Conductors X and R values from SKM Library

[Tainted]

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.

 

[Julius Right]

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.

 

[wwhitney]

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.

https://selinc.com/api/download/2470

Cheers, Wayne

 

[David Castor]

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.