Cable Reactance Issues 1

[Engg101]

How is it that the reactance of a cable per unit it higher than it's resistance per unit? Under what conditions is this possible?

 

[alehman]

They are really unrelated. Reactance depends on the spacing phase-to-phase and phase-to-ground. Large spacing leads to high reactive impedance. In overhead power transmission lines, the reactive impedance is much higher than resistance.

For a 3-phase circuit with conductors in an equilateral triangle for example,
L=2x10^-7ln[D/(re^-1/4)] H/m

where D is the separation in meters and r is the radius of the condcutors

Stevenson, "Elements of Power System Analysis" is the canonical text on the subject.

 

[jbartos]

Suggestion: See Table 9 "AC Resistance and Reactance for 600V Cable, 3-phase, 60Hz, 75°C (167°F) - Three Single Conductors in Conduit
in NFPA 70 National Electrical Code.
The resistance tends to be higher than reactance up to about 250kcmil. Past that, the reactance is higher than resistance.

 

[busbar]

Typically for 600V circuits in all but the smallest conductors, it may be worth noting that circuit reactances are greater with ferrous raceways compared to nonferrous and nonmetallic raceways. [NEC Ch 9 table 9]

 

[jghrist]

Resistance is inversely proportional to cross-sectional area, while reactance is related more to conductor spacing and doesn't vary much with the conductor size. For small conductors, the resistance is usually higher than the reactance, but for large conductors, the reactance is usually higher.