Common mode current on cable harnesses is often responsible for system level EMI, but is difficult to predict early in the design process. Typical component level measurements are unhelpful as common mode currents may change dramatically when the component is placed in the system. A methodology is developed in the following paper to estimate common mode current in system-level applications based on component-level measurements. Estimates are made by assuming that termination impedances can be approximated as shorts or opens with respect to the characteristic impedance of the harness. Furthermore, the dominate sources of common mode current for each set of terminations (open or short) should have similar source impedances. With these assumptions, the common mode current on the harness can be approximated from the sum of currents on two equivalent circuits terminated with an open and a short. The source characteristics of each equivalent circuit can be determined from two measurements of common mode current on harnesses of two different lengths. The equivalent circuits can be used to estimate common mode currents on harnesses of arbitrary length and characteristic impedance.