Motor Nameplate Current vs NEC Tables - STILL CONFUSED

[RDL914]

Just when I thought I had this nailed...kick in the nuts...

One of the items on my list of things to know cold for the exam, without any question, is when to use the nameplate current rating, and when to use table 430.250 or similar for motors.

I cannot find a clear answer on engineerboards...or at least not one that has me convinced....but after going over Art 430.6(A)(1) and (2) with a fine tooth comb, I've come to the following conclusion:

1. Unless the question clearly states a "motor-operated appliance", AND lists the nameplate current and HP, Art 430.6(A)(1) Exception 3 DOES NOT APPLY.
   

HOWEVER,

I am still confused by Art 430.6(A)(2) which says: Nameplate Values. Separate motor overload protection shall be based on the motor nameplate current rating. The handbook goes on to explain that the tables should be used for sizing conductors, branch circuit and ground fault protection, and ampere rating of motor disconnecting means RATHER than using the nameplate current.

So if I am understanding this all, if it's a motor overLOAD device you are sizing, you use the nameplate current regardless. If you are sizing conductors or OCPD for a regular motor, you use the tables regardless. IF AND ONLY IF you are sizing conductors or OCPD for a "motor-operated appliance," AND you are given nameplate current, you use the nameplate current and not the tables.

Please, someone clear this up for me...unless of course, there is no right answer and the next edition will be completely different...

Am I studying for the PE exam or to join the NEC committee?

 

[cdcengineer]

I believe you will always use FLA per NEC 430 Tables except when calculating motor overload size. Check out 430 Part III - 430.31 I think it is.

 

[RDL914]

Thanks for the answer. I agree that you use the nameplate FLA when sizing separate motor overload protection (protection from start failure, overheating, single phase condition, etc). But what about 430.6(A)(1) Exception 3 which states:

For a listed motor-operated appliance that is marked with both motor horsepower and full-load current, the motor full-load current marked on the nameplate of the appliance shall be used instead of the horsepower rating on the appliance nameplate to determine the ampacity or rating of the disconnecting means, the branch circuit conductors, the controller; the branch-circuit short circuit and ground-fault protection, and any separate overload protection.

Once again, I'm back to thinking exception 3 only applies to the key-word: MOTOR-OPERATED APPLIANCE

 

[Wildsoldier PE]

Some times the problem says "per NEC 2011" then you know that you have to use the code for these type of questions

(null)

 

[bobadrew]

The way I understand it is that you are supposed to use the table values for sizing because the motor could be replaced at a future date. It's supposed to account for this.

 

[knight1fox3]

This doesn't necessarily apply to studying for the PE exam since it is pretty clear when they want you to use NEC for a particular problem. But here's my take on things from a practical application (overhead material handling cranes) standpoint. Initially when sizing conductors and components for a project, the motor FLA from the NEC tables can and should be used. However, often times once the motor is manufactured and the 100% full load tests are performed, it's possible for the nameplate FLA to be greater than what is listed in NEC. In that particular instance, the larger FLA value should be used.