MCA & MOCP Calculator

How MCA and MOCP are sized under Article 440

Air-conditioning and refrigeration equipment uses a hermetic refrigerant motor-compressor whose locked-rotor and starting currents are far above its running current. Article 440 handles that the same way Article 430 handles motors: the conductor is sized for a modest margin over the running current, while the branch-circuit short-circuit and ground-fault device is allowed to sit much higher so it rides through starting without opening. The compressor is virtually always the largest load in the unit, so it takes the 125 percent (conductor) and 175 percent (device) multipliers, and every other load is added at 100 percent.

The minimum circuit ampacity is the conductor requirement: MCA equals 1.25 times the largest compressor rated-load current plus the sum of the other loads. Pick the smallest conductor whose Table 310.16 ampacity, after any ambient and conductor-count derating and the 110.14(C) termination cap, is at least the MCA. Because NEC 240.4(G) sends these conductors to 440.22 for overcurrent protection, the 240.4(D) small- conductor limits do not apply, so the device on the circuit can exceed the usual 15, 20, or 30 A caps for 14, 12, and 10 AWG.

The maximum overcurrent protection is the device ceiling: MOCP equals 1.75 times the largest compressor rated-load current plus the other loads. Since 440.22 permits a device not exceeding that value, the standard breaker or fuse is the largest 240.6(A) rating at or below it, so you round down. Only where a device sized at 175 percent will not carry the starting current may the rating be increased, and never past 225 percent. Where the nameplate marks a maximum device (Max Fuse or HACR Breaker, or MOP), that tested value governs.

Article 440 multipliers

The device rounds down to the largest standard 240.6(A) rating at or below the 175 percent value. The 225 percent figure is an absolute ceiling used only where the smaller device will not carry the motor starting current.

Worked example: 24 A compressor, no other loads

1. MCA: 24 A x 1.25 = 30 A. The smallest copper conductor that carries 30 A at the 75 degree C column of Table 310.16 is 10 AWG (35 A); at 60 degrees C it is also 10 AWG (30 A).
2. MOCP value: 24 A x 1.75 = 42 A. That is not a standard rating, so the device rounds down to the largest standard breaker at or below it, 40 A.
3. Starting-current ceiling: 24 A x 2.25 = 54 A. Only if a 40 A device will not start the compressor may the breaker be raised, up to the 50 A standard rating at or below 54 A.
4. Result: 10 AWG copper on a maximum 40 A breaker, with 50 A the absolute ceiling. If the nameplate marks a lower MOP, use the nameplate value.

Frequently asked questions

What is the difference between MCA and MOCP?

MCA sets the conductor: 125 percent of the largest compressor rated-load current plus the other loads (440.33). MOCP caps the breaker or fuse: up to 175 percent of the compressor plus the other loads (440.22), raised to 225 percent only where needed for starting current. Conductor size follows MCA; the device is limited by MOCP.

Do I round MOCP up or down?

Down. NEC 440.22 permits a device not exceeding 175 percent, so you take the largest standard 240.6(A) rating at or below the calculated value. A 42 A result becomes a 40 A device, not 45 A. The only path higher is the 225 percent starting-current allowance.

Why can the wire be smaller than usual?

NEC 240.4(G) protects these conductors under 440.22 rather than the 240.4(D) small-conductor rule, so the 15, 20, and 30 A caps on 14, 12, and 10 AWG do not apply. The conductor is sized on ampacity against the MCA alone.

Is RLA the same as the horsepower full-load current?

No. Article 440 uses the compressor rated-load current (RLA), or the branch-circuit selection current where greater, marked on the equipment nameplate per 440.6(A). The Article 430 horsepower tables (430.248 and 430.250) are for general motors, not hermetic compressors. Use the Motor Breaker Sizing calculator for those.

Does this size the compressor overload?

No. This tool sizes the branch-circuit short-circuit and ground-fault device and the conductor. Motor-compressor running-overload protection (440.52) is a separate, usually factory-set device and is out of scope, as are available fault current and arc flash.

Related calculators

• Motor Breaker Sizing Calculator: motor branch-circuit protection under NEC Article 430.
• Breaker Size Calculator: general-use branch and feeder loads under 210.20(A) and 240.6(A).
• Wire Ampacity Calculator: Table 310.16 ampacity with ambient and conductor-count derating.
• Equipment Grounding Conductor Size Calculator: EGC minimum size from the overcurrent device rating (Table 250.122).