Appropriate method to choose NEC table for voltage drop in different scenarios

[Lott99]

What is the appropriate way to choose an NEC table (chp. 9 table 8 or 9) to use for different scenarios of voltage drop calculations? Chapter 9, Table 8 gives DC 75C resistances, while Table 9 gives very specific 3P AC 75C in conduit resistances, impedances, and reactances. How does one decide what they will do when calculating voltage drop for 1P AC? What about DC in conduit? Or AC in free space?

Additionally, reading through the article found at https://www.mikeholt.com/technnical-voltage-drop-calculations-part-one.php, another question arises. For a 3P circuit example, a resistance value was chosen rather than an impedance in an AC circuit. Why is this? Shouldn't impedance be considered when calculating voltage drop for an AC circuit?

 

[Julius Right]

Table 8 Conductor Properties
There are only the conductor data. Using this table you may calculate the ac resistance and reactance.
However, this table presents "raw material" for use in any case [conduit, open area, cable tray, low and high voltage].A.C. resistance [see Neher & McGrath theory] it is:
Rac=Rdc*(1+ys+yp+yc+ys) where ys=skin effect yp=proximity effect yc=conduit effect ys=shield[or armor] effect.
Skin effect depends on dc resistance at the rated conductor temperature [usually 75oC] For other temperature you have to use the factor k from formula[for copper conductor for instance]: k=(234.5+Tc)/(234.5+75) where Tc is the conductor actual temperature. For proximity effect you need the conductor diameter also.
For reactance you need to know how the conductor is stranded also.
X=2*pi*f*(0.2*ln(2*s/dcond)+K)/10^3 ohm/km
Here K depends on number of strands. For instance if are 3 strands in a conductor K=0.0778 but for 19 strands K=0.0554.
Table 9 Alternating-Current Resistance and Reactance for 600-Volt Cables, 3-Phase, 60 Hz, 75°C (167°F) — Three Single Conductors in Conduit
You may extract from this table resistance and reactance of a cable in specific cases: single core low voltage, in conduit of pvc, aluminum or steel only. Also at 75oC conductor temperature.
Cables in open area, at different distances and other are not in this table and also medium and high voltage cable is not here.
By the way, if you don't know the actual power factor [cosφ] and you will take as maximum [1] then only resistance counts.
See IEEE 141 CH.3.11 Voltage drop formula: V = IRcosφ + IX sinφ
cosφ=1 sinφ=0