The internal 12V-2x6 cable supplied by Seasonic is designed for ATX 3.0 power supplies with native 12V-2x6 connectors. The Corsair HX1500i, although a high-end power supply, is not one of the Corsair models listed as having a native 12V-2x6 connector, as this type of connector is mainly found on certain recent specific Corsair power supplies.

In addition, Seasonic states that their native 12V-2x6 cables are only compatible with their own ATX 3.0 power supplies and are not suitable for non-ATX 3.0 models, which is the case with most mainstream Corsair power supplies, probably including the HX1500i.

In short, this Seasonic 12V-2x6 cable is not compatible with the Corsair HX1500i. For this power supply, we recommend using the cables supplied by Corsair or those explicitly compatible with this model.