Motor Calc Question

[wawireguy]

I pulled this one from the archives. Thank you Casey(OP who posted this many moons ago):

"1) What size inverse time circuit breaker is required for a
feeder supplying the following motors on a 480 volt three
phase delta system? One 15 hp 480v motor, two 10 hp 480v
motors, and one 75 hp 480v motor @ 15 minute intermittent
duty?

Per table 430.150
15hp 460 = 21A
10hp 460 = 14A
75hp = 96A (430.22 E) 96A * .85 = 81.6A

Per 430.52
81.6A * 2.5 =204A next size up 225A

Per 430.62
225A + 21A + 14A + 14A = 274A next size down 250A

250A inverse time breaker"


I had almost this exact question on my test. My question is does table 430.22(E) even apply to this question? Section II is for sizing conductors. If you are applying 430.62 does 430.22 apply? 430.62 is based off the short circuit rating? Is the 15 minute intermittant part of the question extraneous?

 

[wawireguy]

Still need help on this one..

 

[westernexplorer]

Ok, I will play.....


Your 75 HP Motor = 96 FLA, per table 430.250...

96 amps X 250% = 240 amps... round up to the next available size OCPD 250 AMPS.

250 Amps, largest branch circuit OCPD + the remaining motors on the Feeder.

250 Amps + 21 FLA, + 14 FLA, + 14 FLA = 299 Amps....

Round down to a 250 amp Inverse Time Breaker.... 250 is a Standard Size.

*** The question asked you to size the FEEDER (OCPD), nothing else.... reducing the FLA for the 75 H.P. Motor because it is intermittent is not even a factor in the question.... That could be for the Branch Circuit calculation.....***

The Feeder has to carry the ENTIRE load of all the motors whether thay run short time or constantly.... It looks like you got the correct method....


I will go back to what I have been saying all along, people are reading why to much into the questions....

 

[wawireguy]

Thanks Western, I just needed to know the correct way to swing that question as it appears the OP didn't do it correctly. This test doesn't give you time to figure it out. I can't wait to get back at the the test table and kick this things butt!