Confused by hvac mca/mocp

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Mgraw said:
They haven't complied in 45 years.
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Of course they have. Name one HVAC manufacturer that ignores the NEC's mca and mocp requirements.

I could just see the marketing..."Buy our equipment because we ignore the National Fire Protection Association. We'd love to burn down your building."
 
david luchini said:
Of course they have. Name one HVAC manufacturer that ignores the NEC's mca and mocp requirements.

I could just see the marketing..."Buy our equipment because we ignore the National Fire Protection Association. We'd love to burn down your building."
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Any unit with a compressor that uses rla instead of fla in noncompliant.
 
david luchini said:
Have you read any of posts in the thread?

Or any of the NEC sections that were referenced?

The NEC REQUIRES the use of RLA for compressors in the calculation of MCA.

Maybe this rings a bell...
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It has been aa long time since I have read it. I am glad to see he nec catch up.

But how about this example:


Screenshot 2020-09-20 083640.png
 
david luchini said:
If by "catch up" you mean "been that way for the last 50 years," then yeah.

Again, have you read any of the other posts in this thread?
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If you mean your comment that it must be a mistake then yes, but their whole product line is consistent with that chart.

here is another manufacturers chart

Screenshot 2020-09-20 103629.png
 
Here is a few more for you
 

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So, am I correct in assuming (I hate that word) that i can use the manufacturers mca and ocpd numbers in my drawing set regardless of weather or not they make sense when normally calculating ocpd based on mca and be ok?
 
Red Forman said:
Hello all,
I am a bit confused when it comes to specifying for hvac equipment. Going by the spec sheets (for instance) a 3 ton mini-split shows 19.8 mca (compressor 18.25, outdoor fan .83, indoor fan .37 = 19.45) with a 20a mocp. How does this work out. It is my understanding that mocp is supposed to be 125% of the load. By the math this should have a 25a ocpd. Could someone please explain this. Thank you in advance.
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The 125% pertains to the conductor size. The Maximum Overcurrent protection is listed on the nameplate. 440.22 States you to not exceed by multiplying 175% x the RLA and if that isn't sufficient for the starting current you may multiply the RLA by 225% which should match whats on the nameplate.
 
In a HVAC system you have the refrigerant in a closed loop system which includes the "High Side" - liquid and the "Low Side"- suction. The hermetically sealed compressor benefits from the return suction vapor providing cooling to the motor windings. A normal motor does not have this benefit.
 
Generally speaking, the OCPD is sized a bit higher to allow for motor starting and prevent nuisance tripping. If there is something in the design of the A/C unit (soft start, low sKVA compressor, etc.) that permits a smaller OCPD than would be traditionally calculated, why is there any concern?
 
Red Forman said:
So, am I correct in assuming (I hate that word) that i can use the manufacturers mca and ocpd numbers in my drawing set regardless of weather or not they make sense when normally calculating ocpd based on mca and be ok?
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Yes. The wire size must suit the MCA, and the breaker must not exceed MOCP. Trust those two numbers, no calcs needed.

Let us know if you want the code sections that allow exceeding normal breaker limits.
 
We should go back to basics! Everyone follows what the manufacturers say how their products are designed to be used, period! Any deviation from their specifications will void any guarantee or warranties that go with their products, IMHO!
 
Keep in mind the NEC does not require HVAC equipment to be 'listed' other local mechanical codes probably do though.
Jraef said:
The NEC is for when YOU have to determine MCA and MOCP in the field from basic nameplate information. For an EQUIPMENT MANUFACTURER, they need to satisfy UL standards, in this case probably UL1995, which has its own set of rules for determining the motor amp loading based on what the equipment mfr expects in terms of performance, then that is used in their MCA and MOCP data. So once THEY provide you with the MCA and MOCP, you need only follow those values, you do not calculate them again.
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David's right that Equipment Manufacturers still need to follow the rules in Art 440 for an AHJ (in a NEC jurisdiction) to accept their products.
See 110.2,
Just like any other case when the NEC does not require listing manufactures need to follow the NEC labeling rules.

david luchini said:
The NEC says the equipment shall be marked with the minimum supply circuit conductor ampacity, as determined by Part IV of Article 440 of the NEC.
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You are correct that 440.4(B) Requires the manufacturer to mark the 'minimum supply circuit conductor ampacity'
And 440.4(C) Deals with branch circuit selection current.
My question to David is are you saying 440.22 is dictating minimums for the manufacturer to calculate this?
They way I read 440.22 is those are maximum's the manufactures 'minimum supply circuit conductor ampacity' calc cannot exceed.
As per 90.1(B) the NEC wont get involved with this minimum calculation, rather the maximum values that can be considered 'necessary for safety'
Keep in mind UL 1995 4.5 specified the printed instructions are infact included in the listing so 110.3(B) applies.


UL 1995 44.14 states how to calculate the 'minimum circuit ampacity' :
1995_44.14_MCA_calc-t1.png
 
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