This paper presents the Cost Model of Wiring Harness and Tubing Assembly. Specifically, the cost estimating relationships (CERs) developed for this model address: Installation of wire harnesses and cable assemblies. Installation of tubing assemblies, based on three different tubing applications: hydraulic systems, fuel systems, and environmental control systems. The information presented in this paper includes: Methodology for developing appropriate CERs. Specifically, the methodology included identification of requirements, data gathering, and the development and validation of the CERs. Basic mathematical methods and tools required to develop the CERs. Least squares data fitting was accomplished by implementing the singular value decomposition method and the method of Gaussian elimination. CERs developed based on linear logarithmic model. CER performance was evaluated based on constants and variables identified by the responsible manufacturing organizations. Typical variables are related to (1) the set-up time, (2) the number of cables/tubes, (3) the total length, (4) the number of associated documents, and (5) the number of pieces of hardware in the installation kit. Performance statistics for each CER.