Determining Compressor Amperage Ratings

July 31, 2003
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When you are diagnosing a potential compressor problem, you need to measure its amp draw to get a correct assessment of its vital signs.

The best way to determine the correct amperage draw for a particular compressor is by referring to the rating chart for that compressor.

Compressor manufacturers publish a rating chart for each compressor they manufacture. The compressor chart will list the correct amp draw for the compressor under its various operating conditions.

To use the chart, the technician must know the evaporating temperature, condensing temperature, and voltage applied to the compressor. A technician can determine the correct amp draw for the compressor and use that information to accurately troubleshoot the compressor and the system. With the use of the Internet this information is now easier to obtain.

Why Not RLA?

Most compressor manufacturers will stamp an amperage rating on their compressors. They will usually stamp the Rated Load Amp (RLA) of the compressor. However, the technician cannot use this value to determine the correct operating amperage.

Also, trying to determine if a compressor is good or bad using RLA is not correct. It has nothing to do with what the correct amperage draw should be under its various load conditions.

RLA is a mathematical calculation required to meet Underwriters Laboratories (UL) approval. The compressor manufacturer must run a series of tests to determine the Maximum Continuous Amps (MCA) before the overload trips. Once that has been determined, UL says to divide the MCA by 1.56 to determine the RLA. Some compressor manufacturers, such as Copeland and Carlyle, use a different factor. They divide the MCA by 1.44.

If the RLA has any value, it is to determine at what amperage draw the compressor overload will trip, and to determine the fuse/circuit breaker size and the wire size.

Amp Measurements

When measuring the amp draw of a compressor, make sure to read only the amperage drawn by the compressor. Choose a location on the wiring diagram that will isolate the compressor from any other loads, such as the condensing fan motor.

It is important to measure the amp draw of the compressor only when you are trying to determine if it is operating properly.

Often a system problem may seem to be the result of a defective compressor when in fact it is due to some other cause. Using the amperage draw of the compressor will help any technician determine if the compressor is truly defective.

However, the technician must know what the correct amperage should be.

Joe Marchese is owner of Coldtronics of Pittsburgh. He can be reached at 412-734-4433,, or

Publication date: 08/04/2003

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