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Determining reliable cable ampacities for marine High Voltage Cables is currently the subject of significant industry and academic reassessment in order to optimize (maximizing load while maintaining safe operating temperatures) design and reduce costs. Ampacity models can be elaborate, and inaccuracies are increasingly predicated on the uncertainty in environmental inputs. A stark example is the role of ambient temperature at cable depth, which, due to the scale of cables and the inaccessibility of the seafloor, is commonly estimated at 15 °C. Oceanographic models incorporating ocean bottom temperature are increasingly available, and they achieve coverage and spatiotemporal resolutions for cable applications without the requirement for project specific measurements. Here, a rudimental validation of the AMM15 and AMM7 mean monthly ocean bottom temperature models for the NW European Shelf indicates encouraging accuracies (MBE ≤ 1.48 °C; RMSE ≤ 2.2 °C). A series of cable case studies are used to demonstrate that cable ratings can change between −4.1% and +7.8% relative to ratings based on a common static (15 °C) ambient temperature value. Consideration of such variations can result in both significant ratings (and hence capital expenditure and operating costs) gains and/or the avoidance of cable overheating. Consequently, validated modelled ocean bottom temperatures are deemed sufficiently accurate, providing incomparable coverage and spatiotemporal resolutions of the whole annual temperature signal, thereby facilitating much more robust ambient temperatures and drastically improving ampacity estimates. [ABSTRACT FROM AUTHOR]