30amp d/c vs 60 amp d/c for central A/C condenser

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  #1  
Old 01-24-06, 12:40 PM
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Question 30amp d/c vs 60 amp d/c for central A/C condenser

Thanks for the responses to my earlier question regarding central A/C hookups..... for the condenser>>I used #10/2 (2 hots & ground)BX wiring, the outside box is a square D 30 to 60 amp d/c ...my question is @ the main panel do I use a double pole 30 amp breaker or a 60 amp double pole breaker I was planning on using the 30 amp double pole breaker ...each pole is a 30 so it is a 60 @the d/c box ...right??? My main panel just has a neutral bar and I was reading in an earlier post that I could put the ground on that without worry....just checking.....
thanks for your patience,
Regards,Triple B
 
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  #2  
Old 01-24-06, 12:51 PM
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The outside disconnect is rated up to 60A, it doesn't mean the breaker needs to be 60A. It just means the breaker can't be more then 60A.

The breaker is designed to protect the wires connected to it. If you used 10/2 wiring the breaker can't be more then 30A.
 
  #3  
Old 01-24-06, 02:28 PM
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There are two concepts that can be confusing regarding motor driven devices, especially a/c units. First, the disconnect typically needs to be of a larger rating than the ampacity of the circuit. This is because it needs to be able to conduct (and maybe clear) a current larger than what the wiring is rated for. Size the disconnect to be at least as large as the breaker. The second item is that the breaker is usually allowed to be larger for A/C compressors than the ampacity tables would ordinarily allow. My A/C unit calls for just a 19 amp conductor ampacity, but allows up to a 45 amp breaker (HACR rated). This information is on the label on the outside of the unit. At my house; i've got 10AWG wire, a 60amp disconnect and I will run a 40amp breaker. Check your label to get this data on your unit.
 
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Old 01-24-06, 02:35 PM
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the ocpd shall not exceed 175% of the motor-compressor rated load current and can go up to 225% of the load current to allow for the inrush during start up. the disconnect shall be rated at least 115% of the nameplate rated-load current. the branch circuit conductors shall be 125% of the motor rating. this is covered in nec 440.11, 440.22 and 440.32
 
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Old 01-24-06, 02:50 PM
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I find something wrong with using 10 gauge wiring "protected" by a 40A breaker.
 
  #6  
Old 01-24-06, 03:06 PM
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there is nothing wrong with #10 on a 40 for a motor. in fact many times that #10 will be on a 60 amp breaker for a motor. remember the wire is protected by the overloads which are sized 125% or 115% of the nameplate depending on the type of motor. the breaker is for ground fault and short circuit protection but must be big enough to allow the inrush during start up. the breaker or fuses are sized 2.50 times the flc of the motor based on the chart in article 430. so for a motor conductors will be 1.25 times the table value, the ocpd will be 2.5 times the table value and the overloads will be 1.15 or 1.25 of the nameplate which are protecting the branch circuit conductors. this is for a regular motor. a hermetic motor for air conditioning is in article 440.
 
  #7  
Old 01-26-06, 06:30 PM
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project completed thanks to all those who answered with great advice

project completed thanks to all those who answered with great advice!!!

All the best , Triple B
 
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