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# Machine Full Load Amp calculation

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I have been a machine designer and controls engineer for many years. Having started a new position recently, I was hit with a curve ball...justifying machine full-load amp calculation. The root-problem is that my calculation leads to customers having to provide a much larger power drop for one of our machines. I have been pouring over my NEC books and handbooks to find the criteria for calculating machine FLA. My training has been to add up all of the maximum potential current draw of all devices (on branch circuits). What this company has been doing is calculating the maximum current that each device pulls and using that. I have explained to upper management that this practice is not safe...any person could jack up a motor drive speed/acceleration or modify the load and the motor drive will pull more current. Where in the NEC are guidelines for full load amps calculation?

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Hi Kaiser I would say you are correct in using the full load amps of the motors and other equipment in the machine. Each of these devices should have a protection devices which would trip if its device draws more current than it should. I cant see why or how your machine should draw more than the full load currents of all the devices added together.(unless at startup maybe)

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NEC section 430.24 should get you what is needed unless the customer has some more specific rules: 430.24 Several Motors or a Motor(s) and Other Load(s). Conductors supplying several motors, or a motor(s) and other load(s), shall have an ampacity not less than the sum of each of the following: (1) 125 percent of the full-load current rating of the highest rated motor, as determined by 430.6(A) (2) Sum of the full-load current ratings of all the other motors in the group, as determined by 430.6(A) (3) 100 percent of the noncontinuous non-motor load (4) 125 percent of the continuous non-motor load.

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This is a common problem here. Every machine that I have been involved with designing, we have tried to arrive at a sensible estimate of machine load current so our customer can size their supply appropriately. And yet every machine, when we check its actual current draw, it is always less than half of what we calculated. It is commonly said that it is vastly better for it to be this way than otherwise....

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you only need to supply for worst case scenario....not necessarily for maximum of every device. So if you have two pumps, but they never run at the same time, you don't need to size either the wiring or feed for both. If however, all your devices could run simultaneously then you'd need to size appropriately. Most of the time when I run into this its because devices aren't actually using their maximum amps..so putting a current meter on the machine leads to erroneous answers.You don't need to size for inrushes either....so if you have a transformer that will use 10a, its inrush could be 20x that.....you don't need to size the feed for 200a...... bob

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