Cable sizing for LV 600V motor - Electrician Talk - Professional Electrical Contractors Forum
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Old 03-22-2019, 09:34 PM   #1
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Default Cable sizing for LV 600V motor

Assuming that I'm considering motor power factor, efficiency, altitude correction and all those fancy but critical factors... which current should I consider as input to cable sizing by voltage regulation: 600V or 575V?

FLA is defined as the current at full capacity drawn by the motor at rated voltage which is 575V and not 600V. However, my boss told me to consider 600V in order to calculate current consumption from the motor.

I think that considering 600V is wrong as we should take the worst case which is the motor operating under 575V which is standarized y the manufactured with the assumption that a 5% voltage drop will occur from up-stream 600V busbar, and in which that case, the motor will draw more current than if we calculate with 600V.
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Old 03-23-2019, 12:54 AM   #2
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For cable sizing, you'd use the motor current, voltage doesn't matter.
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Old 03-25-2019, 07:11 PM   #3
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I think you have asked this same question in other forums, so are you fishing for a different answer?

It's still the same; you don't "calculate" motor amps at all. You size conductors by using whatever local installation code applies to you. Here in the US we use the NEC, which dictates that the cables must be sized using a CHART based on the nameplate HP. The Canadian Electric Code is (I think) the same. I do not know what code you use in Chile (where you are from based on your other postings), but I know that the Canadians do a lot of mining work down there so that's likely how you ended up with 575V motors. So find a copy of the CEC, look up the motor HP in their chart, use the FLA from that table and multiply it by 1.25.

I'm not going to "vote" on your pole because it is not a valid question. There are TWO CLASSES of voltage; Distribution Voltage and Utilization Voltage.
  • Distribution Voltage is what the utility supplies, +-5%. In your case, it IS going to be 600V.
  • Utilization Voltage is what the equipment manufacturers MANUFACTURE TO, +-10%. In your case it IS going to be 575V, because a 575V motor is DESIGNED to be run on a 600V SYSTEM, expecting that by the time the utility supply voltage gets out to the motor terminals, there will be voltage drop on the circuit, so the motor design voltage is always lower. The same is true for 480V vs 460V; 240V vs 230V, etc.
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low voltage, motor, nec, regulation

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