How to determine MCA and MOCP for packaged AC units

cppoly

Senior Member
Location
New York
Just curious how you would calculate MCA and MOCP for packaged AC units, even though this data is supposed to come from the manufacturer.

For example:

A packaged AC unit has a 5 HP (16.7A) evaporator fan, two 3 HP (10.6A) condenser fans, and two compressors at 19.0A. How would you calculate the MCA and MOCP?
 

GoldDigger

Moderator
Staff member
Just curious how you would calculate MCA and MOCP for packaged AC units, even though this data is supposed to come from the manufacturer.

For example:

A packaged AC unit has a 5 HP (16.7A) evaporator fan, two 3 HP (10.6A) condenser fans, and two compressors at 19.0A. How would you calculate the MCA and MOCP?
The answer for MCA should be fairly simple: 125% of the FLA of the largest motor plus 125% of all non motor continuous loads plus 100% of all other motor or non motor loads. But that may not be what the manufacturer does, especially if they know that some loads will never be concurrent or if they know that two or more motors will start simultaneously.

The answer for MOCP is a whole different animal. It cannot really be calculated without knowing the internal wire sizes and and any integral protection, among other things, and is probably actually something determined by testing of the complete product. You can't do it.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
The answer for MCA should be fairly simple: 125% of the FLA of the largest motor plus 125% of all non motor continuous loads plus 100% of all other motor or non motor loads. But that may not be what the manufacturer does, especially if they know that some loads will never be concurrent or if they know that two or more motors will start simultaneously.

The answer for MOCP is a whole different animal. It cannot really be calculated without knowing the internal wire sizes and and any integral protection, among other things, and is probably actually something determined by testing of the complete product. You can't do it.
I don't see any requirement for 125% of non-motor continuous loads.
 

GoldDigger

Moderator
Staff member
440.22 says 175% of compressor up to 225%.
But some appliance manufacturers seem to be declaring an MOCP which is almost identical to the MCA, even if that is less than 175% of the motor FLA.
175% to 225% of compressor without taking into consideration any other integrated loads does not seem right.
Or are you saying that if 225% of the compressor turns out to be less than the MCA then that device cannot legally be supplied by one branch circuit?
 

ActionDave

Moderator
Staff member
Location
Durango, CO, 10 h 20 min without traffic from wing
Occupation
wire pulling grunt
But some appliance manufacturers seem to be declaring an MOCP which is almost identical to the MCA, even if that is less than 175% of the motor FLA.
175% to 225% of compressor without taking into consideration any other integrated loads does not seem right.
Or are you saying that if 225% of the compressor turns out to be less than the MCA then that device cannot legally be supplied by one branch circuit?
If MOCP is posted on the unit that's what you have to use. NEC gives you a way to select one if it is not.
 

Mgraw

Senior Member
But some appliance manufacturers seem to be declaring an MOCP which is almost identical to the MCA, even if that is less than 175% of the motor FLA.
175% to 225% of compressor without taking into consideration any other integrated loads does not seem right.
Or are you saying that if 225% of the compressor turns out to be less than the MCA then that device cannot legally be supplied by one branch circuit?
MOCP is calculated using rated maximum current, if I remember correctly. It is the maximum current the HVAC manufacturer expects a particular motor or compressor to operate in the unit it is installed in. It is possible for the calculated MOCP to be lower than the MCA. In that case the MOCP would be at least the MCA.
 

GoldDigger

Moderator
Staff member
MOCP is calculated using rated maximum current, if I remember correctly. It is the maximum current the HVAC manufacturer expects a particular motor or compressor to operate in the unit it is installed in. It is possible for the calculated MOCP to be lower than the MCA. In that case the MOCP would be at least the MCA.
I think you do not have a clear understanding of what the Maximum OverCurrent Protection (MOCP) means.
 

Mgraw

Senior Member
I think you do not have a clear understanding of what the Maximum OverCurrent Protection (MOCP) means.
Actually I do. It is calculated using UL 1995 section 36.15.

"The “Motor Rated Current” is sometimes referred to as the FLA (full load amps) of the unit. This can be a source of confusion because this rated current is not the same as the motor FLA shown on the nameplate of the motor itself. Our “Motor Rated Current” is determined during worst-case, high-current test conditions of the complete terminal unit, in accordance with UL1995. The FLA on the motor nameplate is a rating from the motor manufacturer and is of no use in our calculations. "
 
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GoldDigger

Moderator
Staff member
Actually I do. It is calculated using UL 1995 section 36.15.
So the circuit overcurrent device can be set as low as the MCA value even though the MOCP, which you call the "maximum current the unit will operate at" is much higher?
Just why should the OCP be absolutely no larger than the maximum operating current?
I do not have a copy of 36.15 to refer to, but I am hoping that it is your paraphrase of it that is the problem.
 

Mgraw

Senior Member
So the circuit overcurrent device can be set as low as the MCA value even though the MOCP, which you call the "maximum current the unit will operate at" is much higher?
Just why should the OCP be absolutely no larger than the maximum operating current?
I do not have a copy of 36.15 to refer to, but I am hoping that it is your paraphrase of it that is the problem.
No. What I am saying is manufactures don't use FLA when calculating MOCP. They use rated current which is what they have calculated to be the maximum current the motor or compressor will operate in that particular unit. Why, I don't know.
 

iwire

Moderator
Staff member
Location
Massachusetts
MOCP is calculated using rated maximum current, if I remember correctly. It is the maximum current the HVAC manufacturer expects a particular motor or compressor to operate in the unit it is installed in. It is possible for the calculated MOCP to be lower than the MCA. In that case the MOCP would be at least the MCA.
No, I do not believe that to be the case.
 

Mgraw

Senior Member
No, I do not believe that to be the case.
I don't know all the formulas used but what I do know is compressor manufacturers do not use FLA. They use "Rated Load Amps". Rated load amps is determined by dividing the overload trip current(as determined by a specific test) by 1.56-1.44.(the value is determined by the formula used by the manufacturer) This has been required by UL since the early 70's.
 
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