Central Air Compressor Wiring / Breaker

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  #1  
Old 09-25-00, 12:44 AM
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My Compressor for my Central Air says it uses 29 amps, but recommends a 50 amp breaker. Currently it is wired with #10 wire and a 40 amp breaker.
Shouldn't it be #6 wire if I use a 50 amp breaker?
 
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  #2  
Old 09-25-00, 03:41 PM
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Central air units must be wired to manufacturer spececifications, which are found on the nameplate of the unit. If the manufacturer recommends a 50A breaker, that is what you must use. As far as wire size is concerned, #10 is most frequently used, but i would recommend a #8 to decrease voltage drop.

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by play it safe:
My Compressor for my Central Air says it uses 29 amps, but recommends a 50 amp breaker. Currently it is wired with #10 wire and a 40 amp breaker.
Shouldn't it be #6 wire if I use a 50 amp breaker?
<HR></BLOCKQUOTE>

 
  #3  
Old 09-25-00, 05:40 PM
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Dave, I'm surprised. My book says you need #6 wire with a 50-amp breaker. Am I reading it wrong?
 
  #4  
Old 09-25-00, 07:29 PM
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Motors are a different world than any other parts of the electrical design. This is the where "otherwise permitted" you always found in your ampacity charts. See 240-3-G it will refer you to 430-22 for motors=FLC times 1.25% = ampacity of wires serving motors. 430-152 sizes the overcurrent device HARC breaker = FLC of motor times 250% maximum overcurrent device. Thermal in motor serves as your overload sized by 430-32 and 430-34 for minimum and maximum overload sizes

You could legally put a 10 ga wire on a 240 volt 3 phase motor with a 28 amp full load current with a 75 amp breaker serving that 10 ga wire as a maximum overcurrent device and meet the minimum safety standards and meet engineering specs as a safe wiring design. The overload protects the motor and the conductor. This is the one place in the code that you are expected to protect the conductor on the load side of a circuit.

Your Hermatic motor compressor is a little bit different but the wiring design principles are the same.

Your hermatic motor compressor has a built in overload protecting both the motor and the conductor serving the motor. Your overcurrent device is just a short circuit device.


A HERMETIC MOTOR COMPRESSOR DOES NOT HAVE HORSEPOWER, OR FULL LOAD AMPERE (FLA) LIKE A STANDARD MOTOR, ITS CURRENT DRAW CHANGES WITH PRESSURE DEVELOPED BY THE COMPRESSOR.

MINIMUM CIRCUIT AMPACITY [MCA]; THIS IS THE MINIMUM CIRCUIT AMPACITY REQUIREMENT TO DETERMINE THE CONDUCTOR SIZE, AND SWITCH RATING. MCA DATA IS FOUND ON THE NAMEPLATE, AND IS DETERMINED BY THE MANUFACTURER AS FOLLOWS; [RLA X 1.25] + OTHER LOADS. IN OUR EXAMPLE, THE NAMEPLATE READS;SEE SECTION 440/33 AND SECTION 440/35 OF THE NEC.

ALWAYS USE BRANCH CIRCUIT SELECTION CURRENT - VALUE [IF SHOWN ON THE NAMEPLATE FOR SELECTION OF CONDUCTORS AND DISCONNECTING MEANS INSTEAD OF THE RLC CURENT VALUE. SEE SECTION 440/2 AND 440/4C OF THE NEC. IF NOT SHOWN USE THE RATED LOAD CURRENT, OR RATED LOAD AMPS [RLC-RLA] FOR SELECTION OF CONDUCTORS AND DISCONNECT MEANS.

MAXIMUM OVER-CURRENT PROTECTION [MOP]; THE MAXIMUM OVER-CURRENT PROTECTION IS DETERMINED BY THE MANUFACTURER, AND THIS DATA IS FOUND ON THE NAMEPLATE. IF THE MAXIMUM OVER-CURRENT PROTECTION IS NOT FOUND ON THE NAME PLATE, IT MAY BE DETERMINED AS FOLLOWS; [RLA X 2.25] + OTHER LOADS.

OTHER LOADS MIGHT BE COILS, HEATING ELEMENTS, CRANKCASE HEATERS, FAN MOTORS, ETC.

A TYPICAL NAMEPLATE WILL SHOW THE MAXIMUM SIZE FUSE OR MAXIMUM SIZE HACR TYPE BREAKER. IF FUSES ARE SPECIFIED ONLY AND HACR BREAKERS ARE NOT MENTIONED, CIRCUIT BREAKERS ARE NOT PERMITTED ON THAT CERTAIN EQUIPMENT. THE NAMEPLATE INDICATES A MIXIMUM SIZE FUSE, OR HACR BREAKER.
THE LETTERS HACR MEAN HEATING, AIR CONDITIONING, REFRIGERATION. MOST MOLDED CASE CIRCUIT BREAKERS TODAY ARE MARKED HACR. WATCH OUT WHEN CONNECTING HVAC, OR REFRIGERATION EQUIPMENT TO EXISTING ELECTRICAL PANELBOARDS WHERE THE HACR MARKING MIGHT NOT BE FOUND ON THE BREAKER. A SIMPLE WAY TO KNOW IF THE INSTALLATION MEETS CODE IS TO MAKE SURE THAT THE HACR MARKING APPEARS ON THE COOLING UNITíS NAMEPLATE, AND ON THE BREAKERíS LABEL. SEE SECTION 440/22.

Hope this clears up some questions and not muddy the waters for you.

WG
 
  #5  
Old 09-25-00, 07:43 PM
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Thanks WG. You certainly add to the electrical knowledge of almost everybody here, and help us electrical hackers learn more of the code.
 
  #6  
Old 09-25-00, 11:21 PM
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Thanks for the help!
 
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