NEC - Capacitors with motors

[Sthabik PE]

Found somewhere in the board, may be useful for discussion.

"If a motor with 60A needs a feeder ckt of 1.25*60=72A  and a capacitor rated for 60A is connected somewhere near motor terminals ( A hypothetical case for understanding purpose only), will the motor feeder ckt conductors now required be 1.35*60 = 81A rated or 72 A rated. ( reference Articles 460.8.A and 430.22), if yes why, if no why?"

 

[roy167]

72 Amps. The apparent power and therefore current in the feeder/branch conductors should be less because the power factor has improved. Cable should still be rated for 72 A amps, as the capacitor fuse can blow and capacitor not working in which case it will draw the same current as before. I believe you can reduce the overload setting in this case to provide proper overload protection. 

 

[Sthabik PE]

72 Amps. The apparent power and therefore current in the feeder/branch conductors should be less because the power factor has improved. Cable should still be rated for 72 A amps, as the capacitor fuse can blow and capacitor not working in which case it will draw the same current as before. I believe you can reduce the overload setting in this case to provide proper protection. 

I appreciate your views on this.

Here is mine.

First i thought it should be 72A because of the statement in NEC 460.8 "The ampacity of conductors that connect a capacitor to the terminals of a motor or to motor circuit conductors shall not be less than one-third the ampacity of the motor circuit conductors and in no case less than 135 percent of the rated current of the capacitor." So back to question thinking that only conductor from capacitor to motor can be 1.35*60 = 81A rated, but the question is asking for feeder ckt conductor, it will be 72A as it is.

But when i peek at NEC 2011handbook i saw this additional statement that change my mind to 81 A. It states that "The capacitor circuit conductors and disconnecting means must have an ampacity not less than 135 percent of the rated current of the capacitor. The reason is that all capacitors are manufactured with a tolerance of zero percent to 15 percent, so a 100-kVAr capacitor may actually draw a current equivalent to that of a 115-kVAr capacitor. In addition, the current drawn by a capacitor varies directly with the line voltage, and any variation in the line voltage from a pure sine wave form causes the capacitor to draw an increased current. Considering these several factors, the increased current can amount to 135 percent of the rated current of the capacitor." So then i thought the capacitor mentioned in the question may draw 81A from the source, 72A feeder may not be sufficient. Thats why my answer changed to 81A.

I may not be correct. These are just my views.

 

[roy167]

I did not answer your question keeping in mind NEC. I thought this was one of those questions where they just want to check your knowledge on what can happen when capacitor is added at the motor terminals. I will have to look into nEC code. 

 

[roy167]

460.9 Rating or Setting of Motor Overload Device.
Where a motor installation includes a capacitor connected
on the load side of the motor overload device, the rating or
setting of the motor overload device shall be based on the
improved power factor of the motor circuit.
The effect of the capacitor shall be disregarded in determining
the motor circuit conductor rating in accordance
with 430.22.

According to this what I told you would be correct. 

 

[Sthabik PE]

460.9 Rating or Setting of Motor Overload Device.
Where a motor installation includes a capacitor connected
on the load side of the motor overload device, the rating or
setting of the motor overload device shall be based on the
improved power factor of the motor circuit.
The effect of the capacitor shall be disregarded in determining
the motor circuit conductor rating in accordance
with 430.22.

My understanding  may be incorrect but i see the 460.9 statement as following:

"When a capacitor is connected on the load side of the motor overload device, line current will be reduced due to an improved power factor, which must be taken into account when selecting the rating of a motor overload device. 

But this effect of capacitor mentioned above shall not be considered while determining the motor circuit conductor rating. This is for determining motor overload device only. "

 

[LyceeFruit PE]

@Sdhabik you do have a newer version of the NEC available to you, right?

The exam is based off of the 2017 NEC and some articles/tables have been renumbered between the versions. It isn't typically a significant change but you really should get a newer version. 

 

[tpkjr2006]

@Sdhabik you do have a newer version of the NEC available to you, right?

The exam is based off of the 2017 NEC and some articles/tables have been renumbered between the versions. It isn't typically a significant change but you really should get a newer version. 

You have to be careful with the change from 2014 to 2017.

NEC 2017 included alot of items on solar and battery charging sections.

 

[Sthabik PE]

@LyceeFruit @tpkjr2006 Thank you for your kind suggestion. I do have the 2017 version as well.