MCA vs MOCP: AC & Heat Pump Circuit Sizing Calculator (NEC 440)
Reading time: ~8 min read
📋 Key Takeaways
- MCA sizes the wire; MOCP sizes the breaker. Minimum Circuit Ampacity (MCA) is the conductor floor — 125% of the compressor's rated-load current plus all other loads. Maximum Overcurrent Protection (MOCP) is the breaker ceiling — up to 225% of the compressor RLA plus other loads, per NEC 440.22(A).
- The nameplate wins. NEC 440.4(B) requires the manufacturer to mark MCA and MOCP on the unit, and NEC 110.3(B) makes those markings enforceable. Field-calculate only when the plate is missing or illegible.
- A hermetic AC circuit is the legal exception everyone asks about: yes, a 14 AWG copper conductor (20A ampacity) can land on a 25A or 30A breaker, because the breaker only provides short-circuit/ground-fault protection while the compressor's internal overload handles running current (NEC 240.4(G) → Article 440).
- MOCP is a maximum, not a target — installing the smallest standard breaker that holds starting current is fine, and often kinder to the compressor on repeated faults.
- Size the equipment grounding conductor from the breaker rating per NEC Table 250.122 — the calculator returns it with the wire and breaker.
Every condenser nameplate carries two numbers that confuse even experienced installers: MCA and MOCP. They're not interchangeable, they're not the load, and picking the wrong one for the wrong component either fails inspection or leaves a conductor under-protected. The calculator below takes the compressor rated-load amps (RLA), the fan full-load amps (FLA), and any electric heat, then computes MCA per NEC 440.32/440.33, MOCP per NEC 440.22(A), the minimum conductor at 75°C, and the equipment grounding conductor per NEC 250.122.
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MCA / MOCP Calculator
Rated-load amps from the compressor or unit nameplate — not LRA.
Condenser fan full-load amps. Enter 0 for mini-split outdoor units where the plate includes it.
Ampacities from NEC 310.16 at the 75°C column.
Minimum circuit ampacity (MCA): —
Minimum conductor (75°C, from MCA): —
Max overcurrent device (MOCP): —
Equipment grounding conductor (Cu, NEC 250.122): —
Sizing logic: MCA = 125% × compressor RLA + fan FLA + electric heat per NEC 440.32/440.33; MOCP = 225% × compressor RLA + other loads per NEC 440.22(A), rounded down to the next lower standard rating per NEC 240.6(A) when the result isn't standard; conductor from NEC 310.16 (75°C) against MCA; EGC per NEC Table 250.122 from the MOCP value. The equipment nameplate markings govern under NEC 440.4(B)/110.3(B) — use this when the plate is missing or to check a submittal. Informational — the AHJ governs.
Typical Residential Condensers: MCA, MOCP, and Wire
Representative single-speed and inverter condensers at 208/230V single-phase (always verify against the actual nameplate — efficiency class moves these numbers):
| Capacity | Typical compressor RLA | MCA (calc) | MOCP (max) | Min. copper (75°C) | Common install |
|---|---|---|---|---|---|
| 1.5 ton | 9 A | 12.3 A | 20 A | 14 AWG | 14 AWG / 20 A |
| 2 ton | 12 A | 16.0 A | 25 A | 14 AWG | 14 AWG / 25 A |
| 2.5 ton | 14 A | 18.5 A | 30 A | 14 AWG | 12 AWG / 30 A |
| 3 ton | 16 A | 21.0 A | 35 A | 12 AWG | 12 AWG / 35 A |
| 4 ton | 20 A | 26.0 A | 45 A | 10 AWG | 10 AWG / 45 A |
| 5 ton | 25 A | 32.3 A | 50 A | 10 AWG | 10 AWG / 50 A |
Read the 2-ton row carefully, because it's the one that gets red-tagged by installers who don't know Article 440: 14 AWG copper has a 75°C ampacity of 20A, the MCA is 16A (satisfied), and the MOCP is 25A — a combination that is perfectly legal on a hermetic refrigerant circuit even though NEC 240.4(D) would forbid it on a general branch circuit. The conductor is protected against overload by the compressor's internal thermal protector; the breaker only clears short circuits and ground faults.
Where the Numbers Come From: NEC 440 in Three Steps
Step 1 — MCA (NEC 440.32/440.33). For a single hermetic motor-compressor with other loads, the branch-circuit conductor ampacity must be at least 125% of the larger motor-compressor rated-load current, plus the sum of the other loads (fan motors, crankcase heater, electric heat). That's the thermal floor for the wire — the number the conductor table answers to.
Step 2 — MOCP (NEC 440.22(A)). The short-circuit and ground-fault protective device must not exceed 175% of the motor-compressor rated-load current plus other loads; where that's not sufficient for starting current, it may go up to 225% — and if the 225% value doesn't correspond to a standard rating, you round down to the next lower standard size (going down keeps you under the 225% ceiling). Manufacturers typically list MOCP at or near the 225% value so the breaker holds across voltage dips during starting.
Step 3 — Reconcile with the nameplate. NEC 440.4(B) puts the manufacturer's marked MCA and MOCP in charge, and NEC 110.3(B) makes the listing enforceable. When the plate says "MCA 21.6 / Max Fuse 35," you pull wire for 21.6A and install any standard breaker or fuse at or below 35A. Field math is for missing plates, design verification, and catching submittal errors.
Fused Disconnects, Whips, and the Rest of the Circuit
The outdoor disconnect follows the MOCP: a fused disconnect takes fuses at or below the MOCP value, while a non-fused pullout is fine whenever the breaker in the panel handles protection. For the whip, use conductors with an ampacity ≥ MCA — 10 AWG THHN-2 in 3/4" liquidtight covers most 3–5 ton residential units. Don't forget the EGC sized from the breaker rating per NEC 250.122, and torque every lug to its marking per NEC 110.14 — our lug torque chart has the values by brand.
Frequently Asked Questions
What is MCA on an AC nameplate?
MCA — Minimum Circuit Ampacity — is the smallest conductor ampacity the manufacturer allows for the unit's branch circuit. It's calculated as 125% of the compressor's rated-load current plus the full-load currents of all other motors and heaters in the unit (NEC 440.32/440.33). Size the wire from the MCA using the 75°C column of NEC 310.16, and never below it even if the actual measured running current is lower.
What is MOCP, and can the breaker really be bigger than the wire ampacity?
MOCP — Maximum Overcurrent Protection — is the largest breaker or fuse allowed on the circuit, up to 225% of compressor RLA plus other loads (NEC 440.22(A)). Yes, it can exceed the conductor's 75°C ampacity: on hermetic refrigerant circuits the breaker provides only short-circuit and ground-fault protection, while the compressor's internal overload device protects against running overcurrent. NEC 240.4(G) routes these circuits to Article 440 instead of the general 240.4(D) caps.
What size breaker and wire does a 3-ton AC need?
Typically 12 AWG copper on up to a 35A breaker — but read the nameplate. A representative 3-ton condenser with a 16A compressor RLA calculates to an MCA of 21A (met by 12 AWG at 25A ampacity) and a maximum MOCP of 35A. High-efficiency inverter units often draw less; older single-speed units can draw more. The marked MCA and MOCP on the unit govern under NEC 110.3(B).
Does the 240.4(D) small-conductor rule apply to air-conditioning circuits?
No — and this is the most common point of confusion. NEC 240.4(D) caps 14 AWG at 15A, 12 AWG at 20A, and 10 AWG at 30A for general circuits, but 240.4(G) sends air-conditioning and refrigeration circuits to Article 440, which has its own coordination scheme. That's why 14 AWG copper legally lands on a 25A breaker for a 2-ton condenser. The exception does not extend to general branch circuits.
Which number sizes the wire and which sizes the breaker?
MCA sizes the wire; MOCP sizes the breaker. The conductor ampacity (75°C column of NEC 310.16) must equal or exceed the MCA. The breaker or fuse must not exceed the MOCP — anything at or below it that holds the starting current is acceptable, including matching the breaker to the MCA when starting current allows. Size the equipment grounding conductor from the chosen breaker rating per NEC Table 250.122.
Setting a Condenser This Week?
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