Due to the rapid surge in operating frequencies and complexity of modern VLSI designs, fast and accurate EMC/EMI analysis is becoming mandatory. This paper presents a closed-form SPICE macromodel for fast transient analysis of lossy multiconductor transmission lines in the presence of incident electromagnetic fields. In the proposed algorithm, the equivalent sources due to incident fields are formulated analytically in the time-domain, based on the information of line and incident field parameters. The resulting time-domain macromodel is in the form of ordinary differential equations and can be easily included in existing analog circuit simulators such as SPICE. The proposed algorithm, while guaranteeing the stability of the transient simulation by employing passive macromodels, provides significant speed-up for high-speed interconnect networks, especially with large delay and low losses.