Washington State Admin. Exam Inverse time circuit break

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johnweaver

Member
Can anyone Help?
What size inverse time circuit Bkr. for this feeder supplying the following motors on a 480 volt three phase delta system?

One 15 hp, two 10 hp, one 75 hp @ 15 minuite intermittent duty.

I have done it this way, am I wrong?
75 hp Full load amps = 96
96 amps x .85 (intermittent duty cycle art-430.22E)x 250% (inverse time Bkr.Art-430.52)
96 x .85 = 81.6 x 2.50 = 204 amps
Then take 204 amps and add the full load currents of all other motors.
204 amps plus
15 hp = 21 amps
10 hp = 14 amps
10 hp = 14 amps
Add these all up and you get 253 amps
with a feeder I cant go up in bkr. size so I have to step down to a 250 amp bkr. (Art-240 overcurrent protection)
Is any of this right? Thanks John
 
T

traderx

Member
Re: Washington State Admin. Exam Inverse time circuit break

I had this very same question on my test... but the answers were

a:300
b:350
c:400
d:450

But this doesn't match with what you have at 250.

As I would think that this will come up again on my 2nd round ( a variation of it )what would be the correct answer?
 
N

nicknorth

Senior Member
Re: Washington State Admin. Exam Inverse time circuit break

I need help with this also?? Can anybody show how to do this correctly?? This was also on my WA Admin test.
 
K

kw

Member
Re: Washington State Admin. Exam Inverse time circuit break

I got this question right on my test, so I'll see if I can help

using your numbers,

the 204 amps is right for the 75hp motor, here is where I think you might have went wrong.

take the 204 x 1.25 since it is the biggest motor to get 255 round this up to the next standard size which would be 300

now add the 21+14+14 to get 349. This is where they try to play games with you. Round down to 300

I believe this would give you the right answer.

Good Luck,

It took me 3 times to pass, just keep with it.
 
N

nicknorth

Senior Member
Re: Washington State Admin. Exam Inverse time circuit break

Kw do you use table 430.22e for va calc on the service??
 
W

woodymiller

Member
Re: Washington State Admin. Exam Inverse time circuit break

As I recall, don't you take the highest protection device and then add the flc of the other motors? and then round down. And I have a question without answer about receptacle demand factor on this forum under exam prep and also calculation. Any help?

thanks, Woody
 
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