Void fraction covariance has been introduced into the interfacial drag calculation used to close the one-dimensional two-fluid model. A model for void fraction covariance has been developed for large diameter pipes. The newly developed model has been compared with two previously developed models in terms of void fraction prediction accuracy. The effects of these additions on the void fraction prediction uncertainty have been evaluated utilizing a computational tool developed in MATLAB. The results indicate that there are small differences in the void fraction prediction between the models evaluated and the two-fluid model without void fraction covariance. Higher void fractions above 0.7 show the most significant changes. However, the differences in the uncertainty are not significant when compared to the uncertainty in the data used for the comparison. The results highlight a need for additional data for higher void fractions, collected with steam-water systems in large diameter pipes.