600V & MV Cable R & X values

[MyCleveland]

Question 1
NEC Chap 9 table 9
Can anyone know the (R) multiplier for going from PVC values >>> Alum Conduit values >>> Steel conduit values ?


Question 2
MV cables
L or X calculation methods....how do you account for Skin & Proximity Effect on these values ? or is it typically ignored /

 

[Julius Right]

In my opinion, the three single core cables in conduit Rac calculation is based on
Neher and McGrath article The Calculation of the Temperature Rise and Load Capability of Cable Systems published in October 1957. In chapter Calculation of Losses[ eq.14] it is stated:
Rac/Rdc=1+Yc+Ys+Yp where Yc=Ycs+Ycp skin and proximity effect factors,Ys=Ysc+Yse[circulating and eddy current] it is shield [and armor] factor and Yp it is the pipe factor.
For nonmagnetic[metallic] pipe the same [eq.31] as for Yse may be used and for steel pipe eq. 33,34 or 35[for iron it has to be multiplied of 0.8]. Low voltage cable-less than 2000 V rated-is not provided with shield but sometime with armor.
For inductive reactance calculation we may use the well known Nomogram for Determining Conductor Reactance at 60 Hz based on formula:
2*pi()*f*(0.1404*log10(2*g/dcnd)+K)/10^3 Ω/kft
g=[dist1.dist2.dist3...distn)^(1/n)
For 3 lives and one neutral in cradle formation it could be 1.22 times overall cable diameter.
dcnd= conductor diameter.
K depends on no.of strands: solid[1 strand]K= 0.0153 61 strands =0.0155.
No skin or proximity effect is considered.

 

[MyCleveland]

Alternate Method Proposed for Reff

Alternate Method Proposed for Reff

Blocked from PDF add because of file size.
You can find the article I was trying to reference at www.electricalengineeringtoolbox.com, "AC Resistance of a Conductor".
This method uses a similar formula to the Ne-Mc formula #24A.

So many different variations out there for both R and L dependent upon method and units...metric or imperial.

I am hoping to use this method for calculating Reff and modify it to account for "Conduit Effect" for either "Aluminum" or "Steel"

Model I am coming up with for "Steel" is as follows...
Reff = Rdc ( (R/Rdc - 1 + fp) x 1.7 )

1.7 value taken from Ne-Mc paper, second paragraph after formula #35.
Do you agree ?

If yes, alternate factor for "Aluminum" conduit /

Hold for the moment on "L" calculation.

 

[Julius Right]

Sorry. I cannot find "AC Resistance of a Conductor" through the above link.:weeping:
Be so kind and give the definition for Reff, fp and R.

 

[MyCleveland]

Forgive my lack of tech savy

Forgive my lack of tech savy

I just assumed going to the site and typing in the article referenced would bring it up.

Try this please: https://www.electricalengineeringtoolbox.com/2014/07/ac-resistance-of-conductor.html

Reff - eff short for "effective" or Rdc after all adjustments.

R - numerator of AC/DC ratio.

The AC/DC ratio is interpolated from a table of values which yields the Skin Effect portion of the overall adjustment.

fp - is the proximity effect of the overall adjustment.

All these values will be apparent after looking over the article.

Please note in using my proposed formula ... converting NEC chap 9 table 8 Rdc values and comparing them to the table 9 values the total percent error is 4% STEEL conduit and raising the 1.7 multiplier by a factor of 5 for ALUM conduit the total percent error is also 4%. If I take 1000kCMIL the error for both drops to approx. 3%.

 

[Julius Right]

Sorry for the delay.[Sometimes I have so much to do that I do not have time for interesting things].
Thank you for this information.
However, because in these formulas it is used GMR and GMD this calculation fits more for Transmission Lines-I think.
Usually no wire of a transmission line will run through conduits-or ducts.
I had no time to verify how close it is for three single core cables in ducts-including shield-and armor- losses for MV cables. It will take more time.

 

[MyCleveland]

JR...thanks for your interest.
A given formula should be applicable for both LV & MV systems.
We can calculate GMR & GMD for both.
I agree that it has to be adjusted for shields, conduit or armor type.

 

[Julius Right]

I have worked on comparing between skin and proximity factors as per AC Resistance of a Conductor from Learning Electrical Engineering and Neher&McGrath/ICEA P53-426 and IEC 60287-1-1 [chpt.2.1.2-2.1.3]. Skin effect it is the same but proximity is far from it.
I had some problem to calculate GMR for 750 MCM [61 strands] indeed.As I calculated it was 0.02561 but one manufacturer said it is 0.0331 ft.

 

[MyCleveland]

Recall I was only off 2-5% using the link method.
I started over and substituted the Ne-Mc portion for Proximity also and then the Ne-Mc "PIPE" factor and I am down to 1.5% error for both "in-STEEL" & "in-ALUM", 600V.

Can you breakdown the "Shield" effect factor for MV cable ? I have not attempted it yet.