On commercial equipment, there is some information on the data plate of the unit that has to do with safely wiring and protecting the equipment. These ratings are the MCA and MOP.
To start this discussion, we first need to know what MCA and MOP are. The minimum circuit ampacity (MCA) and maximum overcurrent protection (MOP) ratings provide guidance for safely connecting field-wired equipment to the building mains . Understanding these ratings, and their relationship to each other, is critical to properly selecting wire and circuit breaker sizes.
- What is MCA? Minimum Circuit Amps (MCA) is a calculated value that specifies the minimum main power wire size. More specifically, MCA is the highest steady-state electrical current that the rooftop unit should see when operating correctly. The minimum circuit ampacity (MCA) is also used to figure the minimum wire size required for a field wired product. It is chosen to guarantee that the wiring will not overheat under the expected operating conditions. The wire size takes into account the normal current draw, ageing of components and anticipated faults.
- What is MOP? Maximum Over-Current Protection (MOP) is a calculated value that determines the maximum size of the over-current protection device (fuse or breaker). There are different MOP equations depending on your application. The maximum overcurrent protection (MOP) is the maximum circuit breaker size required to properly protect the equipment under anticipated fault conditions. The MOP takes into account startup surges and component ageing.
The chart below gives a very good summary of what these are (click on it to enlarge):
(Chart was provided by Bruce Porter Manager UPG Field Service. Bruce has written many application bulletins for UPG and is well-respected for his expertise in the HVACR field. He also teaches HVACR at Metro Technologies Center HVAC)
Fortunately for all of us, most manufacturers of the equipment does all the calculations and publishes them in both the equipment literature and on the unit data plate so we don’t have to. But sometimes it is nice to know and understand how these are accomplished and calculated or how to do it if it is not published.
So — how are these calculated? (Information below was provided by Jack Bartell, Director, Service & Training for Virginia Air Distributors. It was also his request and suggestion to write this post.)
Supply wiring must be rated to carry at least the amps shown as MCA. By comparison the overcurrent protection device (either a breaker or fuses) must be sized to prevent the unit from drawing more current than the MOP.
Minimum Current Ampacity (MCA)
MCA = 1.25 x [Motor Rated Current + Heater Current]
The Motor Rated Current is sometimes referred to as the FLA (full load amps) of the unit. This can be a source of confusion because this rated current is not the same as the motor FLA shown on the nameplate of the motor itself. Motor Rated Current is determined during worst-case, high-current test conditions of the complete terminal unit, in accordance with UL1995. The FLA on the motor nameplate is a rating from the motor manufacturer and is of not used in this calculation.
Maximum Overcurrent Protection (MOP)
First, a basic calculation is made, and then a number of filters or conditions will alter the computed MOP value to arrive at the final value that appears on a product nameplate. In short, the basic MOP is calculated by multiplying the rated current of the largest motor times 2.25, and adding in all other loads of 1.0 amp or more that could be in operation at the same time.
MOP = [2.25 x (Rated Current of Largest Motor)] + (Other Motor Loads) + (All Heater Loads)
Filter 1: If the MOP value is not an even multiple of 5, the MOP is rounded down to the nearest standard fuse size.
Filter 2: If the MOP is less than the MCA, the MOP is made equal to the MCA and then rounded up to the nearest standard fuse size, typically a multiple of 5. In other words, the MOP shall not be less than the MCA.
Filter 3: If the MOP is less than 15, it shall be rounded up to 15 amps. This is the minimum size of fuse or circuit breaker permitted by code.
Calculations are based on the national standard for Heating & Cooling Equipment, UL1995, a.k.a. CSA-C22.2 No. 236. Also — The calculations for the MCA and MOP are based on requirements of NFPA 70, the National Electrical Code (NEC) and CSA C22.1, the Canadian Electrical Code (CEC).
To sum it all up — The MCA is the minimum wire size needed to guarantee that the wiring will not overheat under all operating conditions for the life of the product. The MOP is the maximum allowable circuit breaker size that will properly disconnect power to the equipment under any anticipated fault condition.
Minimum circuit ampacity (MCA) and maximum overcurrent protection (MOP) are necessary to correctly connect field wired equipment . As their names indicate, the ratings will tell you the minimum wire size and maximum circuit breaker size allowed for the equipment. Higher currents have led to a greater awareness of the risks of overcurrent and fires. The system of MCA and MOP was developed to reduce that fire risk.
One last thing to always remember — all the properly sized wires and circuit breakers are for naught if the unit is not properly grounded. A proper ground provides the safety factor for all equipment. You can’t have a short to ground if there is no ground to short to in the first place. So every unit needs to have the properly sized wire, circuit breaker, and ground.