# How do you properly size for HVAC equipment



## electricista

telecom_guy said:


> I have been reading Article 440 and am not quite understanding how to figure the conductor size and disconnect size. My specific application is:
> 
> Installing a rooftop HVAC unit with these manufactures spec's,
> 
> -minimum circuit amperage of 118 amps
> -maximum overcurrent protection of 125 amps
> -full load amps are 127
> -there are two compressor motors each with a locked rotor amperage of 208 (do you add these to get the units LRA?)
> -voltage is 208 3 phase
> 
> Can someone please explain with the code how to size the disconnect, overcurrent protection and conductor size? Thanks.


Where did you get these specs. Something does not seem right. Normally the max ocp is 175% of the FLA and I would think the min. cir amp. MCA would not be less than the FLA.


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## Minuteman

I'm at my son's house, and do not have my code book handy, but start at 240.4 and look at D and G.

Size the disco and the OCP for your MAX amps, and you may size your conductors and conduit and the MIN amps.

Nameplate gives you all the info that you need.


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## MDShunk

Minimum circuit ampacity = wire gauge

Maximum fuse or breaker size = overcurrent protection size

That's really all you need to know.


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## telecom_guy

I thought the spec's were off too! Here's the link, page 176, unit is a 20 ton with the largest config.

http://www.docs.hvacpartners.com/idc/groups/public/documents/techlit/48pg-5pd.pdf

Thanks for everyones input, much appreciated!:thumbsup:


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## electricista

If that were the info I had I would do as Marc said but I would double check with a phone call.


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## Minuteman

electricista said:


> If that were the info I had I would do as Marc said but I would double check with a phone call.


What??? I said the same basic thing as Marc, and beat him by 5 minutes...


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## MDShunk

Minuteman said:


> What??? I said the same basic thing as Marc, and beat him by 5 minutes...


Awesome. If I could put a smiley face stamper on your post, I would. :thumbsup:


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## telecom_guy

Thank you _all_ for the help, it is much appreciated. By the way Marc, the first time I read one of your posts, I was wearing my pagamas and it was about 2:30 in the afternoon. Your tag line made me feel pretty bad about myself so I went and changed. :laughing: 

Please note: pagamas are not my standard attire.


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## MDShunk

telecom_guy said:


> Thank you _all_ for the help, it is much appreciated. By the way Marc, the first time I read one of your posts, I was wearing my pagamas and it was about 2:30 in the afternoon. Your tag line made me feel pretty bad about myself so I went and changed. :laughing:
> 
> Please note: pagamas are not my standard attire.


Pajamas in the daytime is a veiled reference to spending your days in a psyche ward.


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## william1978

MDShunk said:


> Minimum circuit ampacity = wire gauge
> 
> Maximum fuse or breaker size = overcurrent protection size
> 
> That's really all you need to know.


 This right here is what you need to go by. I would have said the same thing. :thumbsup:


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## telecom_guy

Thank you for that youtube video Marc, I now consider myself educated and speechless, along with a barage of other unexplainable emotions!

In regard to my origional question, article 440.11A it says that: 

"A disconnecting means serving a hermetic refrigerant motor compressor shall be selected on the basis of the nameplate *rated-load current* or *branch-circuit selection current,* whichever is greater, *and locked-rotor current*, respectively, of the motor-compressor as follows.". 

How does the LRA tie into sizing your disconnect? If my LRA is 400amps does that simply mean that my disconnect must be rated for at least 25 hp as according to table 430.251B under 208 volts? Are there hp, RLA and LRA ratings on disconnects that all need to be addressed?


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## MechanicalDVR

telecom_guy said:


> Thank you for that youtube video Marc, I now consider myself educated and speechless, along with a barage of other unexplainable emotions!
> 
> In regard to my origional question, article 440.11A it says that:
> 
> "A disconnecting means serving a hermetic refrigerant motor compressor shall be selected on the basis of the nameplate *rated-load current* or *branch-circuit selection current,* whichever is greater, *and locked-rotor current*, respectively, of the motor-compressor as follows.".
> 
> How does the LRA tie into sizing your disconnect? If my LRA is 400amps does that simply mean that my disconnect must be rated for at least 25 hp as according to table 430.251B under 208 volts? Are there hp, RLA and LRA ratings on disconnects that all need to be addressed?


 
Or you can do what everyone else does an use the nameplate specs as MD and William have already told you.


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## Minuteman

MechanicalDVR said:


> Or you can do what everyone else does an use the nameplate specs as MD and William have already told you.


And Minuteman... (What am I, chopped liver? I was first AND I gave a code reference!!!)


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## MechanicalDVR

Minuteman said:


> And Minuteman... (What am I, chopped liver? I was first AND I gave a code reference!!!)


 
Sorry seemed to have skipped over your post not intentionally. It just seems like such an elementary thing using the equipment manufacturers specs. I wonder if that is why they put them on the plate like that????


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## telecom_guy

telecom_guy said:


> In regard to my origional question, article 440.11A it says that:
> 
> "A disconnecting means serving a hermetic refrigerant motor compressor shall be selected on the basis of the nameplate *rated-load current* or *branch-circuit selection current,* whichever is greater, *and locked-rotor current*, respectively, of the motor-compressor as follows.".
> 
> How does the LRA tie into sizing your disconnect? If my LRA is 400amps does that simply mean that my disconnect must be rated for at least 25 hp as according to table 430.251B under 208 volts? Are there hp, RLA and LRA ratings on disconnects that all need to be addressed?





MechanicalDVR said:


> Or you can do what everyone else does an use the nameplate specs as MD and William have already told you.


To clarify, my question is not concerning overcurrent protection or conductor size, but rather the size of the disconnecting means. In the code referenced above, it says:

"A disconnecting means serving a hermetic refrigerant motor compressor shall be selected on the basis of the nameplate *rated-load current* or *branch-circuit selection current,* whichever is greater, *and locked-rotor current*, respectively..."

How exactly do you figure the LRA into the sizing of your disconnect? I get the picture of using the name plate rating and such, but I want to understand the code and not simply install according to "the rule of thumb", as good as they tend to be. My apologies should that offend anyone.


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## william1978

Minuteman said:


> And Minuteman... (What am I, chopped liver? I was first AND I gave a code reference!!!)


 Is it time for a group hug...





















Just kidding :laughing::laughing::laughing:


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## 480sparky

william1978 said:


> Is it time for a group hug...


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## william1978

480sparky said:


>


 I just knew when I said that you were going to post this.:laughing:


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## Minuteman

Ah... now I feel better! :thumbup:


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## Mike_586

MechanicalDVR said:


> Sorry seemed to have skipped over your post not intentionally. It just seems like such an elementary thing using the equipment manufacturers specs. I wonder if that is why they put them on the plate like that????


Its in the NEC 

*440.4
(A) Hermetic Refrigerant Motor-Compressor Nameplate
*
...and the *CEC*

*2-100 Marking of equipment*


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