question about an exam example question

[bran82004]

This test question example was given and doesnt seem to be correct. the only way i can seem to make it work is by using the amp rating they give in the example. but when using the nec flc chart 430.250 the answer i came up with was 77.5A. which would require a #2 xhhw-2 conductor. any explanation would be great thanks..


3. A feeder supplies three 460-volt, three-phase, 1740 RPM, Design B, alternating-current
motors. The motors have the following nameplate ratings: 25 horsepower, 28.4 amperes; 15
horsepower, 18.2 amperes; and 10 horsepower, 11.3 amperes. Assuming all the motors are
operating under continuous duty, what minimum size XHHW-2 feeder circuit conductors are
required to supply the motor load? The conductor terminations are rated at 75?C.
Answer 4 AWG NEC 430.6, 430.24, 430.150, 310.16

 

[cadpoint]

I don't have a code Book to look at those Articles, I'm going to guess the "B" factor is involves or bad math of what your suppose do to the math with muliple motors...

 

[augie47]

I also came up with the 77.5 amps.
With 75? termination, a #4 Cu rated at 85 amps would be sufficient.
Why did you select #2 ?

 

[Little Bill]

This test question example was given and doesnt seem to be correct. the only way i can seem to make it work is by using the amp rating they give in the example. but when using the nec flc chart 430.250 the answer i came up with was 77.5A. which would require a #2 xhhw-2 conductor. any explanation would be great thanks..


3. A feeder supplies three 460-volt, three-phase, 1740 RPM, Design B, alternating-current
motors. The motors have the following nameplate ratings: 25 horsepower, 28.4 amperes; 15
horsepower, 18.2 amperes; and 10 horsepower, 11.3 amperes. Assuming all the motors are
operating under continuous duty, what minimum size XHHW-2 feeder circuit conductors are
required to supply the motor load? The conductor terminations are rated at 75?C.
Answer 4 AWG NEC 430.6, 430.24, 430.150, 310.16

You need to look at Table 430.250 to get the FLC of the motors listed in the example. Take your largest motor FLC x 1.25% then add the FLC of the other 2 motors. You then go to Table 310.16 under the 75 deg. column and find the ampacity for the total you came up with, then find your conductor size.
I also came up with 77.5A and #4AWG. You got the 77.5A correct, I don't see where you get #2AWG for that.

 

[bran82004]

duh perfect example of reading the whole question

duh perfect example of reading the whole question

Right away when i was reading this question i saw the xhhw-2 and incorectly assumed they wanted the size based of an alum conductor. i now see that #4 xhhw-2 copper is suffecent for 85 amps. but thanks for the help

 

[Finite10]

You may very well se a question like that on a masters exam, but the largest motor is intermitent or short time duty cycle.

Then see 430.24, exc. 1. Pay attention to that word 'or' and then-
Check Table 430.22(E).