), and up to 225 percent where needed for starting. Switch this calculator to the From motor RLA/FLA mode to compute the MCA from the compressor RLA and fan FLA per NEC 440.33, and use the smallest standard breaker at or above that MCA as a conservative choice until the real MOCP is confirmed.

Breaker: 35 A, since 35 A is itself a standard rating in NEC 240.6(A) and may not be exceeded. Wire: with 75 C terminations and THHN/THWN-2 copper in conduit, 14 AWG (20 A at 75 C in Table 310.16) technically satisfies the 19.9 A MCA, and NEC 240.4(G) makes the 35 A breaker legal on it. With NM cable, or 60 C terminations, you need 12 AWG copper (20 A at 60 C). Most installers run 12 AWG copper minimum for condensers regardless, which also buys voltage drop margin on longer runs.