question on what motor parameters to use

[stcalle]

Ok in the NEC 2002, Article 430.6(A)(1) through 430.6(A)(2) says to use the FLA(full load amps) from the tables to size motor circuit components, then use motor nameplate current to size the overload protection.

Just sticking to what the book says this is how I would go about sizing the motor components. Someone at my job said that you would do this in the event that you do not have the nameplate information for the motor.

There point is that if you have the values from the motor nameplate, then you would use those primarily to what the code is saying to do. Meaning when you size your conductors, branch circuit protection, etc. you would use the current rating from the nameplate.

This seems to contradict what the code is saying because it explicitly says that use the FLA from the motor tables to size cetain components and use the nameplate current rating to size the overload protection.

Can anyone clear up this unclarity?

thanks

 

[caj1962]

Re: question on what motor parameters to use

st lets break down what you are trying to do
1. Short circuit ground fault protective, use the correct table and the right multiplier based on your motor. Do not use the name plate.
2. Conductor sizing again refer to the same table you just used and multiply accordingly. Again do not use the nameplate.
3. Forget the tables and use the nameplate to size the overload device.
Keep in mind that this is for non-HVAC motors.
I am sure there will be others to jump in here, but this is the readers digest version with 10 minutes left at lunch.

 

[stcalle]

Re: question on what motor parameters to use

well yea, thats the method I would use when performing these calculations. This is because I just read through the NEC on things I am unsure of(which is alot). The thing is that I am a new employee and I try to go more by the NEC since I do not have the experience of other engineers at my job. And now its the more experienced engineers who are saying to use the nameplate ampere rating for all apects of the motor design as opposed to what the NEC is saying about using FLA from table and nameplate amps for different components. I would go by what the code is sayin but since I am the newguy, I don't feel I am in a position to tell the other engineers what is correct. I figured I'd get your guys input to see if I have any misconceptions in my thinking.

thanks

but caj, I see what you posted is how you would perform these calculations, same as me...anyone else have some input?

[ March 21, 2005, 11:34 AM: Message edited by: stcalle ]

 

[rbalex]

Re: question on what motor parameters to use

Originally posted by stcalle:
... And now its the more experienced engineers who are saying to use the nameplate ampere rating for all apects of the motor design as opposed to what the NEC is saying about using FLA from table and nameplate amps for different components....
thanks

The more experienced engineers are in error. Essentially the only thing the nameplate data can be used for is to determine MOTOR overload protection. The tables are to be used for all other computations.

The reason for this is fairly simple and practical - a motor may be replaced someday with a motor with the same hp rating and frame size but a higher nameplate current. Using the Tables permits continued confidence that the circuit is still adequate.

The Tables are based on the worst case motor that any NEMA Motor Division member makes. This can be verified by reviewing NEMA's Proposal to change the Tables during the 1996 cycle.

 

[jwelectric]

Re: question on what motor parameters to use

2005 code and is the same in 2002
410.6 (A) (1) Table Values. Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate.

 

[rbalex]

Re: question on what motor parameters to use

The "proof of the pudding" is 430.6(A)(2):

(2) Nameplate Values. Separate motor overload protection shall be based on the motor nameplate current rating.

which states specifically where nameplate current IS to be used.