still dont understand voltage drop

Status
Not open for further replies.
T

titan01

Member
3 phase 460v

15hp motor
21 full load amps
110 feet one way distance

i need to find what type of wire is appropriate but im running into some trouble

for voltage drop i have 460 / 3% = 13.8V

should i go with 12.9 resistance for my conductor resitivity?

this is the equation i have - wire size = (conductor res)(amps * 1.25)(distance)(sqrt of 3) / voltage drop
 
Mr. Bill

Mr. Bill

Senior Member
Location
Michigan
titan01 said:
i read somehwere that you have to??
Click to expand...
I think there's a lot of stuff we do just because we heard it somewhere, awhile ago. It may have been added to cover all situations. Continuous and non-continuous loads. It doesn't hurt you to add 25% and lots of engineers enjoy having a comfortable safety margin in their calculations. That might be where it came from.

And if you're calculating single phase remember to multiply the distance by 2. Since the current goes out and back. At least that's what I do. But that might be my comfortable safety margin. Others may disagree.
 
bob

bob

Senior Member
Location
Alabama
DavisIMI said:
#12 wire is fine. 2.9%vd at 110'
Click to expand...

It may be ok for voltage drop but is it ok for 21 amps? You must multiply the FLA x 1.25 to determine the proper wire size for the circuit which = 26.25 amps.
Titan
For continuous loads, which includes motors, you must take the FLA x 1.25 to determinwe the correct conductor size. This conductor size may or may not be the correct size for voltage drop. In this case the #12 cu would have covered the VD but is not the correct size for the circuit.
 
Last edited:
D

DavisIMI

Member
If you would try reading the posts, like #1-#5 it seems, to someone who would pay attention, that he'd already added the 25%.
 
D

DavisIMI

Member
Let me rephrase my last post, after rereading it, it sounds like I was trying to be a smart a$$, which I wasn't, and I'm sorry if it was taken that way.

If you will reread post #1-#5, it looks as if he has already added the 1.25% to the FLA. It seems that way since bphgravity asked him why he was increasing his load by 25%, and he replied " I read somewhere that you have to??".

Looking back, he never actually stated of showed where he added the .25. He just had it in his calculation, he just stated 21 FLA's. So you may be right in your correction.

1st thing my mentor taught me a long, long time ago, B.E. in SC, never assume anything in this line of business. I guess I done went and broke the golden rule.:mad::roll:

Anyway, I didn't want you to take my last post the wrong way, I can be a a$$, and I am a lot, just ask anyone who knows me, but not this time.:smile:
 
Status
Not open for further replies.
Top