Trying to figure out what a compressor’s amp draw should be can be a bit confusing. Most hermetic compressors only have the LRA (locked rotor amps) on the compressor data plate. The locked rotor amps tell you what the compressor will draw if the rotor cannot turn when the compressor is energized. This typically is 5 to 7 times as high as the operating current. But the operating current is not a fixed quantity. Changes in condenser and evaporator pressures change the work load of the compressor, which changes the actual operating current. Equipment manufacturers label their equipment with the compressor RLA (rated load amps). On complete systems submitted to UL, the rated load amp value will be determined by actual system tests. The same model compressor in two different applications can have a different RLA in each application. However, the RLA is not really there to tell you what the compressor amp draw should be. It is there for the purpose of sizing the wire, controls, and overcurrent devices for UL listed equipment.
Underwriter’s Laboratories requires that the compressor motor protection system will not permit a continuous current in excess of 156% of the rated load current. In commercial refrigeration, compressors and condensing units are often sold as separate component parts. Since there is no system to test, the compressor manufacturer must come up with an RLA. Compressor manufacturers work backwards to figure this out. They operate the compressor at different loading conditions to determine the MCC (maximum continuous current.) This is the highest amp draw the compressor can operate at continuously without tripping the overload. Since UL says the overload must trip no higher than 156% of the RLA (rated load amps), the RLA is calculated by dividing the MCC by 1.56. Some compressor manufacturers use lower safety factors of 1.4 to calculate the RLA. Either way, the RLA does not tell you what the amp draw should be for the particular operating condition in which you find the compressor in the field. In the case of individual compressors or condensing units, it does not even tell you what the amp draw should be at design point, because there is not a complete system, and thus, no system design point. To know what a compressor amp draw should be at any particular operating condition, you need a chart or table supplied by the compressor manufacturer. Some now have this data online. Bristol compressors will let you enter a specific operating condition and their web site will spit out all the relevant data – including the operating current.