Need Help

[sparky9669]

Hi I'm a new user of the forum. I have been having trouble with mutiple motor calculations on an exam I'm preparing for. Code references and steps to complete the problems would be greatly appreciated. Here are the two I'm having problems with.

Giving 480V 3Phase delta service; One 20h.p. 3phase motor, two 10 h.p. 3phase motors, One 50h.p. 3phase motor. The 50h.p. motor is rated for 5 min. varing duty. Calculate the size feeder overcurrent protection using an inverse time circuit breaker.

Possible Answers
200A
225A
250A
300A

Next question is

Givin
115V 1phase 2h.p. cont. duty nameplate 19.92A
115V 1phase 3h.p. interm.(5 min.)nameplate 28.22A
230V 1phase 5h.p. cont. duty nameplate 23.24A
460V 3phase 30hp cont.duty nameplate 33.20A
460V 3phase 40hp interm. (5 min.)nameplate 43.16A
460V 3phase 50hp cont. duty nameplate 53.95A

What is the Total Calculated motor load in Volt Amps.

Possible Answers
130,220 VA
133,146 VA
157,651 VA
143,723 VA

Thanks for the time.

 

[bphgravity]

Re: Need Help

1. To find the feeder ground-fault and short-circuit protection, follow the requirements of section 430.62 of the NEC 2002. Select the largest branch circuit overcurrent device as required by 430.52 and add all the other motor F.L.C's connected on the same feeder. The code does not permit going up to the next standard size on a motor feeder.

2. To find the VA of the motor, multiply the connected voltage to the namplate amperage. For three phase circuits, multiply the above value by 1.732. Add 125% to the continuous loads.

 

[sparky9669]

Re: Need Help

Thanks for the references but, I'm still having trouble with the Duty Cycle parts of the problems.

 

[bphgravity]

Re: Need Help

It would not make a difference for the particular questions you asked. Table 430.22(E) is used only for applying conductor sizing, not the OCD. Section 430.33 refers you to 430.52.

 

[sparky9669]

Re: Need Help

I've taken the test again today and I failed these same two questions. Can someone give me step by step instructions on how to figure these questions out.

 

[roger]

Re: Need Help

Sparky9669, a better learning tool would be for you to show your "step by step" method so that only the incorrect step (or steps) can be addressed.

Roger

 

[bphgravity]

Re: Need Help

Using the exact method as I provided in my first post, for question number 1, I get 200A. For question number 2, I get 133,966. What are you figuring?

 

[sparky9669]

Re: Need Help

1.M1=27FLA;M2=14FLA;M3=14FLA;M4=65FLA
65*250%=162.5+27+14+14=217

2.
M1=115*19.92*125%=2863
M2=115*28.22=3245
M3=230*23.24*125%=6,681
M4=460*33.20*1.732*125%=33064
M5=460*43.16*1.732=34386
M6=460*53.95*1.732*125%=53729
133,968VA

This is the answers and the way I went about getting them. Appreciate the help.

 

[bphgravity]

Re: Need Help

You got the same numbers I did and used the same method. Question 1 is obviously 200A, but I'm not sure why question 2 is not coming out exact.

 

[sparky9669]

Re: Need Help

In section 220 it says to use 120, 230, 480 Volt as your Nominal Volatages in your calculations. I've tried that also and I'm still unsuccessful.

 

[fastjoe]

Re: Need Help

Hi
Spent some time working on this last night.
Here is what I think.
"Giving 480V 3Phase delta service; One 20h.p. 3phase motor, two 10 h.p. 3phase motors, One 50h.p. 3phase motor. The 50h.p. motor is rated for 5 min. varing duty. Calculate the size feeder over current protection using an inverse time circuit breaker.

Possible Answers
200A
225A
250A
300A
"

1. FLA from the tables as always on a test.
Largest motor fla times value from 430-52 for inverse breaker. (250%)
2. adjust up to next size per note one of 430-52
ex 1. for standard size breaker in 240-6
if need be.
3. add remaining Fla's of other motors from 430-150

4.round down to standard size breaker from 240-6
per no rounding up per section 430-62 (A)
I got 225 amp.