Ask the Engineer- How do I calculate power from 3-phase LED readings?
Annie Paquette
October 09, 2010
- Categories:
- How To
- Tags:
- 3 Phase PDU
- PDU
Question from Steven G.: How do I calculate the power usage for devices attached to one of your Smart PDUs by looking at the digital readouts on them? I understand the 3-phase power calculation of P=VA*1.73, but is that done on each leg/phase (x,y,z) for the PDU? An example from a CS-24VY-L30MA is X=4.6, Y=4.6, Z=6.3
Answer: the shortcut way to find power consumption is (X+Y+Z)*120V = 1860W for your example. This is only accurate when loads are fairly well balanced. It turns out that even though one leg is 37% higher than the other two legs, your example is balanced well enough that the power calculation is only 1.6% overestimated.
The more accurate calculation of 1831W must be found by first calculating the outlet branch amperages (XY, YZ, ZX) through an iterative process using the equations:
X = sqrt([XY]*[XY] + [ZX]*[ZX] + [XY]*[ZX])
Y = sqrt([XY]*[XY] + [YZ]*[YZ] + [XY]*[YZ])
Z = sqrt([YZ]*[YZ] + [ZX]*[ZX] + [YZ]*[ZX])
These are derived from the law of cosines with the assumption that the three legs are mutually 120 degrees apart.
Thanks for your submission. One of our Power Strategy Experts will get back to you shortly.