The paper describes a modeling tool for the design of a new air-cored current transformer called PRiME (planar Rogowski current measurement) designed on a printed-circuit board (PCB) with the coil made of the PCB printed copper layers. The PRiME technology discussed in this paper exhibits a very original way of getting an efficient external field rejection, allowing the design of air-cored transducers with remarkable overall performances. A theoretical model of the PRiME transducer is presented. The model permits to perform systematic optimization of the design parameters, allowing to tune any of them and analyze their impact on the overall PRiME performances. The model is based on the Neumann's formula which allows a straightforward representation of complex coils. The model and the simulation tool are validated versus experimental measurements carried out for different sensor configurations