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Joined: Jan 2005
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I really ought to already know the answer ... but I'd like to see your take on this issue.
An air conditioner is being installed. The A/C specifies that the circuit have a minimum ampacity of 28 amps, and a maximum OCPD (breaker) of 40 Amps.
The engineer has specified the 40 amp breaker, #8 wire, and a 30 amp disconnect.
In this situation - where the NEC allows for a larger breaker than usual - is it allowed to place a 30 amp disconnect on a circuit protected by a 40 amp breaker?
[This message has been edited by renosteinke (edited 02-26-2007).]
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Joined: Feb 2001
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440.12(A)(1)--115% of the nameplate rated load or branch circuit selection current, whichever is greater.
Most likely the 30amp disco will be ok. Also you only need number 10's for the conductors. There may be some voltage drop engineering invloved though.
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I suspect that you could be in trouble when you figure the HP rating of the 30a. disconnect against the HP rating of the AC. I would ask the responsible party to give me that information.
Edited to add- 28a. would equate to somewhere around 5 HP per T. 430.151A if I read correctly and I don't think a 30a. disconnect is rated that high. Please correct me if I'm wrong.
[This message has been edited by George Little (edited 02-26-2007).]
George Little
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440.12(A)(1)--115% of the nameplate rated load or branch circuit selection current, whichever is greater.
That tells me that 28 X 1.15 = 32.2 amps.
I'd use a 60 amp disconnect.
Plus there's more room in the 60.
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shockme, 440.12(A)(1)--115% of the nameplate rated load or branch circuit selection current, whichever is greater.
That tells me that 28 X 1.15 = 32.2 amps. The 28 amps is the branch circit selection current and you are not required to take it at 115%. You use it at its face value. Don
Don(resqcapt19)
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Don- I agree with you about the sizing based on "branch circuit ampacity". I wonder what your thinking is about the horsepower thing?
George Little
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In which cases do I need to consider the 115%?
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George, I wonder what your thinking is about the horsepower thing? 440.12(A) does not require a horsepower rating for this disconenct. As long as the ampere rating of the disconnect is equal to to or greater than the branch circuit selection current you are in compliance. Don
Don(resqcapt19)
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Stupid attack on my part. oops.
Thanks, George, for reminding me of the horsepower ratings of the disconnect. I will be sure to mention this to the engineer involved. I do not expect that I will have trouble getting paid for the change order.
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Shockme, In which cases do I need to consider the 115%? In those cases where the nameplate does not list a minimum circuit ampacity. Don
Don(resqcapt19)
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