IEEE Red Book Cable Impedances, 600V vs 5kV nonshielded

[JensenDrive]

At the end of chapter 4 of the 1993 IEEE Red Book (Std 141) there are some typical cable impedances (the 1986 version does not seem to have the tables). For a given AWG or kcmil, the tables give the same reactance for 600V and 5kV nonshielded cablereactance, XL. I know the tables is a rough/typical estimate because how cable is lain in the tray/conduit/trench has a large effect on reactance, but still, I am a bit surprised that the two voltage levels have about the same reactance, per the tables. I do not deal with 5kV cable in my line of work, but I would expect it to have thicker insulation than 600V cable. Thicker insulation means larger wire-wire spacing, which will cause higher XL, so you can guess that I am scratching my head; "How can these two voltage classes have (roughly, of course) the same XL for the same AWG?" Two answers come to mind:
a) Maybe 5kV non-shielded insulation is higher grade but not really much thicker compared to the insulation used at 600V.
b) Maybe how the cables are placed in the conduit/trench/tray has a such a huge effect on spacing, relative to insulation thickness, that there is not much point in different tables for the two voltage classes.

Questions for the group:
1) Do either of these answers make sense?
2) Any other explanations come to mind?
3) If cable reactances actually are a little different for the two cable voltage levels, do the tables more closely represent 600V or 5kV cables?
4) Do you feel comfortable using these Red Book impedances in fault and voltage drop calculations?

 

[davidbeach]

I don't have the Red Book at hand, but many tables of impedances list the impedance at a given spacing and then there are additional factors that account for the spacing but are not impacted by conductor size.

 

[dpc]

There are a lot of variables so you have to make sure you're comparing apples to apples.

Three single conductor cables in a conduit will have a different reactance than a three-conductor cable. Spacing assumed between conductors and the assumed configuration will also have an impact. Of course metal non-ferrous conduit will be different than ferrous metal and both will be different than non-metallic or direct buried.

Also, keep in mind that the reactance can be broken into two parts - the self-inductance and the inductance caused by adjacent conductors. The self-inductance is only a function of the conductor diameter (and the stranding to a small degree).

Conrad St. Pierre's book "A Practical Guide to Short Circuit Calculations" covers the calculation in a fair bit of detail and he also has tables of typical values.

I've never heard any issues related to the tables in the Red Book, but I haven't looked at them for quite a while. I'd wager the data in the Red Book will give you reasonable SC and Voltage drop calcs.

 

[cuky2000]

See the enclosed links

http://cuky2000.250free.com/Cable_Resistance_and_Reactance.gif

http://cuky2000.250free.com/Cable_Impedance_AL_1C.gif

 

[JensenDrive]

dpc:
I dug up a copy of Conrad's book from a co-worker, and found that Conrad's table C-3a and C-3b show roughly the same reactance for 600V and 5kV cable, which tends to say that the Red book is OK in listing 600V and 5kV cable having about the same reactance. However, both R and XL was much different in the two sources. For example, for 250MCM in magnetic conduit, 600V, ohms/1000':

Red Book, Table 4A-8: 0.0896+j0.0384 (90deg C)
Conrad, Table C-3a: 0.0445+j0.0621 (temp not specified)

Conrad's R is much lower, but XL is much higher. This sort of makes a case for actually measuring cable impedance if you have a marginal breaker rating or some other issue.

 

[dpc]

The temperature will make a big difference in the resistance.

I believe Conrad's data is at 25 deg C - see Table C2. That probably explains a lot of the difference in the R values.

Again, it will be different for 1/C versus 3/C cable and assumed spacing as well. Shielded versus non-shielded for 5 kV will also have an impact.

Good luck trying to measure it in the field!