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Gary Arthurs

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I am calculating loads for three-phase motors using NEC 430.250.

Can anyone tell me if the amps listed in this table are per phase or the total amps drawn by the motor?

Dennis Alwon

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If you have a 3 phase motor then yes the amps would be the same on each phase but I am not sure that is helpful for your calculation. If the motor per the table draws 50 amps then that is all you need to know-- you don't use 150 amps-- not sure what you are trying to do

Gary Arthurs

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These motors will be run from a 480 VAC three-phase emergency generator that supplies "226 amps". So I need to know, as an example, how many 20 hp motors (at 27 amps from NEC 430.250) I can run on this? Is it on the order of 226/27 or 226/(27*1.732) or 226/(27*3) or something else?

Dennis Alwon

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If all the phases are equal loads then you can distribute the load easily
27 amps 226 amp/ 27= 8.3 so you can put 8 motors on the generator. That is assuming the genset can handle that continuous load.. I would probably not load the generator more than 80% but you need to know the specs of the genset

226 * .8 = 181 amps

181/27 = 6 units

You need to be careful with motors that have a large starting load. If they all start at the same time I bet that will be an issue even with 6 units. I would call the manufacturer to see what they recommend

Gary Arthurs

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Thank you Dennis. This helps. I am new the NEC. We are looking at VFDs or time delays to deal with the starting current.

Dennis Alwon

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You're welcome. The nec has a section for finding the overcurrent protective device needed for feeders for multiple motors.

Take the largest motor (27 * 250%= 67.5) from Table 430.52 for polyphase motors. Thus you can upsize to the next size breaker--70 amps

Then add the sum of all the other motors to the 70 amps.. Yes weird but you use the overcurrent protective device amp and then add the motor amps. So if you have 6 --27 amp motors then you multiply 7 * 27= 162 + 70 amps = 232 amps. You casnnot go up in breaker size so you would have to se a 225 amp breaker for these motors.

Then you have to calculate the conductor size. Lots of work for motor

19delta

Gary Arthurs

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This is appreciate. I have been working through some of the "Tom Henry - Calculations for the Exam" book and this sounds familiar. Thank you.

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