In this paper, characteristics of ignition voltage and current by wire length of HID(High Intensity Discharge) lamp circuit are analyzed. In the construction field, decision of the wire thickness for HID lamp circuit has the problems, and these are presented. Through transmit parameters, equivalent model of the ballast and HID lamp circuit was derived. The graph of voltage reduction about length between the ballast and the lamp is shown. The simulation of proposed model and experimental results are presented in order to validate the proposed method. The proposed model and graph can use to choose the proper length of wire between the ballast and the lamp in the field.