For standard conductors generally the values for GMR are available. These tables are also useful for calculating inductive reactance as well as shunt capacitive reactance and resistance. The units used in these tables are inches, feet and miles as used by the industries in the united states.
Normally value of inductive reactance is required rather than the inductance. The inductive reactance of one conductor of a single phase two conductor is given by,
The GMR found from the tables is an equivalent D which takes into account skin effect which affects the value of inductance appreciably.
Expanding the above equation we have,
Considering both Dm and Ds to be expressed in feets, the first term in the equation is the inductive reactance of one conductor of a two conductor line having a distance of 1ft between them. Hence the first term is called inductive reactance at 1ft spacing Xa. It depends upon the GMR of the conductor and frequency. The second term is called inductive reactance spacing factor Xd. This term is independent of type of conductor and depends only on frequency and spacing.
When Dm = 1, the spacing factor is equal to zero. When it is less than 1, the spacing factor value is negative. The inductive reactance at 1ft spacing value is available from the table. To that value, the value of inductive reactance spacing factor is added which is also available at desired line frequency.
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