Inductive Reactance

An inductor is simply a coil of wire. When current passes through the coil, an electrical field is generated. The field has been "induced." The bigger the coil or the greater the number of turns, the greater the induced field. This phenomenon is called "inductance."

Inductive reactance is the name given to the opposition to a changing current flow. This impedance is measured in ohms, just like resistance.

In inductors, voltage leads current by 90 degrees.

The formula for calculating the inductive reactance of a coil is:

inductive reactance, or XL, is the product of 2 times p (pi), or 6.28, the frequency of the ac current, in hertz, and the inductance of the coil, in henries.

XL =2p x f x L.


XL = inductive reactance measured in ohms
2 = a constant (2 x 3.1416 = 6.28)
f = the AC frequency of the electrical supply in hertz
L = the inductance value of the coil in henries.

A coil with an inductance of 0.3 henries is connected to a 120 volt, 60 hertz AC circuit. To determine the current flow in the wire, first find the inductive reactance of the coil. The inductive reactance equals 6.28 times 60 hertz times 0.3 henries which equals 113.1 ohms. Now use Ohm's Law and divide 120 volts by 113.1 ohms which equals 1.06 amps. Remember the current will lag the voltage by 90 degrees so the current flow is 90 degrees behind the voltage sine wave.