ECN Forum Forums General Topics of the Trade General Discussion Area Motor math question... Feeder Fuse.

I thought I had motors figured out. After some reviewing I'm missing something.

Anyone wanna help?

All conductors are THW 75*C terminations.

25hp, 25hp, 15hp, 5hp
208V three phase
w/ non-time-delay fuse.

What is the Feeder fuse size?

This is how I think the problem is done but Its not Im missing something.

Take the largest motor FLC @ 125% = 93.5A
Get the fuse size for 93.5A = 100A
Now add that 100A with the FLC of the remianing motors.

100
74.8
46.2
+ 16.7
= 237.7A

Round down and the Feeder fuse should be 225A

Thats wrong, apparently the answer is 350A Feeder Fuse.

Help me out here I'm missing something. Thanks.

Trick440 ,, there some figures that dont really ring the bell to me but one question will the largest motors will start the same time or not ??

for me this what i will do is figure each motor seperately

25 hp @ 208v = 74.6 amp [ i did not add 125% yet ]

74.6 +74.6 + 46.2 + 16.7 = 212.1 amps

now i will add extra 25% on largest motor which is 18.65 amp more so the total load is 230.75 amp then go with 175% for time delay fuse which it will say 402 amps but i can down size a little to even 400 amp but if you used the non time delay fuse the fuse size will jump up to 690 amp non delay fuse. the nondelayed fuse is sized at least 250% so that what it get ya on the figures

if more question just post it here

Merci , Marc

This is a text book, masters exam question. There are many more with the same result and the answer in the book is definelty showing 350a feeder fuse.

As far as the 'non time delay fuse' I think that is just thrown in there. You use that for rating motor 'branch circuits'. (short circuit, GFCI protection). I'm don't think its used in the feeder calcs. But I could be wrong.

Heres a copy of the question page & the answer page. Question 7 is the one I posted.

Sorry its messy looking. Not sure how I got the answers the first time around... I might of just copied out of the back of the book.

Bah... It looks like you do use the fuse.

Largest motor X 300% (non-time delay. 430.52)
=224.4A

Now add the remaining.
224.4
74.8
46.2
+16.7
= 362.1, round down and you get a 350A feeder fuse.

I tried to figure this out for an hour last night with no luck... I come back and put 15 minutes into it this morning and get it. new day new ideas I guess.

Trick
couple of good motor reads
http://ecmweb.com/mag/electric_motor_calculations_part_2/

http://ecmweb.com/mag/electric_motor_calculations_part/