Sizing wiring and protection for single motor circuits is pretty straight forward and can be obtained from tables in the electrical code. It gets more complicated with multi-motor devices. Most HVAC RTUs (roof top units) usually come with the two coils one for cooling and one for heating. These equipment usually have multiple motors for the various compressors, and operate in a way such that not all motors are operational at the same time. For example when the cooling coil is being used heating coil will not be used. Modern PLCs control the various motors within these devices thus make it hard for a designer to calculate the equipment’s ampacity and over-current protection. Because of this, HVAC equipment that are equipped with multiple motors usually explicitly state the MCA rating (Minimum Current Ampacity) and the MOCP rating (Maximum Over Current Protection). Note that the MOCP rating is sometimes referred to simply as the MOP rating.
Minimum Current Ampacity – MCA Rating
The MCA rating is calculated and provided to you by the equipment manufacturer. The MCA rating dictates to the designer the minimum ampacity for the wiring feeding power to the equipment. For example, if the MCA rating for an HVAC equipment is listed as 30A, you must use a wire that is capable of handling a minimum of 30A. By refering to CEC Table 2, we see that we must use #10 AWG at a minimum.
Maximum Overcurrent Protection – MOCP Rating
The MOCP or MOP rating is also calculated and provided to you by the equipment manufacturer. The MOCP rating dictates the maximum rating for the protective device used to protect the circuit feeding the equipment. Usually there are two ratings, one if a fuse is used, and another if the circuit breaker is used. Most of the time the ratings are the same regardless of whether it is a fuse or a circuit breaker. For example, if the MOCP for an HVAC equipment is listed as 50A, you must use a circuit breaker that is no larger than 50A.
Where to find MCA and MOCP ratings
For designers, the first place to look is the manufacturer’s cut sheet. For field personal the MCA and MOCP rating will be listed explicitly on the equipment name plate. Be advised the MOCP is sometime listed as “Max Fuse/Breaker” on the nameplate.