What Size Breaker for a 3-Ton AC? (MCA vs MOCP Explained)

Every listed condenser carries both numbers on its data plate, because NEC 440.4(B) requires the manufacturer to mark them. They answer two different questions:

Nameplate value	Question it answers	Rule
MCA — Minimum Circuit Ampacity	How big must the wire be?	Conductor ampacity ≥ MCA
MOCP — Maximum Overcurrent Protection (also "Max Fuse" or "Max Breaker")	How big may the breaker be?	Breaker/fuse rating ≤ MOCP

They come from two different calculations, which is why they're two different numbers:

MCA ≈ 1.25 × compressor RLA + condenser-fan FLA. That's the branch-circuit conductor math from NEC 440.32/440.33 — 125% of the largest motor-compressor's rated-load amps, plus the other loads on the circuit at 100%. Example: compressor RLA 16.7 A, fan FLA 1.2 A → MCA = 1.25 × 16.7 A + 1.2 A = 22.1 A.

MOCP comes from NEC 440.22(A): the short-circuit and ground-fault device is sized at no more than 175% of the compressor's rated-load current — and if a device that size can't hold the starting inrush, the next standard size up is allowed, with a hard ceiling of 225%. On that same 16.7 A compressor, 1.75 × 16.7 A = 29.2 A, so the manufacturer typically prints a 30 A or 35 A maximum.

You don't run either calculation in the field — the manufacturer already did, under lab conditions you can't reproduce. Your job is to obey the printed result: wire sized from MCA, breaker capped by MOCP. In practice the breaker lands between the MCA and the MOCP; many techs simply install the maximum listed, which guarantees the unit starts on a hot day at the end of a long line run.

Ranges you'll actually see on 3-ton (36,000 BTU/hr) residential equipment at 208/230 V single-phase:

Nameplate item	Typical 3-ton range	What it controls
Compressor RLA	~14–18 A	Basis for the MCA/MOCP math
MCA	~18–25 A	Minimum wire ampacity
MOCP (max breaker/fuse)	30–45 A, most commonly 30–40 A	Largest permitted breaker

Real examples from inspection references: a 3-ton American Standard condenser listing MCA 25 A with a 35 A maximum breaker, and 3-ton units with RLA in the 14–18 A range calling for 30, 35, or 40 A breakers. Higher-SEER2 equipment draws less current, so a new high-efficiency 3-ton may want a 30 A breaker where the 20-year-old unit it replaced had a 45 A — one more reason a like-for-tonnage swap still requires re-reading the plate.

Once you have the MCA, pull the wire size from NEC Table 310.16 (copper):

MCA on nameplate	NM cable (Romex, 60°C column)	THHN in conduit (75°C column)
≤ 20 A	12 AWG (20 A)	12 AWG (25 A)
≤ 30 A	10 AWG (30 A)	10 AWG (35 A)
≤ 40 A	8 AWG (40 A)	8 AWG (50 A)

NM cable is limited to the 60°C column per NEC 334.80. On long runs, upsize for voltage drop — the ampacity table only covers heating, not the volts you lose over 80 ft of branch circuit.

On a general-purpose circuit, NEC 240.4(D) caps 10 AWG copper at a 30 A breaker — so a 40 A breaker feeding 10 AWG looks like a violation. On an air-conditioner circuit it usually isn't, and the reason is what the breaker is actually protecting against.

Under NEC Article 440, the breaker on a hermetic motor-compressor circuit provides short-circuit and ground-fault protection only. Running-overload protection — the thing that normally justifies matching breaker to wire ampacity — is handled separately by the equipment itself: the compressor's internal thermal overload and the unit's listed protective devices, per NEC 440.52.

The breaker gets sized loose on purpose. A compressor pulls locked-rotor amps every time it starts — check the LRA line on the same nameplate; it runs several times the RLA. A breaker sized tight to the wire would see that inrush as an overload and nuisance-trip on hot starts. So NEC 440.22(A) lets the breaker run up to 175% of the compressor's rated-load current (225% ceiling), while the wire only has to carry the continuous MCA.

InterNACHI's inspection guidance gives two worked examples: a nameplate with MCA 24.0 A permits 12 AWG conductors with a 40 A maximum breaker, and a nameplate with MCA 30.4 A permits 10 AWG with a 50 A maximum. Both would fail the general rule and both are legal on this equipment, because both jobs are covered — the unit's overloads protect the wire from sustained overcurrent, and the breaker clears faults.

A big breaker on small wire is usually fine on a condenser circuit. These are the actual violations to look for:

1. Breaker larger than the nameplate MOCP. A 60 A breaker where the plate says "Max Fuse/Breaker 40 A" is wrong no matter what wire is attached. This is the classic swap after nuisance trips — someone upsizes the breaker instead of finding the failing compressor or loose lug. Over the MOCP, the protection no longer matches the equipment listing.
2. Wire ampacity below the MCA. 12 AWG NM (20 A at the 60°C column of NEC 310.16) feeding a unit with MCA 23.4 A fails, even if the 40 A breaker itself is within the MOCP.
3. Wrong device type. The overcurrent device "shall be the size and type marked" — if an older plate says maximum fuse only, it means fuses (usually in the disconnect), not a breaker. Most modern plates read "fuse or HACR-type breaker," which nearly all current molded-case residential breakers satisfy — but the plate governs.

The 30-second field check: open the disconnect and the panel, read the plate, then verify breaker ≤ MOCP, wire ≥ MCA, and device type matches the plate. If all three pass, the circuit is compliant — even when the breaker number is bigger than the wire's ampacity.