A/C disconnect size...

[1793]

Would like to know how others size the disconnect, pull out, for A/C's. Do you size for min. or max.? ( min.=16, max=40 )

 

[busman]

The A/C disconnect must be at least the rating of the branch circuit. The BC rating is determined by the size of the primary OCPD protecting the BC. So in your example of min=16 and max=40, if you use a 30A breaker at the panel, then you can use a 30A disconnect. If you go to the 40A breaker at the panel then you would have to use a 60A disconnect (the next size above 30A).

Mark

 

[hillbilly]

Would like to know how others size the disconnect, pull out, for A/C's. Do you size for min. or max.? ( min.=16, max=40 )

I always size the circuit for the Maximum allowed and select the disconnect at or above that number.
Everything that I do that needs a disconnect is generally 60A or less.
Around here a 60A A/C disconnect is less than $10.
steve

 

[LarryFine]

Steve, you've been throwing money away.

 

[hmspe]

I'm not sure I agree with Mark. What about horsepower rating? What I've seen is that standard duty switches generally don't have a high enough horsepower rating to meet 440.12.

Martin

 

[Jim W in Tampa]

We simply slap a 60 amp pull out on wall and forget about it unless this is a really big walk in type

 

[benaround]

Martin, you're 100% correct, unfortunatly the % of electricians who even

check the HP rating, I feel is very low.

 

[monsteriname]

Seeins as the original question of this thread has pretty much been answered and some talk has taken place about a related subject that I am interested in I'll go ahead and ask a question if ya'll dont mind.
When sizing circuits,disconnects,switches for motors, the order I should go in is as follows?:
Circuit ampacity if listed on nameplate, FLA if on the name plate, horsepower/output amps as listed on the motor(Horsepower and output amps have to be adjusted, not used as is)
If I have the jist of it what would be some articles to look at that are the exceptions to the above?
Thanks

 

[hillbilly]

Steve, you've been throwing money away.

That's the way that I operate...It's in the price.
Got more than I can do.
steve
Been thinking about it for a few minutes....
I generally select the branch circuit conductors for the minimum ampacity and use a 60A disconnect.
A 60 costs less than a 30 around here.
Don't know where my head was on my original reply.
steve

 

[hillbilly]

Seeins as the original question of this thread has pretty much been answered and some talk has taken place about a related subject that I am interested in I'll go ahead and ask a question if ya'll dont mind.
When sizing circuits,disconnects,switches for motors, the order I should go in is as follows?:
Circuit ampacity if listed on nameplate, FLA if on the name plate, horsepower/output amps as listed on the motor(Horsepower and output amps have to be adjusted, not used as is)
If I have the jist of it what would be some articles to look at that are the exceptions to the above?
Thanks

Articles 430 and 440 in the NEC.
Also, there have been some recent posts and discussions on this web site that you'll find very helpful. Do a search.
steve

 

[bphgravity]

I bet nobody can tell me how to size the required disconnecting means for fixed electrical space-heating equipment such as a single family dwelling air handler unit...

 

[monsteriname]

Thanks Steve,
I have been studying 430 and 440. Thats where I came up with the generalization I was asking about. I will search more also.

I will be starting a class soon that has motors as part of the curriculam. I am
trying to get a head start so that I wont fall behind the class to much. I have not been to any kind of school since I dropped out of high school 28 years ago. I am a little nervous about it.


bhpgravity, this will be a great one for me. Thanks

Ya'll have a good one.

 

[ceknight]

I bet nobody can tell me how to size the required disconnecting means for fixed electrical space-heating equipment such as a single family dwelling air handler unit...

I thought you just read it off the nameplate.

I don't have my 2005 hymnbook in the office, but if I'm following 1999's 440-33 properly it's the highest motor RLA on the circuit x 125 + FLA of all other equipment motors, rounded up?

 

[kbsparky]

It's been my experience that the motor load is the least of your worries when it comes to fixed electrical space heating in a residential air handling unit. These usually come with what is commonly known as strip heaters installed, with anywhere from 5 kW to 20kW capacity.

The larger units (over 10kW) oftentimes will require more than one circuit, and corresponding disconnect(s). I prefer to wire the units with the built-in unit breakers serving as the required disconnecting means, as they usually do not require an additional external disconnect.

But sometimes the HVAC gang get a case of the cheapies, and use the strip heater assemblies with no unit breakers installed. In those cases, we usually use double-pole 60 Amp rated molded case switches for each 240 Volt up to 60 Amp circuit required. We install the overcurrent protection at the load center, not as part of the disconnecting means.

 

[bphgravity]

I was just pointing out that there is nothing in Article 424 covering this issue.

Look for something in the 2008 that states: "The disconnecting means specified in 424.19(A) and 424.19(B) shall have an ampere rating not less than 125 percent of the total load of the motors and the heaters." (Proposal 17-31)