Condensing Unit Overcurrent Protection

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kwired said:
I would too if guaranteed to be properly selected and adjusted. Other than phase loss protection feature I like thermal element overloads better because you don't have operators tampering with settings, unless you have a setting where there is a supply room and additional elements readily available, did have a case one time where they did just that, because there was easy access to a supply of elements.
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An expirienced electrician will select and adjust the field overloads properly. It must be everyone else that
comes along and messes with it after these adjustments have been made that's got everyone dogging
the Field Overloads Protection.
 
jap said:
That's right, but it's where the Thermal Overload is sensing its heat from, is what makes the difference.

I feel the actual current going through a set of field overloads is more accurate than an internal thermal
overload since the Internal Overload is usually relying on heat applied to the outside of it to open, instead
of the current going through it like on a external overload.

But to each his own.
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Not true. Internal overloads work on external heat and current.
 
On your standard Residential Compressor Motor where all you have is a DP Contactor feeding it?
 
I can't recall ever seeing on a compressor nameplate showing at what amperage the integrated thermal
overload was set open, and take the compressor off line.
 
I have seen a number of residential AC units with CTs in addition to the thermal switch to protect from overload.
 
jap said:
I can't recall ever seeing on a compressor nameplate showing at what amperage the integrated thermal
overload was set open, and take the compressor off line.
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Compressor internal overloads must trip at or below the compressors MCA. That is required by the standards they are manufactured to. Internal overloads will trip because of excessive heating of the compressor or heating of the bimetal strip by overcurrent.
 
Mgraw said:
Compressor internal overloads must trip at or below the compressors MCA. That is required by the standards they are manufactured to. Internal overloads will trip because of excessive heating of the compressor or heating of the bimetal strip by overcurrent.
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Completely true,,,,,,And where can I find that amperage information on the nameplate?
 
Mgraw said:
Compressor internal overloads must trip at or below the compressors MCA. That is required by the standards they are manufactured to. Internal overloads will trip because of excessive heating of the compressor or heating of the bimetal strip by overcurrent.
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So how do we get away with feeding a unit such as this with smaller guage wire, than what is required for
the Short circuit protective device ahead of it, when we dont know for sure at what specific amperage the
internal overload is "Supposed" to trip at?

At least on field installed overloads on a motor starter we expect the overload to trip at what we set it at,
and using the smaller guage wire to the motor is allowed.
If an internal overload must trip at or below the maximum circuit amps then in my mind we could not
install a smaller guage wire to the unit than what the overcurrent device feeding it is set at.
 
jap said:
So how do we get away with feeding a unit such as this with smaller guage wire, than what is required for
the Short circuit protective device ahead of it, when we dont know for sure at what specific amperage the
internal overload is "Supposed" to trip at?

At least on field installed overloads on a motor starter we expect the overload to trip at what we set it at,
and using the smaller guage wire to the motor is allowed.
If an internal overload must trip at or below the maximum circuit amps then in my mind we could not
install a smaller guage wire to the unit than what the overcurrent device feeding it is set at.
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Because all this is considered when the manufacturer gives you a MCA and MOCP on the label.
 
Mgraw said:
Because all this is considered when the manufacturer gives you a MCA and MOCP on the label.
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Then I think they should let me know what they came up with, so I could sleep at night, should I ever
decide to pull a smaller guage wire to a condensing unit where I cant see or adjust the overload protection
on it. :)

I guess that's why I never downsize my wire to a unit such as this, where I can't see, adjust or even
know what the overload protection is set at.
 
jap said:
Then I think they should let me know what they came up with, so I could sleep at night, should I ever
decide to pull a smaller guage wire to a condensing unit where I cant see or adjust the overload protection
on it. :)

I guess that's why I never downsize my wire to a unit such as this, where I can't see, adjust or even
know what the overload protection is set at.
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The MCA on the label tells you all you really need to know to size the wire.
 
kwired said:
The compressor motor continues to get hotter and hotter
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How that can be? The internal thermal overload protector including the branch circuit protective device should prevent that, if they are healthy.

kwired said:
if a capacitor should fail they will just keep recycling and motor will get hot.
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The compressor motor would not start and short cycle with its starting capacitor defective. Under such conditions, it is doubtful the compressor motor would be damaged, if its internal overload protector is healthy.
 
Sahib said:
How that can be? The internal thermal overload protector including the branch circuit protective device should prevent that, if they are healthy.


The compressor motor would not start and short cycle with its starting capacitor defective. Under such conditions, it is doubtful the compressor motor would be damaged, if its internal overload protector is healthy.
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OK the getting hotter and hotter part is maybe a little exaggerated. Any automatically resetting device will continue to keep the motor in a hot state where a manual reset device will allow things to cool until operator intervention comes to reset it.

Most AC compressors only have a run capacitor, they still typically have automatic resetting overload protectors.

There is a difference between the winding getting a little hot and a manual reset device shutting down and allowing for cool down, and an automatic reset device that not only heats up the winding but with the repeated resetting the steel cores, the motor frame, and everything else eventually heats up, and the winding temp does increase some over what it was at the first overload trip. This is true for any automatic resetting overload. Probably the most effective device is a thermostat or sensor embedded in the windings it is protecting. A typical thermal device as found on an AC compressor is in same ambient as the compressor but is not subjected to exactly same temperature as what the windings will actually reach.
 
jap said:
Then I think they should let me know what they came up with, so I could sleep at night, should I ever
decide to pull a smaller guage wire to a condensing unit where I cant see or adjust the overload protection
on it. :)

I guess that's why I never downsize my wire to a unit such as this, where I can't see, adjust or even
know what the overload protection is set at.
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Smaller wire than what, smallest you can run to the unit is based on the MCA.

It already has 125% of full load current designed into it. Your conductor will handle overloading longer than the compressor will, so AFAIC, the conductor is still protected, the motor is what is a little debatable, as seen in this thread.
 
jap said:
To me MCA is what the unit would be pulling in ideal conditions.
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MCA is kind of the same as FLA for 430 applications, except it also contains the rated current of the compressor along with other accessories - primarily the fan motor in most cases. MCA is still derived same way as sizing multiple motors or motors and other loads, 125% of largest motor plus 100% of other motors or other loads that may run simultaneously.

It is not ideal conditions, it would be worst case loading without overloading. Most compressors I have measured never draw what the rated amps are, but I seldom have measured in extreme conditions either. Disconnect the condenser fan and the compressor amps will begin to rise, as there is more head pressure. Increase outdoor temperature with fan still running and head pressure will also increase resulting in compressor working a little harder.

Run an air to air heat pump in really cold weather and compressor draws very little, but is not doing much work either, you eventually reach a point where most of the energy used is just to run the compressor and very little energy is put into pumping the refrigerant.
 
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