Voltage Drop Calculation for Feeders

[MA_PE]

Can someone tell me the correct Voltage Drop Calculation to use for a 3 Phase, 4 Wire Feeder?

Thanks.

 

[chris kennedy]

Can someone tell me the correct Voltage Drop Calculation to use for a 3 Phase, 4 Wire Feeder?

Thanks.

Is there one correct one? Try this. See note under 'select voltage'.

 

[Smart $]

Is there one correct one? Try this. See note under 'select voltage'.

According to the IEEE there is... i.e. if your use of the word "correct" is synonomous with their "exact"

 

[Dennis Alwon]

According to the IEEE there is... i.e. if your use of the word "correct" is synonomous with their "exact"

I'm sure the op will use this formula. :grin:

 

[Smart $]

I'm sure the op will use this formula. :grin:

Assuming the smilie indicates your statement is a sarcastic remark, I have to ask why you believe he would not? ...because it's too complex? ...because there are too many easy-to-use online calculators, and the result need not be "exact"?

In truth, what I fail to understand is why a fairly comprehensive online calculator, like the one you linked to, falls just short of using the exact formula. It's not really that hard once all the parameters are determined or estimated. If the one you linked to added the power factor parameter and slightly changed the math, it'd be there.

 

[Mayimbe]

In truth, what I fail to understand is why a fairly comprehensive online calculator, like the one you linked to, falls just short of using the exact formula. It's not really that hard once all the parameters are determined or estimated. If the one you linked to added the power factor parameter and slightly changed the math, it'd be there.

im estimating those values, im using this formulas

Xaa=0.2794*log(Dt/GMR)

Dt=25920*sqrt(earthresistivity/60Hz)

Xab=0.2794*log(Dt/GMD)

GMD=cubic root(Dab.Dbc.Dac)

Dxx= distance between conductors centers.

i would get a 3x3 matrix. im thinking that i would have to use KRON or CARSON reduction, but i dont have the bibliography for that reduction method, do you know where can i find it? is this an apropiate way to find the value of X?

 

[Smart $]

im estimating those values, im using this formulas

Xaa=0.2794*log(Dt/GMR)

Dt=25920*sqrt(earthresistivity/60Hz)

Xab=0.2794*log(Dt/GMD)

GMD=cubic root(Dab.Dbc.Dac)

Dxx= distance between conductors centers.

i would get a 3x3 matrix. im thinking that i would have to use KRON or CARSON reduction, but i dont have the bibliography for that reduction method, do you know where can i find it? is this an apropiate way to find the value of X?

I'm at a loss in determining what you are attempting to do...

Your formulas do not explain their correlation to the "exact formula" variables... and it appears you may even be using scripting math, which I am not versed in [example: GMD=cubic root(Dab.Dbc.Dac)].

What is Xaa? Xab? GMR? GMD? Why are you using earth resistivity at 60Hz?

In addition, while the exact formula may be "exact", the precision of its result is highly dependent on input precision... and there are way too many real world varibles which affect that precision. Sure you can increase the level of input precision, but after a certain point it becomes moot to any real world application.

Voltage drop calculations happen before the fact. There is no need to do so after the fact because simple empirical measurements can provide that value.

 

[Dennis Alwon]

Assuming the smilie indicates your statement is a sarcastic remark, I have to ask why you believe he would not? ...because it's too complex? ...because there are too many easy-to-use online calculators, and the result need not be "exact"?

In truth, what I fail to understand is why a fairly comprehensive online calculator, like the one you linked to, falls just short of using the exact formula. It's not really that hard once all the parameters are determined or estimated. If the one you linked to added the power factor parameter and slightly changed the math, it'd be there.

I think part of it is the way it is written. Most of us are not dealing with cosine, sine, etc. so yeah the simple online calculator makes life a breeze

 

[gar]

090826-0655 EST

The equation as presented is meaningless without a definition of its intent and what each variable represents.

This is illustrative of the blind use of an equation to get an answer without an understanding of the theory.

The following discussion was very unclear because there was no initial definition of Vd:
http://forums.mikeholt.com/archive/index.php/t-102955.html

Extremely important is the definition of Vd.

.

 

[Mayimbe]

I'm at a loss in determining what you are attempting to do...

im designing conductors... for industrial purposes at 480 V.

Your formulas do not explain their correlation to the "exact formula" variables... and it appears you may even be using scripting math, which I am not versed in [example: GMD=cubic root(Dab.Dbc.Dac)].

What is Xaa? Xab? GMR? GMD? Why are you using earth resistivity at 60Hz?

i dont know what scripting math is.

Xaa is the "own" inductive reactance. or the inductive reactance between conductor "a" and conductor "a".

Xab is the "mutual" inductive reactance. or the inductive reactance between condcutor "a" and conductor "b"

GMR is the geometric medium radio, example: for a solid conductor GMR = R*e^1/4, for stranded conductor i can find it on tables.

GMD is the geometric medium diameter. GMD=cubic root(Dab.Dbc.Dac). Dab is the distance between the center of the conductor "a" and the center of conductor "b", and similar with the rest.

earth resistivity = ro = p

so Dt=25920*sqrt(p/60Hz) [inches]

it was on the "Electric Distribution Handbook" by T. A Short. its fine by me, if the medium of the magnetic field is the earth and not the air, it has to appear somewhere...

so at the end, i would get a matrix compose by this

Xaa Xab Xac
Xba Xbb Xbc
Xca Xcb Xcc

usually Xaa=Xbb=Xcc=Xp and Xab=Xba=Xm1, Xac=Xca=Xm2, Xcb=Xbc=Xm3... so finally i would have something like this:

Xp Xm1 Xm2
Xm1 Xp Xm3
Xm2 Xm3 Xp

theres a way to reduct or convert this matrix into a:

Xa 0 0
0 Xb 0
0 0 Xc

being Xa, Xb y Xc, the values that im going to use on the IEEE Vd formula (for each phase).

im stuck on the way to the reduct this matrix. i believe there iss a method by the name of KRON or Carson, but i dont have enough bibliography to find those. im still searching

Voltage drop calculations happen before the fact. There is no need to do so after the fact because simple empirical measurements can provide that value.

its happening before the fact.

 

[Smart $]

im designing conductors... for industrial purposes at 480 V.

Unless you are running miles of cable, such as in POCO distribution, I think you are getting way too technical on the matter.

i dont know what scripting math is.

The type you use to write calculations up in, say java script. If you are not using scripting math, should I take "GMD=cubic root(Dab.Dbc.Dac)" to mean: GMD=cubic root(Dab ? Dbc ? Dac)

Xaa is the "own" inductive reactance. or the inductive reactance between conductor "a" and conductor "a".

Xab is the "mutual" inductive reactance. or the inductive reactance between condcutor "a" and conductor "b"

GMR is the geometric medium radio, example: for a solid conductor GMR = R*e^1/4, for stranded conductor i can find it on tables.

GMD is the geometric medium diameter. GMD=cubic root(Dab.Dbc.Dac). Dab is the distance between the center of the conductor "a" and the center of conductor "b", and similar with the rest.

Thanks... the acronyms and variables with subscripts don't mean much without a description. I always hate jumping to latter pages of reference books which use acronyms or subscripted variables that are defined only once in an earlier portion of the overall text, and end up reading multitudes of text I didn't need to, just to find the description.

earth resistivity = ro = p

so Dt=25920*sqrt(p/60Hz) [inches]

it was on the "Electric Distribution Handbook" by T. A Short. its fine by me, if the medium of the magnetic field is the earth and not the air, it has to appear somewhere...

AFAIK, earth resistivity only comes into play if you are using the earth as a return conductor, such as in a SWER (single wire earth return) system, unless you are attempting to glean the reactive component of earth with respect to the line conductor(s). In such case, the full resistivity would not be realized, and for most cases, I believe the reactivity of earth is of an insignificant value, other than perhaps in soil having an extremely high ferrous or metallic content. But even then you have to consider that conductors of the same circuit in close proximity to each other have a mutual induction cancelling effect.

Thermal conductivity of earth is a different matter and plays a much more significant role in calculating voltage drop accurately (Note in my last post I used the term precise or variations thereof errantly... should have been accuracy or variations thereof)

so at the end, i would get a matrix compose by this

Xaa Xab Xac
Xba Xbb Xbc
Xca Xcb Xcc

usually Xaa=Xbb=Xcc=Xp and Xab=Xba=Xm1, Xac=Xca=Xm2, Xcb=Xbc=Xm3... so finally i would have something like this:

Xp Xm1 Xm2
Xm1 Xp Xm3
Xm2 Xm3 Xp

theres a way to reduct or convert this matrix into a:

Xa 0 0
0 Xb 0
0 0 Xc

being Xa, Xb y Xc, the values that im going to use on the IEEE Vd formula (for each phase).

im stuck on the way to the reduct this matrix. i believe there iss a method by the name of KRON or Carson, but i dont have enough bibliography to find those. im still searching

I'm quite rusty on just basic matrices... sorry but I can't help in this regard without a crash refresher course

 

[Smart $]

I think part of it is the way it is written. Most of us are not dealing with cosine, sine, etc. so yeah the simple online calculator makes life a breeze

I understand entirely... but we do deal with power factor, so it's just a matter of recognizing that part of it and inputting even a guess-timated value for power factor. The R and X values can be obtained from Table 9.

BTW, cosθ= pf, where "pf" is the power factor, and not really that hard to include in calculations, or incorporate into these online calculators. The sinθ value can be calculated sin(arccos pf). Note arccos is the inverse cosine of the value, sometimes noted as acos, or cos^-1 (which is the same as cos with a -1 superscript).

 

[Mayimbe]

Unless you are running miles of cable, such as in POCO distribution, I think you are getting way too technical on the matter.

guess i am. but i need to do it. this is the project for my degree in Electric Engineer.

AFAIK, earth resistivity only comes into play if you are using the earth as a return conductor, such as in a SWER (single wire earth return) system, unless you are attempting to glean the reactive component of earth with respect to the line conductor(s). In such case, the full resistivity would not be realized, and for most cases, I believe the reactivity of earth is of an insignificant value, other than perhaps in soil having an extremely high ferrous or metallic content. But even then you have to consider that conductors of the same circuit in close proximity to each other have a mutual induction cancelling effect.

Thermal conductivity of earth is a different matter and plays a much more significant role in calculating voltage drop accurately (Note in my last post I used the term precise or variations thereof errantly... should have been accuracy or variations thereof)

AFAIK?

I didnt get all the things that you say here. you dont have to explain yourself if you dont want to. or making it more accesible for humble english ignorant like me.

i have to say this about the earth resistivity. maybe isnt the earth resistivity that we have to take account, but the permeability perhaps.

because L is proportional to the permeability. and X=w*L

 

[Smart $]

guess i am. but i need to do it. this is the project for my degree in Electric Engineer.





AFAIK?

I didnt get all the things that you say here. you dont have to explain yourself if you dont want to. or making it more accesible for humble english ignorant like me.

i have to say this about the earth resistivity. maybe isnt the earth resistivity that we have to take account, but the permeability perhaps.

because L is proportional to the permeability. and X=w*L

AFAIK = as far as I know (touch? on the acronyms )

You seem intelligent... you'll figure it all out in due time... coupled with continued experience, and I can't provide that for you .

While the reference you noted above is a good one, as time and experience progress you will realize many reference texts are written overly technical just to appear as the better reference and bolster the ego of the writer

 

[Mayimbe]

i find how to do it, the matrix reduction, it seems that the expression above of Dt is not right.

the Stevenson - Graiger book shows how to do it in the proper way, and leave no place to doubts.

 

[Smart $]

i find how to do it, the matrix reduction, it seems that the expression above of Dt is not right.

the Stevenson - Graiger book shows how to do it in the proper way, and leave no place to doubts.

Sounds great!