Transmission lines with rectangular cross sections are usually used in integrated circuit (IC) and computer chip problems. In this paper, a full-wave method is proposed based on an efficient wire model to analyze transmission-line circuits, where the cross sections of wires can be arbitrary. Comparing the existing wire models in the method of moments, it has been shown that the best performance occurs when the current is assumed to flow along the electrical axis of a wire and the testing is on the whole surface if two wires are very close. The physical significance of such modeling implies that the surface current on a wire is equivalent to a current filament along the electrical axis. For a single round wire, the electrical axis is exactly the same as its geometrical axis. For two parallel round wires, the electrical axis of each wire is located at the image position of the other wire. In this paper, a general wire model is proposed to determine electrical axes of wires with arbitrary cross sections by solving a static problem. Then, full-wave formulations are derived for wire structures with rectangular cross sections, which are the most important for IC and computer-chip problems. Numerical simulations are given to test the validity and accuracy of the proposed method.