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msamad
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NEC: Motor Overload Protection Settings for a Motor Over 600-V

02/09/2009 7:10 PM

For the conductor size of motors over 600 Volts, NEC Article 430.124 indicates that the ampacity should be not less than the motor overload protection settings. But I didn't find any article in NEC for the guideline of motor overload protection settings for motors over 600 Volts (the all in article 430.32 shall not apply for motors over 600 Volts as indicated in 430.31).

What should I follow then for selecting the motor overload protection settings for a motor over 600 Volts?

Thanks,

- MS

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jerryalan
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#1

Re: NEC: Motor Overload Protection Settings for a Motor Over 600-V

02/11/2009 5:58 AM

the NEC is not a "How To Manual"
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bfmickl
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#2

Re: NEC: Motor Overload Protection Settings for a Motor Over 600-V

02/11/2009 7:22 AM

Article 430.226 might be of some help to you.

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msamad
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#3
In reply to #2

Re: NEC: Motor Overload Protection Settings for a Motor Over 600-V

02/11/2009 10:12 AM

Thanks for the response. The article 430.226 (which is 430.126 in NEC 2002) is helpful but it is not complete to serve my purpose.

In fact, I am designing the cable size of a 350 HP, 4.16 KV motor, SF 1.5. The FLC is calculated 46A. Could you please tell me it is ok to size the cable based on 125% of FLA (as can be done for voltage system less than 600V)? Or, is there any other NEC article that can be applied here?

I know the ultimate sizing will be based on the other de-rating factors (such as temperature correction, number of conductors etc.) and voltage drop calculation.

Please suggest.

- MS

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bfmickl
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Location: Raleigh, N.C.
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#4
In reply to #3

Re: NEC: Motor Overload Protection Settings for a Motor Over 600-V

02/11/2009 10:37 AM

NEC 08, 430.226 states: "The ultimte trip current of overcurrent (overload) relays or other motor-protective devices used shall not excced 115 percent of the controller's continuous current rating. Where the motor branch-circuit disconnecting means is separate from the controller, the disconnecting means current rating shall not be less than the ultimate trip setting of the overcurrent relays in the circuit." As a note, I'd recommend that you get an updated code book or at least a publication with the updates covering the two cycles ahead of your current book. Hope this helps. Be safe.

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sb
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Join Date: Mar 2007
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Good Answers: 103
#5

Re: NEC: Motor Overload Protection Settings for a Motor Over 600-V

02/11/2009 11:26 AM

We had a discussion on the same subject and see post #11 with links

http://cr4.globalspec.com/thread/32804/Full-Load-Current

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