MOP stands for Maximum Overcurrent Protection. It’s a value printed on the nameplate of electrical equipment, most commonly HVAC systems, that tells you the largest circuit breaker or fuse you’re allowed to use on that circuit. If you’ve spotted “MOP” on an air conditioner, heat pump, or humidifier label and wondered what it means, it’s essentially the ceiling for your breaker size.
What MOP Actually Does
When a motor starts up, it briefly draws a surge of current far higher than its normal running load. This initial spike is called locked rotor amps. A breaker sized too small would trip every time the compressor kicks on, even though nothing is wrong. A breaker sized too large wouldn’t trip fast enough during an actual fault, creating a fire or equipment damage risk.
The MOP value solves this problem by defining the sweet spot: a breaker large enough to handle startup surges without nuisance tripping, but small enough to disconnect power under any anticipated fault condition. It also accounts for component aging over the life of the equipment, which can gradually increase current draw.
How MOP Is Calculated
Equipment manufacturers calculate MOP using rules from the National Electrical Code, specifically Article 440. The standard formula takes 225% of the rated load amps for the largest motor in the equipment, then adds 100% of the rated load amps for every other electrical load on that same circuit. This generous multiplier for the largest motor is what prevents the breaker from tripping during that heavy startup surge.
Once the calculation is done, the result is rounded up to the nearest standard fuse or breaker size, which typically comes in multiples of 5 amps. If the calculated value lands below 15 amps, it gets rounded up to 15 amps, since that’s the smallest breaker size permitted by code. You don’t need to run this calculation yourself. The manufacturer does it and prints the final MOP number right on the equipment nameplate.
MOP vs. MCA
You’ll almost always see MOP paired with another value on the nameplate: MCA, or Minimum Circuit Ampacity. These two numbers serve different purposes and apply to different parts of the circuit.
- MCA sizes the wire. It tells you the minimum ampacity your conductors need to handle so the wiring doesn’t overheat under sustained load.
- MOP sizes the breaker or fuse. It tells you the maximum overcurrent protection device that will still safely disconnect power during a fault.
A common example: a piece of equipment might have an MCA of 27 amps and an MOP of 40 amps. You’d need wire rated for at least 27 amps, but your breaker can be up to 40 amps. The breaker is intentionally larger than the wire’s minimum rating because the motor’s startup surge would otherwise trip a smaller breaker repeatedly.
How to Use the MOP Rating
Reading the MOP value is straightforward. Find the nameplate on your equipment (usually a metal sticker on the side or back of the unit), locate the MOP number, and make sure your circuit breaker or fuse does not exceed that value. You can go smaller than the MOP if the breaker doesn’t nuisance-trip during startup, but you cannot go larger.
For example, if the nameplate lists a MOP of 35 amps, you could install a 30-amp or 35-amp breaker. A 25-amp breaker might work too, but if it trips every time the compressor starts, you’d step up. Going to a 40-amp breaker would violate the MOP rating and could leave the equipment unprotected during a fault.
In practice, most installers select a breaker at or just below the MOP value. This keeps the circuit within code, avoids nuisance tripping, and provides the full range of fault protection the manufacturer engineered for. If you’re replacing a breaker on an existing HVAC circuit, checking the nameplate MOP is the fastest way to confirm you’re using the right size.