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In the practical design of a transmission cable, the characteristic impedance and the number of twists per meter in twisted cables are variables that are dependent on imperative conditions, with the disadvantage that these variables are strongly depending to each other. Cable manufacturing companies aim to provide customers' products with very well known physical characteristics. For this reason, a methodology to achieve accurate design of twisted pair cables based on the geometry of wires, before going to the manufacturing process is very desirable. In a previous paper, the deformations and irregularities that can occur in the twisted pair during the manufacturing process and their effects on the geometry of the twisted pair were studied by the authors. This paper describes an improved model for estimating the characteristic impedance of a twisted pair that allows an accurate electrical design. Two methods for a fast design of a cable, in agreement with the demanded specifications (characteristic impedance or number of twists per meter) are developed. Furthermore, a software program that automates all necessary calculations has been made. Thanks to the improved model, the design and production of a cable can be done faster, without the need of a big set of production samples. Results showed in this paper are a part of a larger research project carried out in association with the research and development department of a cable manufacturing company, and these results are being applied into the design department of this company.