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Summary form only given. Many modern pulsed power generators use magnetically insulated transmission lines (MITL) to couple power between the driver and the load. In an MITL the electric field stress on the cathode exceeds the vacuum explosive-emission threshold and electron emission occurs. For sufficiently high current, emitted electrons are magnetically insulated from crossing the anode-cathode gap and flow axially downstream in the direction of power flow. The return current from the total anode current Ia is divided between current Ic flowing in the cathode and current flowing in vacuum electron flow, i.e., Ia -Ic. As a result of the electron flow in vacuum between the electrodes, the impedance of the MITL is altered and, thus, the power flow coupling between the machine and the load changes. The effective impedance is best described by the flow impedance Zf. In a dynamic system where the voltage and currents are changing in time, Zf also varies. In this work a model for dynamic flow impedance is developed for incorporation into a transmission line code (TLC). The model describes both self-limited flow as the pulse initially propagates down the MITL toward the load, as well as, the subsequent electron power flow along the MITL after the pulse encounters the load. Additionally, for low impedance loads, this description must include electron retrapping as the flow is modified by the wave reflection off the load and the percentage of the return current in vacuum electron flow decreases. The objective of this work is to efficiently and accurately simulate power flow in systems with a MITL using a simple TLC rather than a more computer intensive particle-in-cell code. Available results will be presented.