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Whaaat size disconnect? #101240 02/26/07 07:51 PM
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renosteinke Offline OP
Cat Servant
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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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Re: Whaaat size disconnect? #101241 02/26/07 08:24 PM
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Steve T Offline
Member
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.

Re: Whaaat size disconnect? #101242 02/26/07 10:31 PM
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George Little Offline
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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
Re: Whaaat size disconnect? #101243 02/26/07 11:24 PM
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ShockMe77 Offline
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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.

Re: Whaaat size disconnect? #101244 02/27/07 08:11 AM
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resqcapt19 Offline
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shockme,
Quote
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)
Re: Whaaat size disconnect? #101245 02/27/07 09:35 AM
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George Little Offline
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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
Re: Whaaat size disconnect? #101246 02/27/07 09:49 AM
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ShockMe77 Offline
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In which cases do I need to consider the 115%?

Re: Whaaat size disconnect? #101247 02/27/07 10:41 AM
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resqcapt19 Offline
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George,
Quote
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)
Re: Whaaat size disconnect? #101248 02/27/07 10:41 AM
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renosteinke Offline OP
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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.

Re: Whaaat size disconnect? #101249 02/27/07 10:42 AM
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resqcapt19 Offline
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Shockme,
Quote
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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