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  #1  
Old 4/12/06, 9:27 PM
jchamp jchamp is offline
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Default AC Unit Maximum Breaker

I had a question regarding the ac unit maximum breaker. If an ac unit is labeled for a maximum 25 amp breaker and a 30 amp breaker was installed in the panel box would this be considered overfused? If so, should a 25 amp breaker be installed? Please let me know.

Thanks.
Jim
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  #2  
Old 4/12/06, 9:30 PM
Larry Kage, CMI Larry Kage, CMI is offline
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Default Re: AC Unit Maximum Breaker

Yes.
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  #3  
Old 4/13/06, 12:49 PM
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David A. Andersen David A. Andersen is offline
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Default Re: AC Unit Maximum Breaker

Is this new construction?
Yes: Change it.
No: Report it.

Because it is only 5 amps I would look closely at the wire and insure that it is rated for the higher amperage under an inductive load.

It may be wrong IAW the Mfg. but an over current device is there to protect the wire on the branch not the appliance. If the wire to the unit is rated for the breaker size at least you won't have a fire hazzard to the unit.

Also a concern is if the breaker is rated for HVAC use.
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  #4  
Old 4/14/06, 12:25 PM
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roconnor roconnor is offline
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Default Re: AC Unit Maximum Breaker

Also consider that the condenser may have internal wiring matched to the Max Fuse/Breaker Rating indicated on the label, and an older oversized breaker may not trip if the compressor motor locks up ... which could complete fry the equipment ...

http://www.nachi.org/bbsystem/viewtopic.php?t=15285

IMO report it as a defect needing evaluation/repair and call it a day ...
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  #5  
Old 4/14/06, 3:24 PM
Dennis DiVito Dennis DiVito is offline
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Default Re: AC Unit Maximum Breaker

every HVAC guy and electrician I speak to say it is alright to go to the next 5 amp size. They want to see that the wire size is consistent with the higher size. None of them will change the breaker when called out to confirm this.

At ITA I remember them saying that the wire size can be smaller if matched to the minimum ampacity rating on the dataplate.
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  #6  
Old 4/14/06, 4:48 PM
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Default Re: AC Unit Maximum Breaker

Being an HVAC tech for a long time that is what would happen. The tech will say no problem. As long as the breaker is correct for the wire size 5 amps isn't a big deal. Start up amps can be in the 60-amp range. I've seen units under this same scenario (25 amp nameplate) that would cause nuisance trips when a 25-amp breaker was installed and when the 30-amp breaker was put in the trips stopped and there were never any problems.
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  #7  
Old 4/14/06, 7:02 PM
Larry Kage, CMI Larry Kage, CMI is offline
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Default Re: AC Unit Maximum Breaker

It is over fused if a breaker larger than the manufacturer's label calls for is installed.

Here is a good explanation I picked up somewhere:

A/C condensers contain a hermetically sealed compressor motor as well as a fan to circulate air across the coils. The rules for protection to motor circuits are different than for circuits with simple resistive loads. When a motor first starts , it draws a much higher amount of current than it does after it is running. The high "inrush" current can exceed the rating of a breaker or fuse sized to protect the wire. The inrush current lasts typically only about 6 electrical cycles, or 1/10th of a second - less time than it would take to damage the wire or its insulation. However, if the overcurrent device is sized to protect the wire against overloads, the device might trip, and the machine would not be able to start.

Motor circuits get around this problem by dividing the two separate functions of an overcurrent protection device. "Overloads" are currents that can damage a circuit if allowed to continue for a sufficient time, whereas "short circuits" and "ground faults" are high currents that can cause immediate damage. Large motors and air conditioners separate these overcurrent functions. The breaker or fuse ahead of the air conditioner only protects against short circuits and ground faults. Overload protection is built into the compressor itself. The wire to a condensing unit must be large enough to allow the equipment to start. The device is therefore NOT sized to protect the wire against overloads. It is not uncommon to see a 50 Amp breaker on a #10 wire. The wire is protected against overloading by a separate thermal protection inside the compressor.

How do we know what the motor requires for wire size and proper fuse/breaker protection? It's all on the mfg. label attached to the appliance. If the label calls for fuses, there MUST be a fuse in the disconnecting means, not a circuit breaker. If the label calls for a "HACR" type breaker (Heating, Air Conditioning and Refrigeration), then that is the type of breaker that must be used. If the label simply states "maximum sized over current protection" then any form of overcurrent protection may be used. The HACR designation is not always visible on a circuit breaker once it is installed inside a panel or disconnect.

Note on the label taken from a Lennox A/C data plate:
MIN. CKT. AMPACITY - 17.7.
MAX. FUSE - 25 UL (CSA is a Canadian standard. We use UL in the U. S.)
MAX. HACR TYPE CKT. BKR. - 25 UL
MAX. CKT BKR. UL - Not allowed in U. S.

All this means is the MINIMUM wire size required is #12 (solid copper); the MAXIMUM rating of a fuse (if a fuse is used) must be 25 amps and the MAXIMUM rating of a circuit breaker (if used) must be 25 amps AND the breaker must be a HACR type.

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  #8  
Old 4/15/06, 4:13 PM
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David A. Andersen David A. Andersen is offline
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Default Re: AC Unit Maximum Breaker

Quote:
A/C condensers contain a hermetically sealed compressor motor as well as a fan to circulate air across the coils. The rules for protection to motor circuits are different than for circuits with simple resistive loads. When a motor first starts , it draws a much higher amount of current than it does after it is running. The high "inrush" current can exceed the rating of a breaker or fuse sized to protect the wire. The inrush current lasts typically only about 6 electrical cycles, or 1/10th of a second - less time than it would take to damage the wire or its insulation. However, if the over current device is sized to protect the wire against overloads, the device might trip, and the machine would not be able to start.

"it draws a much higher amount of current than it does after it is running."

This is called an inductive load.

"The high "inrush" current can exceed the rating of a breaker or fuse sized to protect the wire. "

This is why HVAC breakers must be rated for HVAC use.

The wire, breaker (to protect the wire), and type of breaker must be correct and is reflected in the "minimum circuit data".
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  #9  
Old 11/27/10, 6:49 PM
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Michael J. Merino Michael J. Merino is offline
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Default Re: AC Unit Maximum Breaker

I ran into this issue today. The AC is rated 30 amp HACR, max fuse is the same. The breaker in the panel is a 50 amp. It used to be OK to overfuse, is this still OK to do? I have been doing research and I am getting conflicting results. I told the client that it was OK because of the inductive load, and the wire is the #8 in conduit. I also am getting the impression from this thread that it is not OK. Which is it? Does it have to match or can it be overfused?



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  #10  
Old 11/27/10, 6:55 PM
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Jeffrey R. Pope, CMI Jeffrey R. Pope, CMI is offline
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Default Re: AC Unit Maximum Breaker

The circuit can be "over-fused," not the appliance.

The max-rating for the appliance is the maximum allowable fuse/circuit breaker. The minimum rating for the appliance accounts for the "undersized" conductors.



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  #11  
Old 11/27/10, 7:40 PM
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Russell J. Hensel Russell J. Hensel is offline
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Default Re: AC Unit Maximum Breaker

Plain and simple. The manufacture puts a Maximum breaker size on the data plate. IF something were to happen while it is in its warranty stages, it could void the warranty. There is not a company in ANY town that can override the manufacturer. If its wrong it wrong. No such thing as a little wrong. That my 2 cents. I always call it out! Tell the company to show you ANYWHERE in writing from the manufacturer that you can put in a "slightly" larger breaker.



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  #12  
Old 11/27/10, 8:34 PM
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Michael J. Merino Michael J. Merino is offline
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Default Re: AC Unit Maximum Breaker

Quote:
Originally Posted by jpope View Post
The circuit can be "over-fused," not the appliance.

The max-rating for the appliance is the maximum allowable fuse/circuit breaker. The minimum rating for the appliance accounts for the "undersized" conductors.
I understand that the circuit is over-fused, but the appliance is connected to that circuit. If for some reason it were to draw more than 30 amps (outside of the inductive load) the breaker will not trip. I am confused by the differing opinions. I have been told by inspectors it is OK, and some say it is not. Some say that you can over-fuse the AC up to 40%. I do not seem to be able to find a definitive answer. I am thinking that if the max breaker is supposed to be 30 amp, then the breaker should be 30 amp not 50 amp, even if the conductor is the proper size.



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  #13  
Old 11/27/10, 9:09 PM
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Christopher Currins Christopher Currins is offline
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Default Re: AC Unit Maximum Breaker

Was there a fused 30 amp AC disconnect at exterior?






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  #14  
Old 11/27/10, 9:13 PM
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Default Re: AC Unit Maximum Breaker

Maximum fuse or breaker is just that.

The maximum allowed by the manufacturer of the equipment.



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  #15  
Old 11/27/10, 9:24 PM
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Michael J. Merino Michael J. Merino is offline
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Default Re: AC Unit Maximum Breaker

Quote:
Originally Posted by ccurrins View Post
Was there a fused 30 amp AC disconnect at exterior?
There is not a fuse in the disconnect. Just the pull out disconnect.



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