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?

- Thread starter cppoly
- Start date

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?

- Location
- Placerville, CA, USA

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.

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 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.

- Location
- Connecticut

- Occupation
- Engineer

I don't see any requirement for 125% of non-motor continuous loads.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.

- Occupation
- wire pulling grunt

440.22 says 175% of compressor up to 225%.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.

- Location
- Placerville, CA, USA

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.440.22 says 175% of compressor up to 225%.

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?

- Occupation
- wire pulling grunt

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.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?

- Location
- Opelousas, Louisiana

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.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?

- Location
- Placerville, CA, USA

I think you do not have a clear understanding of what theMOCP 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.

- Location
- Opelousas, Louisiana

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

"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. "

Last edited:

- Location
- Placerville, CA, USA

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?Actually I do. It is calculated using UL 1995 section 36.15.

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.

- Location
- Opelousas, Louisiana

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.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.

- Location
- Massachusetts

No, I do not believe that to be the case.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.

- Location
- Opelousas, Louisiana

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.No, I do not believe that to be the case.