Standard Load Calculations - 25% Largest Motor

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TwoRivers

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Washington State
When trying to find the largest motor in the house for your 25% addition to your calculation, can you use the heat pump if it was eliminated due to the heating being higher? For example if the house has a 15kw heater but only a 4kw heat pump, you would choose the heater for your calculations. But now you have to find the largest motor in the house, do you use the heat pump or because it was eliminated due to the heat, do you ignore that motor and go to the second largest motor? One source I read said to do this because the heat pump is no longer the largest "true" motor. But I read another source that said this still counts as the largest motor. Any thoughts?
 
Dennis Alwon

Dennis Alwon

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Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
My thought is that it will probably make little difference but I also think that the largest motor is the largest motor. If it is the heat pump then that is what I would use
 
kwired

kwired

Electron manager
Location
NE Nebraska
But if arranged so that heat and compressor will never run at same time and heat is a significantly larger load 125% of that compressor rating is meaningless to the total load calculation.

Say you have a 14.4 or even 19.2 kVA heat load and only a 3-3.5 ton compressor - that compressor load is only maybe 17-22 amps and the heat load is 60 to 80 amps.

Many heat pumps will temporarily switch to cooling mode and turn on aux heat during defrost cycle, doesn't last very long but in that case you do have both loads running at same time.
 
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