Confused by hvac mca/mocp

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wwhitney said:
My point still stands that the manufacturer can label the nameplate with a lower MOCP than Section IV allows.

Cheers, Wayne
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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.
 
Red Forman said:
Thank you all for your input and quick responses.
So what I'm getting out of this is, Just to go by whats in the cut sheet as the manufacturer SHOULD have it right.
I must admit I'm a little worried about putting a 19.8a load on a 20a breaker.
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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!
 
From an HVAC mfr manual
  • Rated Load Amperage (RLA). As it relates to HVACR, Rated Load Amperage (RLA) is a mathematical calculation used to get Underwriters Laboratories (UL) approval for a certain compressor motor. It should not be confused with Full Load Amps (FLA). The term Full Load Amps (FLA) has not been used by compressor manufacturers since 1972, when UL changed the term to Rated Load Amps (RLA).
    [*]Maximum Continuous Current (MCC). Compressor manufacturers will conduct many tests on a compressor motor to find the maximum continuous current value the compressor motor will handle before its protector (overload) opens or trips, thus stopping the compressor. This maximum current value is referred to as the Maximum Continuous Amperage (MCA) or Maximum Continuous Current (MCC). MCC and MCA mean the same. It is the actual maximum continuous current value that the compressor’s motor protector will carry without opening. Any additional amperage over the MCC will cause the overload protector to open.
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So again, THEIR rules are different.
 
david luchini said:
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.
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But, does it say who can or must perform that calculation?

I'm happy to use the manufacturers' numbers. It makes life simple.
 
Jraef said:
From an HVAC mfr manual
So again, THEIR rules are different.
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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.
 
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
 
david luchini said:
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.
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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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LarryFine said:
But, does it say who can or must perform that calculation?

I'm happy to use the manufacturers' numbers. It makes life simple.
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The manufacturer performs the MCA calculation if the manufacturer is providing the nameplate. The calculation must be per the NEC requirements.
 
Mdsparky said:
It also specifies fuses
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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.
 
david luchini said:
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.
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I doubt the manual was part of the instructions contemplated by 110.3 (b) but, the nameplate undoubtedly was
 
Mdsparky said:
I doubt the manual was part of the instructions contemplated by 110.3 (b) but, the nameplate undoubtedly was
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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.
 
david luchini said:
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.
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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 .
 
Mdsparky said:
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 .
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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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