Reactance In General:

Now you must get a superior idea of why capacitive reactance X_C is considered negative. In logic, it is an expansion of inductive reactance X_L into the realm of negatives, in a manner that usually cannot take place with resistance. Capacitors act such as "negative inductors." Interesting things take place whenever capacitors and inductors are joint.

Reactance can differ from extremely large negative values, through zero, to very large positive values. Engineers and physicists always calculate reactances as imaginary numbers. In mathematical model of impedance, capacitances and inductances manifest themselves perpendicularly to the resistance. Therefore ac reactance occupies a various and independent dimension from dc resistance. The common symbol for reactance is X; this includes both inductive reactance X_L and capacitive reactance X_C.