Red Forman
Member
- Location
- Longview,Texas
- Occupation
- Electrical Designer
What would really explain things would be if someone in the industry explained how hvac manufacturers calculate their numbers.
Confused by hvac mca/mocp
- Thread starter Red Forman
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GeorgeB
ElectroHydraulics engineer (retired)
- Location
- Greenville SC
- Occupation
- Retired
It would be my OPINION that their engineering department used internal wiring and devices that might not support the current available by a higher OPD with some potential internal fault.
The MOP sizing changed because the manufacturer is using the new Type J compressors and also using inverter-based controls which reduce maximum overload protection by up to 29%. Please read the manufacturer's explanation on page 19 here!
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
From an HVAC mfr manual
- Location
- Connecticut
- Occupation
- Engineer
What you quoted doesn't say anything about calculating minimum circuit ampacity, which the NEC requires to be calculated per the rules in the NEC.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
But, does it say who can or must perform that calculation?
I'm happy to use the manufacturers' numbers. It makes life simple.
My own understanding is that UL wants RLA values on compressors instead of FLA! As per definition, RLA is the computed value at which the compressor can be run without its built-in thermal protection tripping! It is a fitting decision as the motor windings of hermetic compressors are in contact with the lubrication oil in the compressor and are being kept cool by the lube instead of the usual air cooling! Additionally, the cool suction temps of the refrigerant add to the cooling of the whole assembly. A hermetic motor winding will likely be capable of running at higher load amps (RLA) than if they were run air-cooled (FLA)! And, RLA will always be greater than the FLA of the motor windings.
Eddie702
Licensed Electrician
- Location
- Western Massachusetts
- Occupation
- Electrician
If it is a packaged unit with multiple loads, fans, heaters, multiple compressors etc the manufacturer is supposed to provide the MCA and MOCP on the equipment nameplate. as far as I know it has always been 125% of the largest load + the full load amps of the other loads to get the MCA.
A single compressor is a different story
A single compressor is a different story
ramsy
Roger Ruhle dba NoFixNoPay
- Location
- LA basin, CA
- Occupation
- Service Electrician 2020 NEC
I've seen spec sheet data diverge from nameplate before.
Notice spec sheet top right shows 2 different models, which would violate Nameplate data, per 440.4(A).
Only equipment nameplate needs to match model number, and actual components installed.
Proper Nameplate data that matched an installed compressor model was not provided in this discussion.
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- Location
- Connecticut
- Occupation
- Engineer
The manufacturer performs the MCA calculation if the manufacturer is providing the nameplate. The calculation must be per the NEC requirements.
Mdsparky
Member
- Location
- massachusetts
- Occupation
- electrician
It also specifies fuses
- Location
- Connecticut
- Occupation
- Engineer
No it doesn't. This is a misconception. Some manufacturers use 'MFS' instead of 'MOCP.'
The installation manual tells you the overcurrent protection can be fuse or circuit breaker, so they obviously don't mean MFS to mean fuses only.
- Location
- Connecticut
- Occupation
- Engineer
The two model numbers are for the indoor unit and the outdoor unit. Together they make up the heat pump system.
Mdsparky
Member
- Location
- massachusetts
- Occupation
- electrician
I doubt the manual was part of the instructions contemplated by 110.3 (b) but, the nameplate undoubtedly was
Mgraw
Senior Member
- Location
- Opelousas, Louisiana
- Occupation
- Electrician
Yes and RLA is NOT FLA. RLA is a useless number to anybody but the manufacturer.
- Location
- Connecticut
- Occupation
- Engineer
Of course it was. If part of the instructions associated with the unit listing say the unit is to be protected with fuses only, the installation manual would tell you that.
The nameplate has nothing to do with the listing instructions. It provides the information that the NEC says it must provide.
- Location
- Connecticut
- Occupation
- Engineer
RLA is not a useless number to anybody but the manufacturer. RLA is used to calculate the minimum supply conductor ampacity, as directed by the NEC.
Mdsparky
Member
- Location
- massachusetts
- Occupation
- electrician
I had a go around with an inspector and Climate Master .. the end result was that, I was told by a senior member of the engineering department , that the manual was not part of listing and labeling instructions.. only the nameplate . His explanation was that if the manual were part of the listing , they would not be able to change or edit them without the whole shooting match going back through the testing agency . ($$ )In my case based on a lengthy discussion once they agreed thier manual was not consistent with what is allowed by the NEC , they immediately changed the electronic version and also changed the printed .
- Location
- Connecticut
- Occupation
- Engineer
You were told wrong. Neither the nameplate nor the installation instructions are part of the listing and labeling instructions.
Nor does this nameplate say you have to use fuses for overcurrent protection .
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Mgraw
Senior Member
- Location
- Opelousas, Louisiana
- Occupation
- Electrician
It is used by the manufacturer to calculate MOCP and MCA as required by the applicable standard not the NEC.
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