Natural convection heat transfer is experimentally investigated in an enclosed horizontal rod bundle, which characterizes a spent-fuel assembly during transport and some dry storage scenarios. The objective of this experimental study is to obtain convection correlations that can be used to easily incorporate convective effects into analytical models of horizontal spent-fuel systems and also to investigate the physical nature of natural convection in enclosed horizontal rod bundles in general. The resulting data consist of correlations of convective Nusselt number, which are defined in terms of the maximum and average assembly temperatures. The correlations have been corrected for radiation heat transfer using a numerical technique. The data suggest the presence of conduction and convection regimes, distinguished by a critical Rayleigh number. The correlation of the convection regime suggests turbulent flow conditions. Predictions of maximum assembly temperature using the presented correlations are compared with additional experimental data obtained in a horizontal enclosed rod bundle. Further comparisons are made with predictions from the widely used Wooten-Epstein equation and a recently developed theoretical approach based on an effective thermal conductivity model. Favorable results are obtained, especially for thermal conditions that favor natural convection, such as relatively low enclosure temperatures and abovestandard atmospheric pressure.