RTU MCA and MOCP

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Linz

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480V 3phase roof top unit (RTU) has the following with one electrical service:
Compressor #1 FLA = 7.7
Compressor #2 FLA = 7.7
Electric Heater FLA = 39.7
Indoor Motor FLA = 5.6
Outdoor Motor FLA = 7.4
Electric Heater Number of Stages = 2

Could all 5 loads start and/or run at the same time?

How would you calculate the MOCP based on this information?
 
Welcome to the forum!

Rooftop units will have a nameplate with the MOCP on it, calculated by the manufacturer.

Whether the loads all run at the same time depends on if it's a heat pump, air conditioner with reheat, or just an air conditioner with electric heat. In the first two cases, all the loads can run at once. In the third, the compressors might be considered non-coincident with the heater. Does the unit have a single point of connection? Sometimes there is a separate circuit for the heater.
 
JoeStillman said:
Welcome to the forum!

Rooftop units will have a nameplate with the MOCP on it, calculated by the manufacturer.

Whether the loads all run at the same time depends on if it's a heat pump, air conditioner with reheat, or just an air conditioner with electric heat. In the first two cases, all the loads can run at once. In the third, the compressors might be considered non-coincident with the heater. Does the unit have a single point of connection? Sometimes there is a separate circuit for the heater.
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The MCA listed on the cutsheet says 80A and the MOCP listed on the cutsheet says 80A. (We sometimes find errors on the MCAs and MOCPs listed on cutsheets so were are trying to understand how they are typically calculated so we can keep an eye out for mislabeled MCAs and MOCPs.)

It says the heating type is heat pump and the heat control is two-stage cooling / one circulating.

It says it has a single point of connection.

And thank you! I've definitely learned a lot from this forum. : )
 
Ok I see it is a heat pump. I posted as you did. So does the heat pump come on when the heat strips are on? These units usually can get wired either way. It sounds like the load is figured with the heat pump and heat strips on together.

If you figure 125% of the heat strips and add the other loads you get 78 amps, however it should be marked on the unit as others have said.
 
Dennis Alwon said:
Ok I see it is a heat pump. I posted as you did. So does the heat pump come on when the heat strips are on? These units usually can get wired either way. It sounds like the load is figured with the heat pump and heat strips on together.

If you figure 125% of the heat strips and add the other loads you get 78 amps, however it should be marked on the unit as others have said.
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It does have a line item for "Disconnect FLA - 78" and Disconnect LRA - 159" which we have not seen listed out like that before. Thank you. I see now mathematically where the 78.025A comes from.
 
Since you asked,
MCA = 1.25 x (Load 1, + Load 2 + Load 3 etc.), but non-coincidental loads can be taken into account, which is why it is incumbent on the EQUIPMENT mfr to do it because THEY determine / know which loads will never run at the same time, you can only ASS-U-Me. You could do it without that information, but will end up with a higher MCA. That's only important in terms of cost (and install issues)

MOCP = (2.25 x largest load) + Load 2 + Load 3 etc. Again, some issues apply with regard to non-coincidental loads.
 
Jraef said:
Nope, it's 1.25 x everything. The largest load only is used in the MOCP. I did it wrong for a few years until I was caught, that's why it is burned into my mind...
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Jraef, I am quite certain that is not correct. I just looked at a submittal for a 40 ton package unit with multiple compressors and the MCA number matches that when you only multiply the largest motor times 1.25. Additionally, this is from UL 1995 section 36.14 which covers how MCA is calculated:

Minimum Circuit Ampacity 36.14 The minimum circuit ampacity (MCA) required by Clause 36.3(h) shall be determined as follows. All concurrent load conditions are to be considered in the determinations, see Figure 36.1. Whichever load condition provides the highest value shall be used. a) For a motor group only, a load consisting of two or more motors, the rated current of the largest motor or branch circuit selection current, if marked, (see Clauses 36.11 and 36.12), multiplied by 125 per cent, added to the rated currents of all of the other motors. b) For a combination load, a load consisting of one or more motors, electric heaters, and any other loads, that incorporates one or more compressor motors, the rated current of the largest motor compressor or branch circuit selection current, if marked, (see Clauses 36.11 and 36.12), multiplied by 125 per cent, added to which shall be the value obtained by multiplying the rated current of the electric heaters by 125 per cent, and adding to that total the sum of the ratings of all other loads. c) For a combination load, a load consisting of one or more motors, electric heaters, and any other loads, not involving a hermetic refrigerant motor compressor, the sum of the rated currents multiplied by 125 per cent. Exception: The rated current of the heater load may be multiplied by 100 percent, rather than 125 per cent, provided that 1) the rated heater load at a field wiring terminal is 50 kW or more; 2) the minimum conductor size that may be field-connected to such terminal is still to be marked; and 3) the heater element circuits connected to the field wiring terminals are subdivided as specified in Clause 30.8 and are arranged to be controlled by one or more temperature-actuated devices to reduce the likelihood of continuous simultaneous operation of all of the element circuits. 82 CSA-C22.2 No. 236 ‚ UL 1995 September 29, 1995 Figure 36.1 – Minimum circuit ampacity calculation On equipment with multiple modes of operation, every mode shall be considered for each individual field wired hazardous voltage circuit. Whichever load condition provides the highest value shall be used for Clause 36.14, in the applicable circuit. (Clause 36.14(a)) MCA = (1.25× LOAD1) + LOAD2 (Clause 36.14(b)) MCA = (1.25 × LOAD1) + (1.25* × LOAD3) + LOAD2 + LOAD4 (Clause 36.14(c)) MCA = 1.25 × (LOAD1 + LOAD2 + LOAD4) + (1.25* × LOAD3) where MCA = Minimum circuit ampacity per Clause 36.14 LOAD1 = Current of largest motor or branch circuit selection current, if marked, (see Clauses 36.11 and 36.12) LOAD2 = Sum of currents of all motors not including largest motor LOAD3 = Current of electric resistance heater LOAD4 = Any other load rated 1.0 A or more *If exception in Clause 36.14 applies, use 1.0 instead of 1.25
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There needs to be a distinction between standard NEC code requirements and the HVAC requirements.
We are now dealing with "Electronics" lets get up to speed.
 
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