Impedance is the total opposition to the flow of current in an AC circuit. A DC circuit is only opposed by the resistance of a circuit (wire and and other device in the ckt) an AC circuit is opposed by the resistance of the wire, coupled with the capacitive reactance (two wires, carrying current = capacitance) and the inductive reactance of the ckt (such as the pipe the wires are in, bx skin, bldg. steel, etc.). you need to be concerned with it in all ckts, and design that way. Reluctance is alas an OLD electronics term I probably should not have used, the proper more up to date term is reactance. Sorry, geezerism strikes on occasion [Linked Image]