Electrical Definitions RLA ,MCA, MOCP, LRA

Issue:

Electrical Definitions RLA ,MCA, MOCP, LRA

Resolution:

There are many electrical terms used with centrifugal chillers during the design process. It seems that often these terms get misinterpreted and misapplied. First we will define some of these terms and then discuss their use.

Amps, Primary Rated Load Amps (RLA) - also commonly referred to as the selection RLA, and unit RLA. This is the amperage that is drawn when the chiller is at full cooling capacity in terms of cooling tons (kW). RLA is listed on the TOPSS selection and unit nameplate, and it is the key number used to size the starter, disconnects, circuit breakers.

Minimum Circuit Ampacity (MCA) - appears on the chiller nameplate and is used by the electrical engineer to determine the size and number of conductors(wires) needed to bring power to the starter/motor. The MCA is 125% of the motor design RLA plus 100% of the amperage of other loads (sump heater, oil pump motor, and purge, etc.) The MCA is listed on the TOPSS chiller selection report.

Locked Rotor Amps (LRA) - a specific locked-rotor amperage value associated with each specific motor. This is the current draw that would occur if the rotor shaft were instantaneously held stationary within a running motor. LRA is typically five to seven times the motor full load amps (FLA). LRA is also used commonly in discussing different starter types and the inrush amperages associated with the motor start. For example a Star (Wye) Delta starter will typically draw approximately 33% of the motor LRA to start. A solid-state starter will draw approximately 45% of LRA to start. The LRA is listed on the TOPSS selection report.

Maximum Over-Current Protection (MOCP) - The electrical engineer often wants to know the MOCP when selections are being made. It is used to assist in the sizing of up stream fuses or circuit breakers. MOCP is an output of the TOPSS selection program. Use of improperly sized circuit breakers can result in nuisance trips during the starting of the chiller.

Discussion - Customers typically have the most trouble with the term maximum over-current protection or MOCP. Generally the customer must supply some type of overcurrent device upstream of the starter. The MOCP is a term that the chiller manufacturer is required to put on the nameplate and it is MAXIMUM for overcurrent, not a recommended size. We suggest that the customer be given the chiller RLA and MOCP along with the predicted inrush current (based on starter type, and % LRA). With this information, the customer can decide on what size of overcurrent protection is required for upstream use. The cables/wires are not to be sized to this number, they are sized based on MCA only. See the CenTraVac Starter Engineering Bulletin (CTV-PRB004-EN) for more details on this topic.

Notes:

Changes:

Other Question/Problem/Cause Statements:

• Electrical Definitions RLA ,MCA, MOCP, LRA

Facts:

• LRA
• MOCP
• RLA
• MCA