Another Exam Prep Question

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BobbyBJr

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Upstate, SC
I have Mike's Electrical Exam Preparation based on the 2002 NEC and on page 241 there is a motor calculation steps example that I am having problems with.
Step 1 is to find the motor FLC from the tables (430.147, 148 or 150)
Step 2 is to calculate the overload protection (FLC x 1.15)
Step 3 is calculate branch circuit conductors (FLC x 1.25 & table 310.16)
Step 4 is branch circuit protection (FLC x Table 430.52)

Step 5 is figuring feeder conductors is where I am running into a problem. The example gives one 10 hp, 208v, 3 phase motor and three 1 hp, 120v, 1 phase motors to work with. I thought you took the FLC of the largest motor and multiply by 1.25 and add the FLC's of the other three motors;

(30.8 x 1.25) + 16 + 16 + 16 = 86.5

but the book is only adding one of the smaller motors for an answer of 54.5 and it does the same in Step 6 figuring the feeder protection. Which is correct? I did print out the corrections to the book, but this wasn't mentioned there that I saw. Any help is appreciated...

thanks,
Bobby
 
BobbyBJr said:
Okay, I see the motors of the same phase now, correct??

thanks,
Bobby
Click to expand...
I don't have the book, but I'm assuming the single phase motors are connected in a balanced arrangement... i.e. one per phase conductor. In your calculations you are only determining the current on any one phase conductor... so you only use the FLC of one 120V, 1? motor.
 
Thanks. I have been going over and over that section for the past couple hours and I now see where I was going wrong. One little word or phrase misread blows everything. Thanks for your reply.

Bobby
 
Here's another one that I don't get. On page 265 question 18 is:

Wireways shall be supported where run horizontally at each end and at intervals not to exceed ___, or for individual lengths longer than ___ at each end or joint, unless listed for other support intervals.
a) 5 ft b) 10 ft c) 3 ft d) 6 ft

The answer is given as c, but I don't agree. Anyone??

thanks,
Bobby
 
Thanks. I did print out the corrections and went through my book marking them all, but the answer to that question is still given as c. I am thinking it is a mistake, but I have found others that I thought there was a problem with until I asked here and got a explanation. Thanks for your time.

Bobby
 
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