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#101240 - 02/26/07 04:51 PM Whaaat size disconnect?
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
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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#101241 - 02/26/07 05:24 PM Re: Whaaat size disconnect?
Steve T Offline
Member

Registered: 02/14/01
Posts: 312
Loc: Oak Park, IL, USA
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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#101242 - 02/26/07 07:31 PM Re: Whaaat size disconnect?
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
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).]
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#101243 - 02/26/07 08:24 PM Re: Whaaat size disconnect?
ShockMe77 Offline
Member

Registered: 06/11/05
Posts: 823
Loc: Rahway, New Jersey
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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#101244 - 02/27/07 05:11 AM Re: Whaaat size disconnect?
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
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
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#101245 - 02/27/07 06:35 AM Re: Whaaat size disconnect?
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
Don- I agree with you about the sizing based on "branch circuit ampacity". I wonder what your thinking is about the horsepower thing?
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#101246 - 02/27/07 06:49 AM Re: Whaaat size disconnect?
ShockMe77 Offline
Member

Registered: 06/11/05
Posts: 823
Loc: Rahway, New Jersey
In which cases do I need to consider the 115%?

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#101247 - 02/27/07 07:41 AM Re: Whaaat size disconnect?
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
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
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#101248 - 02/27/07 07:41 AM Re: Whaaat size disconnect?
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
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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#101249 - 02/27/07 07:42 AM Re: Whaaat size disconnect?
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
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
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