Calculating the kva load of mechanical equipment

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I am trying to figure out the correct kVA value to enter into an electrical panel schedule for various mechanical equipment based on the info given on mfg cut sheets. For example, the tech data sheet for a 3 phase split AC unit lists the following info:
  • Total power (kw) = 6.2
  • Voltage = 460V/3P/60hz
  • COMP 1 RLA = 9.7 (compressor rated load amps)
  • COMP 1 LRA = 62 (compressor locked rotor amps)
  • OFM (ea) WATTS = 325 (I have no idea what "OFM" stands for)
  • OFM (ea) FLA = 0.8 (full load amps)
  • MCA = 13.7 (min circuit amps)
  • FUSE = 20 (max overcurrent device rating)

My question is: How to calculate the correct value of kVA to use in the panel schedule?

Do I multiply 480V*(COMP RLA + OFM FLA)*1.732/1000 to get 3 phase kVA?
or do I get kVA by dividing the given kW by an presumed power factor (say 0.8)?
or....???
 
jumper said:
Charlie help me out here.

I did the math (I used 460V) and it equals the 6.2 kW listed above.

How does that help me determine kVA with no PF?
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How did you get the math to come to 6.2kW?

You have volts and amps as a given...that will give you VA.
 
Couldn't there be some controls in this HVAC unit too? Those don't tend to appear on the nameplate. Seems like it would be easy (and conservative) to just use the MCA value for the amps portion of KVA. Obviously, there is a 25% factor buried in that from the largest motor which maybe you could subtract out since the give you RLA.
 
suemarkp said:
Couldn't there be some controls in this HVAC unit too? Those don't tend to appear on the nameplate. Seems like it would be easy (and conservative) to just use the MCA value for the amps portion of KVA. Obviously, there is a 25% factor buried in that from the largest motor which maybe you could subtract out since the give you RLA.
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I agree.

(13.7 - 25%*9.7A) * 480V * sqrt(3) = 9,374VA
 
Unless it's an electric heater, the kW is not a useful number for the electrical design. It's all about the amps. You can do just as well at predicting your power factor with a crystal ball and tarot cards.
 
JoeStillman said:
Unless it's an electric heater, the kW is not a useful number for the electrical design. It's all about the amps. You can do just as well at predicting your power factor with a crystal ball and tarot cards.
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How do you design by adding up amps, kva is the way to go.
 
To elaborate, I meant that Amps are the primary design criterion for selecting wires, breakers and panelboards. We don't use kVA to select those things, but we do add up kVA to get the total amps on a panel. You don't need to know kW or power factor in order to decide if you want a 100A, 225A or 400A panelboard.
 
jumper said:
Charlie help me out here. . . .How does that help me determine kVA with no PF?
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The full load amps (or running load amps) includes the effect of both resistive and reactive loads. Multiplying the volts times the RLA times the square root of 3, then dividing by 1000, gives you KVA, not KW.

Whether to use the value 460 or 480 for the voltage is a matter of some debate.

 
JoeStillman said:
To elaborate, I meant that Amps are the primary design criterion for selecting wires, breakers and panelboards. We don't use kVA to select those things, but we do add up kVA to get the total amps on a panel. You don't need to know kW or power factor in order to decide if you want a 100A, 225A or 400A panelboard.
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I am still confused and that is likely a failure on my end. :huh:

I don't understand how I find the panel size I need without using at least KW.

How do I add up a mix of three phase and single phase load amps to figure out what feeder/panel size is needed.

:?
 
iwire said:
I am still confused and that is likely a failure on my end.:?
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Not at all. I was getting ahead of myself. I will try to be more plain.

iwire said:
I don't understand how I find the panel size I need without using at least KW. How do I add up a mix of three phase and single phase load amps to figure out what feeder/panel size is needed.
Click to expand...

When I fill in a panel schedule, I use VA instead of watts. Like you, I add the single phase and three phase to get the total. The reason I use VA instead of Watts is because it makes my calculation more conservative. (kVA is always a larger number than kW.) Also, when estimating the load on a panel, I have no way to know what my operating power factor will be. Your engineering textbook will present you with problems where all the loads have a known power factor, but in real life, it's almost always a WAG. If you keep all your numbers in VA, just divide by V or (V x 1.73) and you have your amps, which is what you wanted all along anyway. If you add up Watts instead, you have to assume a power factor and divide by that too. Why bother when its only a guess anyway?

The OP was confused because the mechanical department gave him amps and kW, and he needed to figure out which to use. The kW is published for the mechanical engineers to use so they can pick the most efficient compressor for a given cooling load. But it's a rare ME that bothers to even compare them. Instead, they see kW and think "Oh, this is for the wire-nuts". They look at me cross-eyed when I say, "Thanks for the kW, now give me what I really need: RLA, FLA, MCA and MOCP. I'll take HP if you've got that too." I use the RLA (compressor) and FLA (fans) to calculate kVA and put that on my panel schedule. No power factors are harmed in the making of this panel schedule.

Note that the Code gives us load estimating figures in VA (180 per receptacle) or VA/ft2 (table 220.12). No power factor is assumed or necessary, because the number we're trying to get to is in Amps.



P.S. Heaters and ranges are rated in Watts, but since they are purely resistive, their kVA is the same as their kW anyway.
 
JoeStillman said:
Not at all. I was getting ahead of myself. I will try to be more plain. . . . "Thanks for the kW, now give me what I really need: RLA, FLA, MCA and MOCP. I'll take HP if you've got that too."
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OK, my next question is related to the blower unit - all I have there is hp (=1) and voltage (=480/3P). I believe the NEC table for 3 phase motors shows the FLA to be 2.1 (@460V). So to get the 3 phase kVA for that equipment I simply multiply (2.1)*(480)*SqRt(3)/1000 to get 1.745 kVA? Is that correct?
 
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