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#8727 - 04/01/02 08:58 PM 3 phase voltage drop calculations
sparkyga Offline

Registered: 04/01/02
Posts: 15
Loc: V.K.

My name is Greg and I work over in Bosnia on a Canadian S.F.O.R. base for a company out of Calgary.As much as I try to do my best to keep to the C.E.C. when doing work, it is not always possible to do, due to supplies or manpower. I found this site yesterday and look forward to using it often for keeping up with my code and hopefully for some help when I get stuck. I have a formula out of the N.E.C. for 3 phase voltage drops that talks about a Q factor for cables above 2/0 .1)What is the Q and how do i use it ?
2) Is there a better calculation I can use for 3 phase calculations? Many of our runs are very long, and alot of our services are large so I like to be right before I purchase cables
Thank you for your time and look forward to hearing some results
Greg Smith
Camp Black Bear

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#8728 - 04/02/02 04:03 AM Re: 3 phase voltage drop calculations
Redsy Offline

Registered: 03/28/01
Posts: 2138
Loc: Bucks County PA
Do you mean "K"?

#8729 - 04/02/02 07:30 AM Re: 3 phase voltage drop calculations
The Watt Doctor Offline

Registered: 12/23/01
Posts: 443
Loc: Mont Belvieu, TX
Hey Greg,
Greg here. Just remember that VD is equal to (2) "licks" over a "Cool Mama". Now this applies to your personal life as well as electric work.
The above formula is for single phase applications.
For three phase applications use the following:
VD=Voltage Drop (or it could be you know, V.D.)
L=length of circuit from source to load (one direction only, thus the "2" in the formula)
I=current in amperes
K=constant for either copper or aluminum
Acm=Area of conductor in circular mils (or it could be "A cool Mama")
The .866 is thrown into the formula for 3 phase applications, and is equal to the square root of 3 divided by 2.
The constants for copper and aluminum may vary depending on who you ask, but I was taught that Copper is 10.4, and Aluminum is 17.1.
If you want to avoid "VD" in your personal life, stay away from those "Cool Mamas", but if you want to avoid "VD" in your electrical work use the above formulas, and I'm sure that you will be O.K.
Always, always remember, "Proper electric today, keeps The Watt Doctor away."
And by the way, The Watt Doctor only does check ups on one kind of VD, and it has to do with the kind where electrons, conductors, difference of potential, etc are involved, not the other.

Best Voltage Dropological Regards,
The Watt Doctor
Altura Cogen
Channelview, TX

#8730 - 04/02/02 07:59 AM Re: 3 phase voltage drop calculations
eman1963 Offline

Registered: 01/24/02
Posts: 22
Loc: rhode island
Nice answer doc. lol

#8731 - 04/03/02 05:34 PM Re: 3 phase voltage drop calculations
HotLine1 Offline


Registered: 04/03/02
Posts: 6792
Loc: Brick, NJ USA
You may want to see if you can locate a great little calculator called "ELECTRICALC"
Matter of fact an ad just popped up to the left of this page. I think it's in the $100 range, but I think it' is well worth it.

Does a whole lot....check it out

#8732 - 04/03/02 06:25 PM Re: 3 phase voltage drop calculations
Ron Offline

Registered: 03/13/02
Posts: 577
Loc: White Plains, NY
Check toward the bottom of the web page below for volt drop calcs in feet or meters.

#8733 - 04/04/02 05:22 PM Re: 3 phase voltage drop calculations
Scott35 Offline

Broom Pusher and

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
"Q" [Figure Of Merit most of the time] can either be applied to the VD formula as Skin Effect Ratio / Calc, or used in conjunction with Reactances [Xl &/or Xc] to determine an Impedance.

It's in one of my EE books some place

"K" is typically for "Constant", as indicated by Doc, and is Conductor dependent.

Are you planning to calc a Voltage Drop using the "Complete AC" method - which applies the Power Factor, Total Conductor Impedance [Resistance and Reactance(s)], Conductor Type [Skin Effect] and Magnetic / Non-Magnetic raceway?

Scott SET
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!


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