Thread: Condenser amperage draw

Many years ago I was taught to check each leg of the condenser circuit for amperage draw and compare with the RLA rating on the unit (easy to do when I have the panel open anyway). I had a question about a result from yesterday, but in perusing this section of the forum, it doesn't seem to be a common approach; the common focus seems to be temp drop.

This is not my primary area of expertise (obviously) and, believe it or not, most houses I see do not have central A/C, but I've used a 65%-75% of RLA as a baseline, significantly higher or lower indicating potential issues (usually low refrigerant charge or failing motor).

Please enlighten me!

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Originally Posted by Mark Fisher

Many years ago I was taught to check each leg of the condenser circuit for amperage draw and compare with the RLA rating on the unit (easy to do when I have the panel open anyway). I had a question about a result from yesterday, but in perusing this section of the forum, it doesn't seem to be a common approach; the common focus seems to be temp drop.

This is not my primary area of expertise (obviously) and, believe it or not, most houses I see do not have central A/C, but I've used a 65%-75% of RLA as a baseline, significantly higher or lower indicating potential issues (usually low refrigerant charge or failing motor).

Please enlighten me!

Amperage and temperature differential are both indicators of the condition of the system. The 65-75% is a good average but you have to add in the fan RLA. Just like the temperature differential you have to take into account other factors that can effect the readings. Ambient air temps, humidity levels, length of time the system has been running, and air flow. The things we as home inspectors can see or measure are only estimates of the condition and should be used to determine whether we recommend further evaluation or not. One of the things many inspectors do not use as a tell is the direction of airflow at the outdoor/condenser unit. If the air flow is horizontal there is a good chance the coils are blocked by dirt or bent fins, it can also indicate missing cowling or too small of fan blades. A piece of surveyors tape on your inspection probe shows the direction of airflow with an easy swipe over the top of the unit.

Originally Posted by Robert Meier

Why would the unit only be operating at 65-75% of the RLA?

RLA is rated load amps or 100% of what the motor can handle. You wouldn't run your car at red line all the time would you?

Originally Posted by Robert Meier

RLA is the actual running current of the compressor when normally loaded. It should be operating at or near it's 100% rating. The MCA is used to determine the conductor size and has a factor of 25% added by the manufacturer of the equipment. The two terms are not interchangeable.

With your logic and interpretation the current should be 125% or more on hot humid days. I, and I'm sure Mark Fisher, can tell you the measured current of a properly running system is normally in the 65-75% range.

Measured at the breaker I would expect to find 14-15 amps on that unit.

Originally Posted by Robert Meier

I'm just wondering if we're talking about the same thing, meaning the RLA. Here's a photo from an AC unit. The RLA of the compressor is 19.9 amps. Are you saying under normal conditions this will only draw 65-75% of it's rating?

Most of the time, yes, that is a typical reading for a system cooling nicely. Much lower usually means a low refrigerant charge. Much higher I think of as the compressor motor 'straining' (bearing going or some such - like I said, not my area of expertise). Rarely see anything at or over the plate RLA value and those have been bad compressor motors.

That's been my experience.

Vern, my original reason for posting was this: I typically have only a tenth or two amperage difference between legs. Monday was .5 Amps.

What does that tell you and how big a difference is an issue?

Originally Posted by Mark Fisher

Vern, my original reason for posting was this: I typically have only a tenth or two amperage difference between legs. Monday was .5 Amps.

What does that tell you and how big a difference is an issue?

The difference between legs is a result of 120v loads in the unit or leakage. If the unit doesn't have a neutral then there is a current leak somewhere.

Current leak?

Originally Posted by Mark Fisher

Current leak?

Most likely leak source would be the compressor windings. Worst enemy to the compressor is moisture which converts into acid when mixed with the refrigerant. The acid deteriorates the varnish like insulation of the winding wires and with the refrigerant cooling the windings by flowing over them there is a resistive path for current flow to the case of the compressor.

Interesting. So how big a difference between the legs indicates an issue?

Originally Posted by Mark Fisher

Interesting. So how big a difference between the legs indicates an issue?

I don't have a number that I can give. I'm sure the manufacturer would say any current leak must be fixed, meaning replace the unit. The practical answer would take into account the size and age of the compressor, will it last out its normal lifespan before going all the way to the bad? Judgment call, in my opinion .5amp is starting to look a little dodgy.

Thanks, Vern. Great info!

Guys, this simple test can tell you a lot about the compressor motor. We already have the breaker panel open, so why not? Temp drop tells us about the overall system, but the compressor motor is the high-dollar item in the mix.