Specific Conductor Sizing

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Sparky2791

Sparky2791

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Ran into a situation regarding circuit sizing on a project.

Reference 2008 NEC

Does 240.4 G Overcurrent for specific conductor applications which includes Article 430 Motors in the table allow the branch circuit conductors feeding a motor to be sized per 430.22 or 430.24 instead of 310.15?
 
jumper said:
Your question is a bit confusing to me.

Here is a couple of articles that may answer you question.

http://ecmweb.com/design/motor-calculations-part-1-motors-and-branch-circuit-conductors

http://ecmweb.com/content/motor-calculations-part-2-feeders

http://ecmweb.com/code-basics/motors-motor-circuits-and-controllers-oh-my
Click to expand...

What I am asking is if , in the case of a motor, the conductor ampacity can be lower than the overcurrent protection device ampacity?

240.4 tells us to size conductors based on the ampacity of the overcurrent device ie.. 30A C/B = #10 Wire (copper of course) However 240.4 (G) offers overcurrent protection for specific conductor applications. Article 430 motors are included in this list of items which protection for specific conductors applies. What is the meaning of this? Does it mean I can size the motor circuit conductors based on the Full Load Current per 430.22 even if this conductor size is less than the ampacity of OPD protecting it.

For Example;

208V 3 Phase 20HP motor has full load current of 59.4A. Breaker size required to allow the motor to start and run is max 125A. The breaker is much larger than the full load current due to motor in rush so can the conductor be sized on the 59.4A load using 430.22 or do they need to be sized per the OPD as per 240.4
 
LibertyEngineering said:
What I am asking is if , in the case of a motor, the conductor ampacity can be lower than the overcurrent protection device ampacity?

240.4 tells us to size conductors based on the ampacity of the overcurrent device ie.. 30A C/B = #10 Wire (copper of course) However 240.4 (G) offers overcurrent protection for specific conductor applications. Article 430 motors are included in this list of items which protection for specific conductors applies. What is the meaning of this? Does it mean I can size the motor circuit conductors based on the Full Load Current per 430.22 even if this conductor size is less than the ampacity of OPD protecting it.

For Example;

208V 3 Phase 20HP motor has full load current of 59.4A. Breaker size required to allow the motor to start and run is max 125A. The breaker is much larger than the full load current due to motor in rush so can the conductor be sized on the 59.4A load using 430.22 or do they need to be sized per the OPD as per 240.4
Click to expand...

240.4 does not apply here. 430 rules apply to this per 240.4(G). The minimum conductor ampacity would be 74.25 amps and an inverse time breaker could be as large as 150 amp.
 
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LibertyEngineering said:
What I am asking is if , in the case of a motor, the conductor ampacity can be lower than the overcurrent protection device ampacity?

240.4 tells us to size conductors based on the ampacity of the overcurrent device ie.. 30A C/B = #10 Wire (copper of course) However 240.4 (G) offers overcurrent protection for specific conductor applications. Article 430 motors are included in this list of items which protection for specific conductors applies. What is the meaning of this? Does it mean I can size the motor circuit conductors based on the Full Load Current per 430.22 even if this conductor size is less than the ampacity of OPD protecting it.

For Example;

208V 3 Phase 20HP motor has full load current of 59.4A. Breaker size required to allow the motor to start and run is max 125A. The breaker is much larger than the full load current due to motor in rush so can the conductor be sized on the 59.4A load using 430.22 or do they need to be sized per the OPD as per 240.4
Click to expand...

Remember that the OCPD is not there to protect the conductors from a current overload condition, the OCPD is there to provide short circuit and ground fault protection.
 
texie said:
240.4 does not apply here. 430 rules apply to this per 240.4(G). The minimum conductor ampacity would be 74.25 amps and an inverse time breaker could be as large as 150 amp.
Click to expand...

what rule limits it to 150A? 250% of 74.25 A would be 185A and you can go to the next standard size of 200A.


in fact, one could argue that the provisions of 430.52 (C) (1) exception #2 would allow one to go to 400%.
 
petersonra said:
what rule limits it to 150A? 250% of 74.25 A would be 185A and you can go to the next standard size of 200A.


in fact, one could argue that the provisions of 430.52 (C) (1) exception #2 would allow one to go to 400%.
Click to expand...

208V 3 Phase 20HP motor has full load current of 59.4 amps.

59.4 * 250% = 148.5 amps, next size up 150 amp.
 
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