Alternating-Current Resistance and Reactance for 500-Volt Cables, 3-Phase, 60 Hz

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AWinston

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Table 9 of the NEC only shows the resistance and reactance for 600V cables. Where can I find the same information but for medium volt cables? Specifically, I need information for MV-90 cables. I am dealing with a 4,160V system.
 
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AWinston

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luckylerado said:
Pretty standard I would think for UL listed cable.
Click to expand...
I'm having some trouble with my calculations. I have a 1,700 ft run. The transformer output will be 4160V at 41.54A on the primary. Using below as an example, I am getting outrageous resistance values. Something has to be wrong.

#8 three conductor wire (DC resistance(R) = 0.647 ohm/km, Inductive Reactance = 0.2635 ohm/km, Capacitive Reactance = 9815 ohm/km)

Z = R cos(theta) + Xsin(theta)
theta = cos-1(.85) = 31.78 deg
R = 0.647 ohm/km
X = XC-XL = 9815 - 0.2635 = 9814.7 ohm/km

Z=5169 ohm/km
Z = 5169 ohm/km / 3280.84 = 1.57 ohm/ft
 
bob

bob

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Location
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Forget Xc. it is the value of capacitance between cables. From the table given, R =2.4 ohms Xl = 0.40 ohms/ km.
Where did you get your figures?
 
Julius Right

Julius Right

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Electrical Engineer Power Station Physical Design Retired
About Centelsa catalogue:
In my opinion, the data from the table on page 9 are wrong.
See data from page 26[ three conductors 90oC] and
you have to correct the resistance of 20oC to 90oC K=(234.5+90)/(234.5+20)
 
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Phil Corso

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Location
Boca Raton, Fl, USA
A.Winston...

There's a table in The IEEE Std-141 (IEEE "RED" book) Table 4A-7! R is fairly constant regardless of voltage-rating! X, however, changes with voltage-rating. Unfortunately the change increases in some cases, but decreases in others! The difference being whether the conduit is magnetic or non-magnetic!

Regards, Phil Corso
 
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Phil Corso

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Forgot what I was to send to you! Table 4A-7 Data, for #8, Cu, 3 single conductors:

600V & 5kV in Mag Duct, Non-Shielded Cable.
R=0.811, X=0.0754

5kV in Mag-Duct, Shielded Cable.
R=0.811, X=0.0860

600V & 5kV in Non-Mag Duct, Non-Shielded Cable.
R=0.811, X=0.0603

600V & 5kV in Non-Mag Duct, Shielded Cable.
R=0.811, X=0.0688

Do you want data for 3-conductor cable?

Caveat: X- Values depend on insulation used. Suggest you get Mfg data!

Phil Corso
 
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AWinston

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Location
Murrieta, Ca
Phil Corso said:
Forgot what I was to send to you! Table 4A-7 Data, for #8, Cu, 3 single conductors:

600V & 5kV in Mag Duct, Non-Shielded Cable.
R=0.811, X=0.0754

5kV in Mag-Duct, Shielded Cable.
R=0.811, X=0.0860

600V & 5kV in Non-Mag Duct, Non-Shielded Cable.
R=0.811, X=0.0603

600V & 5kV in Non-Mag Duct, Shielded Cable.
R=0.811, X=0.0688

Do you want data for 3-conductor cable?

Caveat: X- Values depend on insulation used. Suggest you get Mfg data!

Phil Corso
Click to expand...
It looks like I can make my calculations based on R and XL and ignore XC.
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
AWinston said:
I got my figures from the PDF link posted earlier.
Click to expand...

You are not using the capacitive reactance correctly.

This term is a _shunt_ term. As bob said, " it is the value of capacitance between cables. From the table given, R =2.4 ohms Xl = 0.40 ohms/ km."

In other words, the capacitive reactance is not in series with your load and doesn't directly cause voltage drop at the load.

Instead it represents a path for current flow from one cable to the other, and means additional current flow supplied to the cable set. For most installations done under the NEC you can ignore it.
 
Julius Right

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
On AWinston post of 11-04-15, 02:41 PM it is mentioned as following:
“#8 three conductor wire (DC resistance(R) = 0.647 ohm/km, Inductive Reactance = 0.2635 ohm/km, Capacitive Reactance = 9815 ohm/km)”
It seems to me it is extracted from the table MV-90 5 kV Three conductors, 90°C
100% Insulation Level on page 27. In my opinion, all data included in this table are wrong(!!!).
For instance: conductor diameter 134 mm[134/25.4=5.28 inches!!] but cross section area 0.0130 mm^2[!?].
The table on page 26 data it seems to me close to reality.
So R=2.12 ohm/km [2.12*.3048=0.646 ohm/1000 ft.] at 20oC.
XL=0.1638 ohm/km [0.1638*.3048=0.0499 ohm/1000 ft.]
According to Neher&McGrath R=1.02*roc/CI [10] where roc=10.371 circular mils. ohm/foot at 20oC [Table I].
R=1.02*10.371/16.51/10^6*10^3=0.641/10^3 ohm/ft.[0.641 ohm/1000 ft.].
[UL1072 Table 6.1 #8 =16.51*10^3 circ. mils].
Maximum permissible resistance UL1072 Tab.7.1 R20oC=0.652 ohm/1000 ft.
Neglecting skin and proximity effect Rdc=Rac at 90oC R90oC=0.652*(234.5+90)/(234.5+20)=0.831 ohm/1000 ft.
UL1072 Table 8.2-8.5 for #8 conductor diameter could be from 3.38 to 3.71 mm.
Insulated core for 5 kV [90 mils ins.thick.=2.29 mm] could be 3.71+2.1*2.29+1=9.5 mm and the diameter over the shield could be 9.5+4*.15=10.1 mm. So the distance between conductor center lines could be 10-11 mm.
XL=2*pi()*f[K+0.2*ln(2*s/dc)]*10^-3 ohm/km See[for instance]:
http://www.openelectrical.org/wiki/index.php?title=Cable_Impedance_Calculations
For 7 wires conductor K=0.0642 ; s=11;dc=3.38 [minimum conductor diameter for maximum XL]
XL=2*pi()*60*(0.0642+0.2*ln(2*11/3.38))/10^3=0.1654 ohm/km
So R=0.831/.3048=2.726 ohm/km XL=0.1654 ohm/km
VD=sqrt(3)*(2.726*0.85+0.1654*sqrt(1-0.85^2))*41.5*1700/1000*.3048=89.54 V
VD%=89.54/4160*100=2.15%
 
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