Maximum Overcurrent Protection: A Guide for Electricians
Published on October 5, 2026 by Staff Writer
Maximum overcurrent protection (MOP) is the largest fuse or circuit breaker allowed for a piece of equipment, such as an HVAC unit. Manufacturers print it on the nameplate, and the National Electrical Code (NEC) requires electricians to follow it.
MOP vs. MCA on Equipment Nameplates
MOP works alongside minimum circuit ampacity (MCA), which sets the minimum ampacity for the conductors that feed the equipment. Both values appear on the nameplate. The table below compares them.
Factor | MOP | MCA |
Definition | Largest fuse or breaker size allowed | Minimum circuit ampacity used to size the supply conductors |
Purpose | Protect the branch circuit from short circuits and ground faults | Ensure safe current-carrying capacity |
Risk if Not Followed | An oversized breaker or fuse may not trip in time, creating a fire hazard | Undersized wire can overheat and damage insulation |
Location of Values | Found on equipment nameplates | Found on equipment nameplates |
Installation Consideration | Breaker or fuse must be at or below the MOP rating | Conductor ampacity must meet or exceed the MCA rating |
Equipment manufacturers calculate MOP using NEC 440.22. The breaker or fuse can be sized at up to 175% of the largest motor-compressor's rated load amps (RLA), plus the other loads. If that isn't enough for the compressor to start, the limit can rise to 225%. The result is rounded down to a standard size and printed on the nameplate.
Pro tip: Size the breaker or fuse from the nameplate MOP, and never go above it. If you're working out a value yourself and it falls between standard sizes, round down. This is the opposite of wire sizing, where you round up to meet the MCA.