Norton's theorem for electrical networks states that any collection of voltage sources, current sources, and
resistors with two terminals is electrically equivalent to an ideal current source, I, in parallel with a single
resistor, R. For singlefrequency AC systems the theorem can also be applied to general impedances, not just
resistors. The Norton equivalent is used to represent any network of linear sources and impedances, at a given
frequency. The circuit consists of an ideal current source in parallel with an ideal impedance (or resistor for
nonreactive circuits).
 Wikipedia
The Norton Equivalent of a circuit consists of a
Norton current source in parallel with a Norton resistor and is valid for any load. In AC circuits a Norton
equivalent circuit is valid for a single frequency.
The Norton current is the shortcircuit current at the
output  the same as what is calculated for the Thévénin shortcircuit current (see Thévénin Equivalent page).
The Norton resistance is the same as the Thévénin resistance.
Original Circuit 

Norton Equivalent Circuit 

