capacitor quesion

[wexley]

the ampacity of the capacitor conductors of a 21 amp rated capacitor connected to the motor terminals of a 25 hp, 3 phase, 208 motor shall not be less than a) 26.5 amps b) 30.85amps c)33.35 amp d) 35.85 amp

460.8 says not less than 135% of the rated current of the capacitor, but the math does not work 21 x 135% = 28.35

What am i doing wrong HELP

 

[charlie b]

I think this is an unfair question. I also think I know how their answer was determined, presuming their answer was (C).

460.8 also says that the ampacity of the conductors to the capacitor must be at least one third the ampacity of the conductors to the motor. Not the full load current of the motor, nor the amount of load you must use in your service calculation for the motor, but the ampacity of the motor conductors. Here are my steps:

• Looking at Table 430.250, the full load current of a 25 HP, 208V motor is 74.8 amps.
• You must size the conductors at 125% of this value.
• 74.8 times 1.25 is 93.5 amps.
• You must use a conductor with an ampacity of at least this value.
• The smallest conductor with an ampacity of 93.5 is a #3, with an ampacity of 100 amps.
• One third of 100 is 33.33.
• Therefore, you must size the conductors for the capacitor at the larger of two numbers: the 33.33 that I just calculated, and the 28.35 that you calculated.
• That makes their answer (C) the closest of the available choices.

I really wouldn't mind being wrong about this one. As I said, its a bad question.

 

[wasasparky]

Search the NEC forum for

Size of Conductors - Capacitor Motor Connection

Maybe someone can link it...