Mgraw
Senior Member
- Location
- Opelousas, Louisiana
- Occupation
- Electrician
They haven't complied in 45 years.
Confused by hvac mca/mocp
- Thread starter Red Forman
- Start date
- Status
- Location
- Connecticut
- Occupation
- Engineer
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."
Mgraw
Senior Member
- Location
- Opelousas, Louisiana
- Occupation
- Electrician
Any unit with a compressor that uses rla instead of fla in noncompliant.
- Location
- Connecticut
- Occupation
- Engineer
Non-compliance with what?
Mgraw
Senior Member
- Location
- Opelousas, Louisiana
- Occupation
- Electrician
NEC requires using fla to calculate mca. Manufacturers use rla instead.
- Location
- Connecticut
- Occupation
- Engineer
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...
Mgraw
Senior Member
- Location
- Opelousas, Louisiana
- Occupation
- Electrician
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:
- Location
- Connecticut
- Occupation
- Engineer
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?
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
So, is it expected that compressor RLA is the same for 208V vs 240V? Or is that unusual?
Cheers, Wayne
Cheers, Wayne
Mgraw
Senior Member
- Location
- Opelousas, Louisiana
- Occupation
- Electrician
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
- Location
- Connecticut
- Occupation
- Engineer
Their whole product line? See the unit cutsheet in post #35.
Mgraw
Senior Member
- Location
- Opelousas, Louisiana
- Occupation
- Electrician
Red Forman
Member
- Location
- Longview,Texas
- Occupation
- Electrical Designer
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?
goldsborough
Member
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.
mtnelect
HVAC & Electrical Contractor
- Location
- Southern California
- Occupation
- Contractor, C10 & C20 - Semi Retired
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.
Coach Mike
Member
- Location
- Rock Hill, SC, USA
- Occupation
- Training-Tech Support-Engineering
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?
- Occupation
- Licensed Electrician
I would use the manufacturers specs no matter what else didn't make sense.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
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.
Keep in mind the NEC does not require HVAC equipment to be 'listed' other local mechanical codes probably do though.
See 110.2,
Just like any other case when the NEC does not require listing manufactures need to follow the NEC labeling rules.
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' :
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.
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' :
- Status