ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat
Recent Posts
Grounding electrodes
by gfretwell - 09/07/21 03:41 PM
Looking For Electricians in the Midlands
by Alex247 - 09/06/21 05:26 AM
Backup Generator Done Right
by timmp - 08/31/21 04:44 PM
New in the Gallery:
Now you know.
Now you know.
by Tom_Horne, September 7
February, North East Indiana
February, North East Indiana
by timmp, July 25
Who's Online Now
0 members (), 34 guests, and 14 robots.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Joined: Aug 2005
Posts: 250
T
Member
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.

Last edited by Trick440; 06/13/07 09:07 PM.

Shake n Bake
Joined: Feb 2003
Posts: 939
F
Member
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


Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

Joined: Aug 2005
Posts: 250
T
Member
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.


Shake n Bake
Joined: Aug 2005
Posts: 250
T
Member
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.

[Linked Image from i146.photobucket.com]

[Linked Image from i146.photobucket.com]


Shake n Bake
Joined: Aug 2005
Posts: 250
T
Member
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.


Shake n Bake
Joined: Jul 2002
Posts: 680
W
Member


Link Copied to Clipboard
Featured:

Tools for Electricians
Tools for Electricians
 

* * * * * * *

2020 Master Electrician Exam Preparation Combos
2020 NEC Electrician
Exam Prep Combos:
Master / Journeyman

 

Member Spotlight
RonKipperDatacom
RonKipperDatacom
Kansas City, KS
Posts: 30
Joined: January 2013
Top Posters(30 Days)
timmp 3
Popular Topics(Views)
282,903 Are you busy
216,666 Re: Forum
203,097 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.5