Flow headers are commonly used in nuclear reactors, boilers and heat exchangers to distribute fluid to branches or to combine flow from the branches along the header. In CANDU reactors the main heat transport system divides the flow from the pumps into approximately 120 individual feeder pipes which then direct the flow into separate fuel channels. Historically, nuclear safety analysis has been performed using one-dimensional averaged system codes, and as such the headers are cross-sectionally averaged. In this paper, flow distribution and pressure gradients along a multi-branch header have been predicted using the three dimensional computational fluid dynamics software FLUENT and were compared to results obtained from experimental data obtained from literature for single phase conditions. In order to assess FLUENTs capabilities this study was performed by comparing the predictions against separate effects experiments conducted on a smaller sized header available in literature. For these experiments, water inlet flow rate was varied and flow rates in the header branches were measured. The aim of this work is to validate FLUENT software for predicting flow distribution and pressure gradients in single phase flow in such a multi-branch geometry. The effects of flow model, grid density, convergence criteria, flow inlet velocity and header size on the computational results were studied. Vortex formation and flow separation were also studied and compared to the experimentally observed flow behaviour.