Heat pump calculation

[Strathead]

Can someone explain to me how you could come up with this FLA MCA ?

Heat pump sits in the attic with cooled water circulated to it as the cooling/heating medium:

Fan 1/8 HP
Condenser 3/4HP
Aux heat 1.6 KW
208V/1Ø
FLA 8.5A
MCA 6.3A
MOCP 15A

Also, why would FLA be larger than MCA regardless? This must be a terminology issue. Would this happen when a motor is more efficient than the appropriate 430 table, so MCA is calculated and FLA is actual?

 

[Dennis Alwon]

I don't know where those numbers came from but they can't be right

 

[charlie b]

Are all of these components powered from the same circuit? If so, none of it makes sense. A 3/4 HP, 208V, 1 ph motor draws 7.6 amps by itself. A 1.6 kw heater draws another 7.7 amps. Those two items put you beyond the FLA and MCA values, and also beyond a 15 amp branch circuit breaker. The fan will add another 1.6 amps or so.

 

[Strathead]

Are all of these components powered from the same circuit? If so, none of it makes sense. A 3/4 HP, 208V, 1 ph motor draws 7.6 amps by itself. A 1.6 kw heater draws another 7.7 amps. Those two items put you beyond the FLA and MCA values, and also beyond a 15 amp branch circuit breaker. The fan will add another 1.6 amps or so.

Thank God I am not a total idiot. I couldn't see how this information could be correct. I am attaching a PDF of the plan page (I hope). Hopefully I cropped out the Engineer information etc. but this is from a job set of plans. I was in the same place. The only sane possibility I could come up with was that the "heat pump" designation was misleading and the heat was not "auxilliary" so it was either or. Even then though, I couldn't come up with those MCA and FLA, unless I did creative things like, assume the strips are actually 1.6KW at 240 volts so they are lower draw, and also that section 440 had some section that allowed the motor to be rated by its efficiency in lieu of the table in 430. It isn't my problem, but I just need to know.

 

[david luchini]

The 6.3 mca is a typo in the engineer's schedule. The unit's spec say a 10.6 mca. The unit has a 5.0A compressor, 0.7A blower and 7.8A electric heat. The heat and compressor appear to be non-concurrent.

 

[Strathead]

The 6.3 mca is a typo in the engineer's schedule. The unit's spec say a 10.6 mca. The unit has a 5.0A compressor, 0.7A blower and 7.8A electric heat. The heat and compressor appear to be non-concurrent.

Or wishful thinking. As I indicated this is in reality the Mechanical Contractor's fight, I am always curious myself, so I thought I would see what people here had to say. I talked again to the Mech contractor... Subsequently, the Engineer did state that he didn't intent the heat pump and the aux heat to run concurrent. I guess that is why he called it aux heat and heat pump instead of a condenser:slaphead:. Even then I will be curious when the approved submittals come back because, with one 60A feeder feeding two suites hence two units, it will be interesting to see if there is enough ampacity in the riser for the substitute system.

 

[david luchini]

Or wishful thinking. As I indicated this is in reality the Mechanical Contractor's fight, I am always curious myself, so I thought I would see what people here had to say. I talked again to the Mech contractor... Subsequently, the Engineer did state that he didn't intent the heat pump and the aux heat to run concurrent. I guess that is why he called it aux heat and heat pump instead of a condenser:slaphead:. Even then I will be curious when the approved submittals come back because, with one 60A feeder feeding two suites hence two units, it will be interesting to see if there is enough ampacity in the riser for the substitute system.

The engineer is using the unit as designed by the manufacturer. The unit is a water source heat pump that has an option for auxiliary electric heat. The unit is designed so that the compressor and electric heat do not run concurrently. From the manual...

It is comprised of a single stage of electric heatdesigned to invoice an electric heater in place of the compressor in the event entering watertemperature falls below 55°F

and

In heating mode, when the water temperature falls below 55°F (factory setting), the electric heateris energized, locking out the compressor

 

[Strathead]

The engineer is using the unit as designed by the manufacturer. The unit is a water source heat pump that has an option for auxiliary electric heat. The unit is designed so that the compressor and electric heat do not run concurrently. From the manual...



and

thank you for the clarification. But your statement that the 6.3 being a misprint is still accurate? If so, what is the MCA? And on the separate subject, why is MCA lower than FLA?

 

[david luchini]

thank you for the clarification. But your statement that the 6.3 being a misprint is still accurate? If so, what is the MCA? And on the separate subject, why is MCA lower than FLA?

FLA is 8.5A,

MCA is 10.6A.

MCA is not lower than FLA.