New 220v circuit for Outdoor AC condenser

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  #1  
Old 08-04-07, 05:31 PM
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New 220v circuit for Outdoor AC condenser

Hello,

I ran a 220volt, 40 amp circuit for a new outdoor condensing unit using 2 #8 hot wires and 1 #10 ground wire. In my subpanel, where the neutral and grounding buss bars are separate, I hooked the ground on the new circuit from the ac unit to the grounding buss. Was this the correct hook-up, or should it be on the neutral buss? Thanks.
 
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  #2  
Old 08-04-07, 06:50 PM
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Correct, in a subpanel, all loads have their grounds connected to the ground bus and their neutrals connected to the neutral bus. In your case, you're using only 240v (instead of 240/120v), and therefore you don't have/need a neutral.

The neutral bus should already be electrically insulated from the ground bus and the panel cabinet. And of course in the main panel, the ground and neutral busses are the same.
 
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Old 08-04-07, 07:14 PM
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However, your air conditioner needs a 120 volt receptacle nearby for servicing. Do you have one, or do you now have to add either a neutral or a separate circuit?
 
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Old 08-05-07, 12:26 AM
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check the nameplate rating. Look at nameplate rating and see what the RLA is. You'll need to multply that number by 125% to allow for inrush current at start up. Then you will need to size your breaker by multiplying a min of 175% to a max of 225%. The nameplate should tell you the min and max size of fuses for your disconnect.

Example of how to do this: Let's say that your a/c RLA on the nameplate is 40 amps. 27 amps x 125% = 33.75 in which you would need a #8 wire for your unit. x 1.75% = 59.06 which you would use a 60 amp breaker.
 
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Old 08-05-07, 06:54 AM
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jamead65,
The 175% number is NOT the minimum overcurrent rating, it is the maximum, provided the unit starts reliably. If the unit does not start reliably then you can go up to 225% MAXIMUM.

See:

440.22 Application and Selection
(A) Rating or Setting for Individual Motor-Compressor
The motor-compressor branch-circuit short-circuit and ground-fault protective device shall be capable of carrying the starting current of the motor. A protective device having a rating or setting not exceeding 175 percent of the motor-compressor rated-load current or branch-circuit selection current, whichever is greater, shall be permitted, provided that, where the protection specified is not sufficient for the starting current of the motor, the rating or setting shall be permitted to be increased but shall not exceed 225 percent of the motor rated-load current or branch-circuit selection current, whichever is greater.
 
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Old 08-05-07, 12:17 PM
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Instead of doing all those calculations just check the nameplate for the minimum ckt size required and size your wire that way.

It certainly won't hurt but you could have run a smaller size wire and still been Code compliant.
 
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