An important parameter involved in characterizing the electrical properties of conductive fibrous composites is the percolation threshold. In this study, a 3D percolation model was developed and used to predict the critical fiber volume fractions corresponding to the percolation zone in carbon fiber (CF)-reinforced cement-based materials, generally used as sensors. This model can eliminate or reduce the necessity for performing meticulous experiments to estimate the percolation threshold. Results from the model were found to be comparable with experimental and theoretical predictions in the literature. Established trends with respect to fiber aspect ratios were also confirmed. The effect of fiber breakage was found to be particularly important due to the brittle nature of CFs; an effective length should, therefore, be used in predicting the percolation threshold.