A/C Condenser disconnect Wiring

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  #1  
Old 03-20-06, 10:34 PM
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A/C Condenser disconnect Wiring

My A/C condenser wiring from the unit to the disconnect breaker is 10 GA. and the breaker is 35 Amp which matches the Max Amp Breaker allowed on the data plate. Shouldn't the wiring be 8 GA. or is there an allowance for start-up amps associated with motor/compressors? Thanks in advance.
 
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Old 03-20-06, 11:56 PM
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Are you talking about the disconnect on the outside of the house near the unit? Or the branch circuit breaker in your main panel? If Main main panel then...

If you look at the data plate and give us the rla of the compressor and the fla of the fan motor we can calculate your conductors ampacity. The nameplate may also give a minimum circuit ampacity that will work too as it takes into consideration the compressor and fan motor.

I can give you a reasonable example from the data you have supplied....

The max. circuit breaker (35 amps) suggests that the rla of the compressor is somewhere near 20 amps. since maximum breaker is 175% x RLA. The minimum circuit ampacity would be Rla x 125% + other loads (condensor fan). So if the rla is around 20 amps and the condensor fan is around 1.5 amps typical then the branch circuit conductors would be (20 x 1.25) + 1.5 = 26.5 amps requiring #10 copper conductors rated 30 amps from 60 C column of table 310.16. And the maximum breaker is 1.75 x 20 amps (rla) = 35 amps.

On motors or hermetic compressors breakers can exceed the ampacity of the conductor's to withstand inrush currents during start-up. Generally speaking you dont have to put the max. breaker on the conductors you could have started with a breaker in my example at 30 amps... if the 30 amp tripped on start up then you could move up to 35 amps which is the next standard breaker size but you could not exceed that 35 amps due to manufacturers max. breaker requirement on the nameplate.
 

Last edited by Roger; 03-21-06 at 12:06 AM.
  #3  
Old 03-21-06, 04:47 AM
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Wow. Awesomw answer....I actually was talking about the wiring from the condeser to the exterior located disconnect. Does this change the answer?
 
  #4  
Old 03-21-06, 08:07 AM
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No it doesnt change the conductor ampacity.... most ac disconnects are rated 60 amps or 30 amps and there is a way to calculate the size of disconnect needed. I wasnt sure what you were wanting but the branch circuit conductors dont change to a different size after the disconnect. Generally you just hang a 60 amp rated Ac disconnect as this will cover almost all residential installations and then install the correct HACR rated breaker or dual element time delay fuses in the disconnect. This doesnt mean they are 60 amp just that the disconnect is rated up to 60 amps. It is also common for the disconnect to have a horsepower rating equal to or greater than the loads served by that disconnect for ac equipment.

So if we look at the example the combined load of the compressor and the fan motor is 26.5 amps so the disconnect must be rated a minimum of 115% of that plus rated in hp from the nec tables.

1.15 x 26.5 = 30.5 amp rated disconnect and it must be equal in horsepower to one single phase motor for the combined loads of the compressor and fan. If you were to look at the correct table in the NEC you would see that this falls in between a 3 and 5 hp rated motor. You must use the higher horsepower rating of 5 hp.

30 amp disconnects are rated 3 hp so you would have to get a disconnect rated more than 30 amps and at least equal to 5 hp. Since AC disconnects pretty much come 30 and 60 amp rated you would get a 60 amp minimum 5 hp rated disconnect (allows for locked rotor amps) and install most likely a 35 amp double pole breaker in it or fuses depending on what the manufacturer specifies for the disconnect.

Curious as to what breaker size the electrician chose for the main panel?

Hope this wasnt too confusing
 

Last edited by Roger; 03-21-06 at 11:45 AM.
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