Just wondering

[Don Gorham]

NEC 430.6(A)(1).... use the table not the name plate on the motor for ampacity x 125%. Then, 430.32(A)(1) use the name plate on the motor for motor branch OL protection. Then 430.52(C)(1) .....not exceeding the value calculated according to T430.52 which says "Full Load Current". Which full load current? The one on the name plate or the one from the table? And why can't the manufacturer of the motor determine his motor FLA. Don

 

[raider1]

The only time you use the FLC listed on the motor nameplate if for sizing the motor overloads.

The reason that the NEC requires that we use the motor tables in the back of Article 430 for full load current is that the tables have the worst case full load current for the HP rating of the motor. In the future a new motor may replace the motor that you are currently installing and the branch circuit conductors, feeder conductors, and short circuit and ground fault protection should be sized for the worst case scenario motor replacement.

Hope this makes sense.

Chris

 

[Don Gorham]

Thanks Chris, not to sound smart aleck but where is that information posted or are we assuming this to be the case from previous installation experiance. Don

 

[raider1]

Your welcome.

430.6(A)(1) specifically states that we use the Table values given in Part XIV for determining the ampacity of conductors, branch circuit short circuit and ground fault protection instead of the actual current rating marked on the motor nameplate.

As far as the reason for doing this, the NEC does not specify any specific reason. It is my opinion that this is due to the likelihood that at some point the specific motor will be replaced with another motor that might not have the same efficiency and power factor as the original motor. Therefore the conductors that supply the motor should be sized using the worst case senario.

Chris

 

[Don Gorham]

Thanks again, in section IV when calculating the rating or setting for individual motor circuits as per 430.52(A),or(B),or(C)(1), where do you find evidence of which FLA to use, the Tables or the motor nameplate datum? Don

 

[raider1]

Thanks again, in section IV when calculating the rating or setting for individual motor circuits as per 430.52(A),or(B),or(C)(1), where do you find evidence of which FLA to use, the Tables or the motor nameplate datum? Don

You use the Table values to size the short circuit and ground fault protection for an individual motor circuit.

Again if you read 430.6 this will tell you what ampacity to use.

Chris

 

[Don Gorham]

Thanks again, after pulling my head out and reading the full article I see the light in 430.6(a)(1). Don

 

[dlarkin]

Low speed motors

Low speed motors

Hi. I'm new here. Seems like a good forum!

Just wanted to point out that for low speed motors (less than 1200 RPM), 430.6(A)(1) directs you to use the nameplate current ratings. I would think in that case that it would be prudent to check the nameplate rating and use the higher of nameplate vs. table current ratings.

 

[raider1]

Thanks again, after pulling my head out and reading the full article I see the light in 430.6(a)(1). Don

Your very welcome.

Chris

 

[raider1]

Hi. I'm new here. Seems like a good forum!

Just wanted to point out that for low speed motors (less than 1200 RPM), 430.6(A)(1) directs you to use the nameplate current ratings. I would think in that case that it would be prudent to check the nameplate rating and use the higher of nameplate vs. table current ratings.

Welcome to the forum.

Yes, you are correct. For low speed motors, high torque and multi speed motors you would use the nameplate values, for all other motors you use the Table values in accordance with 430.6.

Chris