Another Exam Prep Question, Help!

Static Design

Got another for you guys, can you please show the math with a code reference?



What is the name plate FLA for a 20hp, 208V, three phase motor with 90% power factor and 80% efficiency?

mhillc

AH,AH, WHAT

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Magnettica

This is a simple one. Even carpenters and sheetrockers know how to do this!

Haha, just kidding.

Go to table 430.250 for 3Ø motors.

20 HP motor at 208 volts uses 59.4 amps.

59.4 amps x 90% (59.4 x .90) = 53.46

53.46 x 80% (53.46 x .80) = 42.76, or 43 amps.

The tables for motors are used for everything except sizing overloads.

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Static Design

I couldn't find that damn table, thanks man! it's all clear now, stupid NEC haha


Oh, I must be really stupid. I have a B.S. in architecture, do carpentry and I can sling mud:but couldn't find a damn table, makes me look good!

thanks again

Magnettica

the NEC is your bible

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Static Design

I came up with something different maybe you can check my math.

3 phase (%Eff/Pf)= HP x 746 watts/ E x I x 1.73

We are looking for amps so rearrange the giving formula and get.

I= (HP x 746 watts) / ( E x 1.73 x PF x Eff)

So pluging in the givin numbers I get.

I= (20HP x 746 watts) / ( 208v x 1.73 x .90PF x .80Eff)
I= (14920)/ (259.0848)
I= 57.6 A


I think I just confused myself again......... I believe it's time to go to bed.

Magnettica

Scrap the watts terminology and look at 430.6 Ampacity and Motor Rating Determination.

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Static Design

Well 43 amps isn't an option for me to click on holts free practice exams and I don't have the answers to know how he is figuring them. The online free test gives 51a, 58a, 65a, 80a for the selections, so I don't know. I see your way and the code references that comply to that maybe I am just missing something. I am going to go smoke and then go to bed and study on tomorrow haha.

Thanks for the help.

Magnettica

No problem.

Are you sure the motor is three-phase?

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Static Design

Yeah it's question 82, first section I believe on the free 05 edition test.

http://www.mikeholt.com/examprep.php...linejourneyman

Magnettica

bump

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TinkerGA

full spool 57.6A is the right answer and that is the formula