ECN Forum Forums Code Related Discussion NEC & other Code issues Whaaat size disconnect?

Don- Look at this one again- I could be mistaken (it has happened) but we do need to take into consideration the HP of the load based on the rest of 440.12.

[This message has been edited by George Little (edited 02-27-2007).]

George,
Only if you are using a "motor circuit switch" or have two or more compressors on a single disconnect.
Don

Thanks Don.

You need to show me Don, I thnk the disconnect needs to be able to handle the HP involved. Need page and verse.

I believe the typical type disco for a residential A/C unit is not a type that requires HP rating such as 430.109 (2) or (3), in which case determining the equivalent HP in complying with the requirements of 430.109, as stated in 440.12(A)(2), is not necessary.

Of course if you do you a type of disco that requires sizing according to HP rating, then you have to follow 440.12(2).

Can anyone tell me for sure what type of disco it is with the pull out handle that you reinsert upside down to assure it is off, typically seen for res. A/C's?

Also, does 430.109 apply to A/C units, specifically? Section 440 does not refer to 'type' of disco as 430.109 does.

When an A/C unit lists the branch circuit selection load the %'s do not apply. It is easily understandable why most people would interpret the phrase ...'115% of the nameplate rated current or branch-circuit selection current, whichever is greater,...
It sort of reads like--115% of the nameplate rated current or 115% of the branch-circuit selection..; but this is the incorrect interpretation. Branch-circuit selection current is taken at face value as Don pointed out.

This is why you can use #10's and don't need #8's. You don't take 125% of the branch-circuit selection current, you use the number they give you. #10's in the 60 degree column is good for 30 amps.

Also the NEC is not allowing a larger breaker than usual. The conductors are allowed to be protected at the motor, instead of the supply point. The breaker is strictly short circuit protection.

Reno, I think I know what you meant by what you said, but it could be misinterpreted, so I just wanted to clarify.

Gentlemen- I respectfully remind you that we are talking about the ampere rating and horspower rating of the disconnect for air conditioning equipment in Part H of Article 440 and more specifically 440.12(A)(1) talks about the ampere rating and 440.12(A)(2) talks about the horsepower rating. Now it's a case of both not either or when we size the disconnect. The reference in 440.12(A)(2) for calculating the horsepower sends us back to Article 430 and the tables. Usually if the ampacity is correct the horsepower rating is also but the example given of 28a. might not be the case.

George,

Part H? I am looking at the 2002 NEC. Was the layout of the code changed? The Part in the 2002 code Article 440 is--II. Disconnecting Means. (Sorry for my ignorance and lack of an '05 book handy.)

440.12(A)(2) refers back to specifically 430.109. 430.109 has only a few instances where you have to determine the HP rating of a disco such as 430.109(A)(1), or 430.109(C)(3). If 430.109 does not require the disco to be sized using an HP rating then only 440.12(A)(1) is applicable.

Steve- Sorry about the error, - should be "II" not "H", Bad lighting and old eyes I guess. But I still maintain that there is a need to consider the HP when selecting a disconnect or they would not talk about the "locked rotor current" in A. I need to get to the bottom of this because both you and Don are disagreeing with me. I'm still researching it.

A/C disconnect rated at 60A last time I bought 1 about $13.

Phil (Ob)http://www.squared.com/us/squared/corporate_info.nsf/unid/ECA90110AB7098CA85256A3A007091D7/$file/productsa2zFrameset.htm

George,
After taking another look, I think that you are correct in the cases where the disconnect serves an unit that has both a hermetic refrigerant motor-compressor an another motor such as a blower motor or condenser fan. This would cover most commonly installed units. I was only looking at 440.12(A) and that only applies where there is a single hermetic refrigerant motor-compressor without other loads. Sorry for the misinformation.
Don