Reading an AC Nameplate: RLA, LRA, FLA, MCA and MOCP (Breaker and Wire Size)
The data plate on the outdoor unit tells you how to wire it. Most wiring mistakes come from applying general circuit rules to air conditioning equipment, which follows its own rules in NEC Article 440.
What the numbers mean
| Marking | Meaning | What you use it for |
|---|---|---|
| RLA (rated load amps) | Compressor current at rated conditions | Comparing to running amps; sizing a contactor; manufacturer uses it for MCA and MOCP |
| LRA (locked rotor amps) | Inrush current when the compressor starts or is stuck | A compressor drawing LRA and not dropping is locked or not starting |
| FLA (full load amps) | Fan motor current at full load | Comparing to fan running amps |
| MCA (minimum circuit ampacity) | Minimum ampacity of the supply conductors | Wire size |
| MOCP / MOP / max fuse or breaker | Largest overcurrent device allowed | Breaker or fuse size (maximum) |
| Min fuse or breaker (sometimes) | Smallest device that lets the unit start | Breaker or fuse size (minimum) |
| HACR | Heating, air conditioning and refrigeration breaker type | Breaker type required |
The manufacturer already did the Article 440 math. MCA already includes the 125% factor on the largest motor. You don't need to recalculate it.
Sizing the wire: MCA
Choose a conductor whose ampacity is at least the MCA, using the correct temperature column (NEC 110.14(C): on circuits of 100 A or less, the 60°C column unless the terminals and conductors are rated and marked for 75°C), then adjust for long runs (voltage drop), conduit fill and ambient temperature.
Example from EC&M: a nameplate with an MCA of 24.5 A needs a conductor with at least 24.5 A ampacity; with 60°C rules that's 10 AWG copper (30 A).
Sizing the breaker: MOCP
Use a breaker or fuse no larger than the MOCP. It does not have to match the wire's "normal" breaker size. EC&M's example: same unit, maximum 40 A on the plate, so a maximum 40 A breaker on that 10 AWG conductor is allowed.
That's the part that confuses inspectors and electricians: a 40 A breaker on 10 AWG looks wrong by general rules, but Article 440 allows it for motor-compressor equipment because the breaker only needs to provide short-circuit and ground-fault protection. The compressor's own internal protector and the unit's design handle overload. InterNACHI's guidance for home inspectors makes the same point: the usual breaker-to-wire rules don't apply to AC and heat pump circuits; the nameplate does.
You can go smaller than MOCP as long as the breaker holds through the compressor start (and isn't below any "minimum" rating on the plate). Going larger than MOCP is a violation.
Fuse vs. breaker vs. HACR
- "Maximum fuse size": use fuses (dual-element or time-delay as marked). A breaker isn't a substitute.
- "Maximum fuse or circuit breaker" / "maximum overcurrent protection": either is allowed.
- "HACR type breaker": the breaker must be HACR rated. Most residential breakers are, but check the breaker label.
Room air conditioners and window units
Different rule: if a room AC is the only load on a circuit, its marked rating can't exceed 80% of the circuit's overcurrent device rating (NEC 440.62(B)).
Using the plate when troubleshooting
- Running compressor amps above RLA for long periods means high head pressure, low voltage, overcharge or a failing compressor.
- Amps near LRA that don't drop quickly mean it isn't starting: see AC compressor not starting and how to test a compressor.
- Fan motor amps above FLA: check the capacitor, bearings and blade. See condenser fan not spinning.
- Breaker tripping: compare the breaker to MOCP and MCA, then hunt for a short: see AC breaker keeps tripping.
- Contactor size: must be rated at or above the compressor RLA plus any fan on the same contacts. The replacement part finder does that math.
FAQ
Can the breaker be bigger than the wire rating on an AC?
Yes, up to the nameplate MOCP. NEC Article 440 allows it for motor-compressor equipment because the manufacturer has accounted for motor starting current.
What size wire do I need for my AC?
Enough ampacity to meet the nameplate MCA, using the proper temperature column for the terminals, then upsized for long runs or conduit fill.
What's the difference between RLA and LRA?
RLA is the compressor's rated running current. LRA is the much higher current it pulls at the instant it starts (or if the rotor is locked).
Does MCA already include the 125%?
Yes. The manufacturer calculates MCA per Article 440, including 125% of the largest motor's current.
Sources
- EC&M: A/C and refrigeration equipment (NEC 440 sizing examples)
- EC&M: Properly sizing conductors and protective devices for A/C and refrigeration equipment
- IAEI Magazine: Air conditioning and heat pump equipment nameplate information
- InterNACHI: Inspecting oversized AC breakers on small-gauge wires
- HVAC School: Why the breaker may be lower than the MCA
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