ac breaker size

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from Mike Holt's recent newsletter

from Mike Holt's recent newsletter

Single Motor-Compressors. Branch-circuit conductors must have an ampacity not less than 125 percent of the motor-compressor rated-load current or the branch-circuit selection current, whichever is greater [440.32].
For these same conductors, size the short-circuit and ground-fault protection between 175 percent and 225 percent of the rated-load current [440.22(A)].
Let?s work another problem. Question: What size conductor and short-circuit and ground-fault protection device are required for an 18A motor compressor?
(a) 12 AWG, 30A (b) 10 AWG, 50A
(c) a or b (d) 10 AWG, 60A
Answer: (a) 12 AWG, 30A
Step 1: Determine the branch-circuit conductor [Table 310.16 and 440.32]:
18A x 1.25 = 22.50A, 12 AWG, rated 25A at 75?C [Table 310.16]
Step 2: Determine the branch-circuit protection [240.6(A) and 440.22(A)]:
18A x 1.75 = 31.50A, next size down = 30A
If the 30A short-circuit and ground-fault protection device isn?t capable of carrying the starting current, you can size the protection device up to 225 percent of the equipment load current rating. 18A x 2.25 = 40.50A, next size down 40A.
A 30A or 40A overcurrent device can protect a 12 AWG conductor against ground-faults and short circuits for an air-conditioning circuit. See 240.4(G) for details.
 
billsnuff said:
Single Motor-Compressors.
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Bill very few of us wire "Single Motor-Compressors" we most often wire factory produced HVAC units and for those we have no calculations to perform. We take the minimum circuit ampacity as given at 100% and use the over current protection indicated on the units label.
 
CooperTone,
Your post was spot on. But, I still strongly suggest to those that don't
understand the exceptions to the "Small Conductor" rules 240.4 (D).
After understanding that A/C units do the math for you,next move to motors,
where YOU have to do the math. Once you understand the motor calcs,
the A/C math that is already done for you, will make more sense.
When I completed a correct Feeder calc for 10 motors of different HP's
including the OL's for each motor. That was when I truly understood why
I can have a 14AWG protected by a 40 amp breaker.

Yes people will think you're crazy putting a 14AWG under a 40 amp breaker
for a motor load ,but that is because they don't understand the math and rules. One of my favorite sayings is', "The smarter I get,the dumber I sound."
 
And I as an old motor man thats connected tons of motors as well as HVAC units just love to crush the egos of the inspectors who wrongly write up a correction for 14 on a 30 or 12 on a 40. All I do is in a diplomatic way take them thru the calculations as well as the 4 or 5 code sections that get you to to the bottom line and hope they get it. Some do, some dont. The ones that dont usually will have a senior inspector or lead that knows the rules though.
 
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