NCEES 510

[countrytoad]

I thought we multiply by 1.25 to get the FLA of a CD (continuous duty) motor, but that doesn't appear to be the case. When do we multiply by 1.25? And why don't we here?

So I calculated: 65 Amps x 1.25 x 1.15 = 93.4 Amps

(65 Amps from NEC Table 430.250) (1.15 from NEC 430-32a1)

The correct solution is: 65 Amps x 1.15 = 74.75 Amps =(to maximum 5 Amp increment)=> 70 Amps

The original question #510: "A 50-hp, 460-V, 3-phase induction motor is served by a single feeder conduit... and it is rated for continuous duty... Starter running overload protection is available in 5-A increments... the initial maximum sized protection device (amperes) that can be applied is most nearly"

Thanks!

 

[Ship Wreck PE]

You use the 1.25 if you are sizing the wire size.

And this problem is sizing the motor overload heaters that go in the starter, and for these motor

Parameters it is 1.15.

 

[EEpowerOK]

NEC 430.32 (A) (1) all other motors (less than 1.15 service factor) the rate is 115%

 

[JB66money]

According to the question they asked for the maximum amount which means that we should use NEC article 430.32 © and multiply the Full Load Current by 1.30 instead of 1.15. I believe that the solution to problem 510 is incorrect. I am currently taking a course in the NEC and I will ask my instructor tonight to verify it. My instructor is very knowledgable of the code he has a master electrician for 23 years and is currently an electrical inspector. Also he has read the entire code book from cover to cover. He is a walking NEC.

 

[JB66money]

I checked last night with my instructor and he said that the question asked for the initial maximum which means that 430.32 (A) is used and 1.15 is multiplied by the Full Load Current.

 

[EEpowerOK]

I checked last night with my instructor and he said that the question asked for the initial maximum which means that 430.32 (A) is used and 1.15 is multiplied by the Full Load Current.

According to 220.5B you can round up to 75 amps.

According to 240.4.B < 800 Amps next higher standard overcurrent device is permitted

According to 240.6 Standard amp rating, the next higher size is 80 amps.

Answer is C 80 amps, book says answer is B 70 amps.

Ask your instructor about that.

 

[iahim]

According to 220.5B you can round up to 75 amps.

According to 240.4.B < 800 Amps next higher standard overcurrent device is permitted

According to 240.6 Standard amp rating, the next higher size is 80 amps.

Answer is C 80 amps, book says answer is B 70 amps.

Article 220.5B applies to branch circuit, feeder and service calculation. This is a motor overload protection problem, so that article does not apply. Same for the other 2 articles you mentioned. You have to round down, since 430.32 (A) specifies the maximum value.

 

[JB66money]

Article 430.62 (A) states that you have to go down to standard size for a feeder, but you can go up for a branch circuit.