We evaluate the relative impact of three sources of epistemic uncertainty on probabilistic seismic hazard analyses in California: source model uncertainty, ground motion model (GMM) uncertainty, and site parameter uncertainty. Seismic source model uncertainty is inherently contained in the source model framework applied by the USGS in the 2023 National Seismic Hazard Model (NSHM23); we have added tools to extract this uncertainty for California sites in the open-source seismic hazard software OpenSHA. GMM uncertainty is generally accounted for using alternative models in PSHA or a single backbone model with a defined uncertainty. Site parameter uncertainty refers to uncertainty in the shear wave velocity of the upper 30 meters of the site profile (VS30) and potentially other independent site parameters. We demonstrate the impacts of these major sources of epistemic uncertainty at the sites of two UC campuses, Berkeley, which is located near the active Hayward fault, and Davis, which is located in the relatively quiescent Central Valley. We investigate potential correlations between the different sources of uncertainty and find that source uncertainty is practically independent of GMM and VS30 uncertainty at Berkeley but dependent on GMM and VS30 at Davis. At both locations, GMM and site parameter uncertainty are correlated (i.e., inter-dependent). We represent epistemic uncertainty in ground motion with a period-dependent log-normal standard deviation term that is specific to a given site location, site condition, and exceedance frequency. We show that at Berkeley, the total epistemic uncertainty can be well approximated by the square root sum of squares (SRSS) of source uncertainty (i.e., uncertainty in ground motions related solely to the source model) and the combined GMM and site parameter uncertainty. We find that combined GMM and VS30 uncertainty is comparable to or greater than the source uncertainty at many oscillator periods at both sites. Combined uncertainties range from natural log standard deviations of about 0.2 at short periods to 0.6 at Berkeley and 0.3-0.7 at Davis at long periods.