Condensing Unit Overcurrent Protection

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jap said:
They ought to just tell it like it is on the nameplate.
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What are they not telling you that you need to know?

I don't care what setting the overload device actually trips at, it is part of the listed unit intended to protect its associated motor, it inherently protects the conductor that they also told on the nameplate what the minimum ampacity is to be.

Same goes for a motor with internal overload protection. I don't need to know what the overload actual trip point is to determine what size conductor to run to the motor.

I don't need to know overload trip point when selecting conductors for any other art 430 installation either. If I know I am going to have a 7.5 hp motor I select conductors and overcurrent accordingly to tables. I may not know actual nameplate FLA until a later time, then I will select overload protection device, but conductor was already selected before based on the worst case 7.5 hp motor that may be connected to the circuit via the values in the tables.
 
jap said:
Everything you all have been explaining in the last several posts.
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:roll: Considering what I have been explaining, maybe a better question is why do you need to know?

Is it that important to know if an overload for a compressor will trip at 22 or 23 amps, or even 28 amps?

Also keep in mind those overloads are not going to instantly trip at a specific setting - a time factor is involved creating a trip curve. The higher the overload the faster the trip time is.

Even a fuse or circuit breaker will have similar trip characteristics - as in a trip curve for overload and another trip curve for short circuits or ground faults.
 
kwired said:
:roll: Considering what I have been explaining, maybe a better question is why do you need to know?

Is it that important to know if an overload for a compressor will trip at 22 or 23 amps, or even 28 amps?

.
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It is to me.
 
At times a 6 Amp Swing one way or the other is a completely different Overload Relay with different settings.
 
jap said:
At times a 6 Amp Swing one way or the other is a completely different Overload Relay with different settings.
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And your point is? The overload was specifically chosen to protect the unit it is installed on, not the supply conductors. The supply conductors are inherently protected from overload because of the overload device protecting the motor. If the unit is drawing 6 amps too much it probably trips, once the trip curve has met the current and time needed to trip it.
 
kwired said:
And your point is? The overload was specifically chosen to protect the unit it is installed on, not the supply conductors. The supply conductors are inherently protected from overload because of the overload device protecting the motor. If the unit is drawing 6 amps too much it probably trips, once the trip curve has met the current and time needed to trip it.
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I've almost forgot what my point was, but, I think it was that I felt a motor starter with an overload relay protecting a motor was
a more accurate means of motor protection than the Thermal Overload found integral to a motor, but thats just my opinion.
Adjustable overload relays have incremental dials where you can dial in pretty close to where you want your protection to be.
To me it would be more accurate than a thermal overload that would let the current swiing to the upside as much as 6 amps continously without tripping as you have stated. The Adjustable overload shouldnt allow that much overcurrent to stay in play without tripping, otherwise, what's the point of dialing it in.
 
jap said:
I've almost forgot what my point was, but, I think it was that I felt a motor starter with an overload relay protecting a motor was
a more accurate means of motor protection than the Thermal Overload found integral to a motor, but thats just my opinion.
Adjustable overload relays have incremental dials where you can dial in pretty close to where you want your protection to be.
To me it would be more accurate than a thermal overload that would let the current swiing to the upside as much as 6 amps continously without tripping as you have stated. The Adjustable overload shouldnt allow that much overcurrent to stay in play without tripping, otherwise, what's the point of dialing it in.
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AFAIK those overloads integral to the unit are selected just as carefully as you decide where to set an adjustable overload. They have a heater that responds to motor current and a bimetal spring with a contact attached to it. The heater heats up in similar fashion to motor windings, the more current the faster it heats up, same as windings.

I don't know where you got this 6 amp figure from. True you may find a unit that will draw 6 amps too much, but it should eventually trip. When depends on where that 6 amps puts the trip point within the trip curve. Even a solid state overload would let it draw 6 extra amps for certain amount of time before tripping that time could be shorter or longer than the other overload - all depends on the trip curve.
 
kwired said:
I don't know where you got this 6 amp figure from. True you may find a unit that will draw 6 amps too much, but it should eventually trip. When depends on where that 6 amps puts the trip point within the trip curve. Even a solid state overload would let it draw 6 extra amps for certain amount of time before tripping that time could be shorter or longer than the other overload - all depends on the trip curve.
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I got the 6 amp figure from you when you said this:

Is it that important to know if an overload for a compressor will trip at 22 or 23 amps, or even 28 amps?


I'd hope my field adjusted overloads would trip if I had a 6 amp overcurrent. They always have.
 
JDB3 said:
I understand this about an air conditioner, but, does the same apply if the unit is a heat pump? What articles apply, please? Thanks.
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Same criteria for both, a heat pump is the same piece of equipment with a defrost board,reversing valve and second refrigerant metering device with check valve.
 
JDB3 said:
I understand this about an air conditioner, but, does the same apply if the unit is a heat pump? What articles apply, please? Thanks.
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Gregg Harris said:
Same criteria for both, a heat pump is the same piece of equipment with a defrost board,reversing valve and second refrigerant metering device with check valve.
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As far as the power circuit goes, there is no difference, controls are more complex with the heat pump.

Some heat pumps will have a crankcase heater, but I have also seen this on AC only units that are intended to run in cold weather.

I think you do see less case heaters though with the scroll compressors.
 
kwired said:
As far as the power circuit goes, there is no difference, controls are more complex with the heat pump.

Some heat pumps will have a crankcase heater, but I have also seen this on AC only units that are intended to run in cold weather.

I think you do see less case heaters though with the scroll compressors.
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Kwired, You should become a Moderator. You seem to be very knowlegeable on a vast amount of electrical subjects.

I mean that sincerely.

JAP>
 
jap said:
Kwired, You should become a Moderator. You seem to be very knowlegeable on a vast amount of electrical subjects.

I mean that sincerely.

JAP>
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Thanks, I think:D.

I have challenged some of the well respected engineers that lurk here, and have eaten crow a few times as well as had some of them credit me with something. There are some areas I just am not quite on with but do know just enough to be in a conversation about. I do find those areas worth reading most of the time and sometimes learn something.
 
All I'm saying is,

I size my overload protection to the full load amperage of the motor when I field install heaters in a starter.
It was stated in a previous post that the Minimum Circuit Amps indicated on a Compressor name plate
is actually worst cast scenario plus 25% and thats what your basing the wire size on, which is fine.

The thermal overload inside the compressor is more than likely set to drop out at a point much lower than
that.

It's just that when I install a Starter with Overloads I do it all myself. I'm not only trying to protect the conductors, I'm also trying to protect the motor when I set the overloads.

On the compressor I have to take thier word for it that they have the motor protected, since this is not
indicated by the "Worst case scenario plus 25%". thats all.
 
I dont reduce the wire size to the motor until I'm on the Load Side of the overloads.
but in the condensing unit situation the wire is being reduced on the line side of the overload all the way
back to the Breaker Panel.

Most feeders to A/C disconnects are sized to the Max Overcurrent Protection listed on the unit if on a new
install.

It seems the only time someone wants reassurance that it will be ok to run a unit where they think the wire
is too small (like the OP) is when they start looking more towards the Minimum Circuit Amps and the
exceptions to see if they'll be ok.

It's not common to see #12 wire on a 35 amp breaker in a panel right from the start, Its usually #10 for
30amp #8 for 40 amp and so on.
 
jap said:
I dont reduce the wire size to the motor until I'm on the Load Side of the overloads.
but in the condensing unit situation the wire is being reduced on the line side of the overload all the way
back to the Breaker Panel.
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A motor overload can go anywhere in the circuit, and on a single phase motor only needs to be in one conductor of the circuit. Entire circuit is protected from overload by this one device whether located immediately near L1 terminal of the branch circuit breaker or is internal to the motor.

If motor is overloaded the entire circuit will see the overload, is why it doesn't matter where it is located.

Three phase motors is different, and short circuit and ground fault protection is different.

You can put the solid state three phase overloads anywhere in the circuit also, as long as all three phases pass through the overload. It can be immediately after the breaker, ahead of contactor, after contactor, near the motor, basically anywhere it will be seeing the motor current and you do not need different sized conductors before or after the overload. Conductors sized to either MCA for AC equipment or to full load currents (as we are instructed in 430) are all that is required both before and after the overload device.
 
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