A/C Electrical


  #1  
Old 01-14-01, 10:47 AM
Guest
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I have a new roof-mounted 5-ton 220 volt A/C unit. It is wired with 2 No. 8 wires and a No. 10 ground. Distance from the panel to the unit is 75 feet. The circuit breakers (double for 220) are 50 amps. When the A/C unit starts, the circuit breakers chatter for about 1/2 to 1 second. This is very loud because the panel is next to the bedroom. Can this chattering be corrected, and how? The electrical people I have talked to, including contractors and my power company, don't have the slightest idea what is causing the chattering, or if they do, they won't tell me. However, I believe that chattering of circuit breakers on high loads is somewhat common. It's common, probably, because nobody knows how to fix it.
 
  #2  
Old 01-14-01, 01:08 PM
Wgoodrich
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The following is an explaination that should help you discover your chattering problem of your breaker and resolve it within the limits of the NEC.

The name plate rating will tell you the size branch circuit conductor required for a/c unit. This minimum branch circuit conductor size is marked in amps on the name plate rating of your A/C unit. MINIMUM CIRCUIT AMPACITY [MCA]. This tells you the minimum amapacity required for a conductor to serve this A/C unit. Then go to table 310/16 and that chart will tell you the conductor size that coincides with that ampacity. Remember to use the column for 60 degree C. for conductors smaller than # 1 whether copper or aluminum due to the terminal temprature limitations.

Now this subject is one of those other areas that allows the conductor protection on the load side. Your A/C unit has an overload built into the unit. This overload is the protection of your branch circuit conductor, not the breaker or fuse. The breaker or fuse can be sized as large as the MAXIMUM OVER-CURRENT PROTECTION [MOP] stamped on your name plate of the A/C unit. Regardless of the ampacity of the branch circuit conductor, you may size the fuse or breaker up to the MAXIMUM OVER-CURRENT PROTECTION [MOP] stamped on the A/C unit. Remember that the overload of the hermatic motor [compressor of A/C] is designed to protect the branch circuit conductor.

The reason that your breaker is chattering is either that it is too small to take the inrush of starting the hermatic motor or you are using a breaker that is not HACR rated as required on A/C hermatic motors, or you A/C unit is not allowed to use breakers at all. The HACR breaker must be mentioned on the name plate at you Maximum over-current protection section or a breaker must not be used. Most A/C units are approved for HACR breakers but this is required to be mentioned on the A/C unit's name plate.

I suspect that your breaker is HACR rated, that the A/C unit is approved for use with an HACR rated breaker, but your breaker is not sized large enough for your inrush at the startup of your A/C unit.

Remember that you are supposed to size the breaker as small as possible but not over the maximum overcurrent device rating MOP on the name plate.

Hope this helps

Wg
 
  #3  
Old 01-14-01, 03:47 PM
tammi ann
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Look at the rating plate like Wg said, I think you will find that wire size and breaker size are to small, especially considering the length of the wire.
 
  #4  
Old 01-14-01, 07:18 PM
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AC Breaker

Thanks to you both for the fast reply. This installation complies with Los Angeles City Electrical Code, and was inspected. I have since found out that many installations of AC in Los Angeles have chattering breakers, simply because the code does not require a large enough wire size. Most people, however, do not know about the chattering because their entrance panel is not right next to the bedroom as mine is.
The breaker size is 50 amps, which is the specified size on the AC unit. And No. 8 is the required wire size for a 50 amp breaker. Interestingly enough, I have not been able to find any mention in the Los Angeles electrical code for length of wire. The inspector even told he he only looks at breaker size and wire size. The problem I have now is: How much larger in wire size and breaker should I go to stop the chattering? Maybe going up to No. 6 won't solve the problem. Maybe even No. 4 won't. I don't know.
 
  #5  
Old 01-15-01, 12:41 PM
Wgoodrich
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You are still trying to size the breaker with the conductor. This is a required wiring design in Chapters 1 through 3 of the NEC.

However you are dealing with a motor. Chapter for is a whole different world compared to electrical design. The maximum overcurrent protection MOP on your name plate rating does not set the wire size. The minimum circuit ampacity MCA on the name plate sizes the conductor.

When it comes to motors it is perfectly legal to install a 10 ga. 35 amp rated copper wire with a 90 amp breaker serving that conductor because the conductor is protected by the OVERLOAD OR HEATER required by article 430-34 of the NEC. Again this wiring design is in chapter 4 not chapter 1 through 3.

Again the breaker must be HACR rated. This HACR must be stamped on the breaker.

I suspect that the #8 copper is the required minimum branch circuit conductor size stated in the MCA part of your A/C. I suspect that you should increase the breaker size to the MOP section of the name plate of you A/C.

Remember that the overload built into the compressor inside the compressor protects the branch circuit conductor not the breaker. The breaker only serves as a short circuit device. This breaker does not act as a fuse but only uses the interupting rating of the breaker to kick off just in case there is a short circuit.

You are not to size the breaker by the conductor size. You are to size the breaker as close as the MOP rating says in the name plate of your A/C unit as needed to stop the chattering and the breaker from unnecessary kicking only. The breaker is not used as a fuse to protect this A/C branch circuit conductor.

Check out below;

440-6-A

(a) Hermetic Refrigerant Motor-Compressor. For a hermetic refrigerant motor-compressor, the rated-load current marked on the nameplate of the equipment in which the motor-compressor is employed shall be used in determining the rating or ampacity of the disconnecting means, the branch-circuit conductors, the controller, the branch-circuit short-circuit and ground-fault protection, and the separate motor overload protection. Where no rated-load current is shown on the equipment nameplate, the rated-load current shown on the compressor nameplate shall be used. For disconnecting means and controllers,


Your chattering is very possibly the breaker not rated for the inrush of a motor. HACR rating is required.
Wg
 
 

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