Conductors X and R values from SKM Library

Tainted

Tainted

Senior Member
Location
New York
Occupation
Engineer (PE)
In SKM I selected NEC 310.16 conductor library 4-1/C+G, 500KCMIL THWN (in non-magnetic raceway)

The SKM libaray sets these conductors with the following positive sequence R and X values:

R = 0.0276 ohms/ 1000ft
X = 0.0373 ohms/ 1000ft

and zero sequence R and X values:

R = 0.0438 ohms/ 1000ft
X = 0.0999 ohms/ 1000ft

My question is:

Is this accurate? How did SKM get these values for conductors? These values do not match in NEC chapter 9 table 9, this table doesn't even show zero sequence impedances either.
 
Positive X and R
From X value, it seems to me, the frequency is 50 Hz. The conductor is ASTM class B stranded.
The conductor diameter as per UL 1581.
The insulation -Class XL or EPCV under no covering-as per UL 44.
The core layout factor =2.32- something between 2.15-for 3 conductors and 2.42 for equal 4-
and 37 strands per conductor.
For resistance, if we take the maximum as per UL1581, and, this time, frequency 60 Hz, and conductor temperature 90oC, we get yskin=0.01493 and yproxy=0.00863 and the resistance of 0.0276 ohm/1000. ft
Zero R and X
It is more complicated because the data presented is insufficient.
 
The positive sequence numbers are pretty close to the NEC Table 9 numbers, which are (for PVC conduit):

R = 0.027 ohms/1000 ft
X = 0.039 ohms/1000 ft

As to the zero sequence impedances, my familiarity with symmetrical component analysis is minimal, but a quick read of this document (random google hit) indicates that they differ from the positive sequence impedances by 3 * M, where M is the mutual impedance between a pair of conductors. But I'm not clear on why M is non-zero or how to calculate it.


Cheers, Wayne
 
The zero sequence impedance depends heavily on the assumed return path, so it can vary significantly. I'm sure SKM got their data from a particular cable vendor. I would just ask SKM about the source of their data. I doubt if it's a secret. There will never be 100% agreement for cable impedance data. For the resistance, you have to consider the assumed conductor temperature. I would tend to give more credibility to data from reputable vendors than the NEC tables.
 
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