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).]

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.

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).]

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.

shockme,
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- I agree with you about the sizing based on "branch circuit ampacity". I wonder what your thinking is about the horsepower thing?

In which cases do I need to consider the 115%?

George,
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

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.

Shockme,
In those cases where the nameplate does not list a minimum circuit ampacity.
Don

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

Thanks Don, you and I and so many others really benefit from interacting on this BB, it caused us to compare notes as it were and dig out the old code book. I have had to change my thinking more than once- listening to you, Bob, Roger and the rest so let's keep it going.

George

I've read the section about 30 times. I see what you are getting at George. I agree.

I saw a 30 amp disco, DU221RB, rated at 3 HP.

I still have a question though. Is the intent of 440.12(B) [and 430.110(C) for that matter] trying to say 'figure the HP rating twice(using the motor tables), both by the rated current of the motors and by the locked rotor current of the motors; whichever is the highest HP rating figured is the required rating of the disco?

If that's what is suppose to be done, it doesn't say that very well. If I still don't get it--HELP!

Also, can I still use (trust) the equipment current rating or do I have to check the ampere & LR rating of the compressor and blower motors separately?

i once priced 30amp and 60 amp disconnects and got a better price on the 60 amp one.

I went out and looked at the label of my Trane and the LRA is 91a. How does that relate to disconnect size?

Greg- 91a. would equate to 3 HP per Article 430 T. 430.251(A). I have a 60a. fused switch in the garage that is labeled 3 HP using Standard fuses and 10 HP Maximum. My understanding is that Standard fusing is one time fusing and Maximum is using time delay. (hope I don't have that backwards).

Thanks, that is what I believed but it is good to hear someone else say it.

I ended up with a 60a rated disconnect too since the breaker in it is 40 (per the O/C device spec on the label).

In a disconnect without O/C protection I agree with those who say you will need one that is HP rated, like any other disconnect. I fear a "30" might be aimed at water heaters if it is not specifically labelled with a HP rating. If it is thusly labelled, go by the label. I am sure most pullouts can open LRA but you should be wearing gloves and sunglasses

Hard to beat $8 for a 60A NEMA-3 HVAC disconnect regardless of what ampacity or use. Mass production and economy of scale does have SOME advantages!