Common-mode current on cable harnesses is often the primary source of radiated emissions below several hundred megahertz. It is difficult, however, to estimate common-mode currents that will occur in a system from component level tests because the common-mode impedances are different in the system. Here, a method is proposed to use simple component level tests to estimate equivalent common-mode source voltages and source impedances responsible for generating common-mode current. This source information can be used to predict the common-mode currents on harnesses of different lengths or characteristic impedance. The common-mode load impedances are assumed to be approximately open or short compared to the characteristic impedance of the harness. Equivalent sources are predicted for those circuits terminated with an open and those terminated with a short. Source characteristics are found from the magnitude of the common-mode current measured at multiple locations along the harness while varying the component height above the return plane (i.e., by varying the common-mode source impedance). The proposed method was used to predict the common-mode current on multi-wire harnesses of different lengths. The common-mode current was predicted within a few decibels except at frequencies where the harness was resonant during source estimation.