The Hubbell HBL105 marine adapter connects a boat or appliance with a standard 15A or 20A straight-blade plug to a 30A, 125V locking dockside receptacle. It is a yellow, moulded, watertight adapter: a 30A locking male plug on one end, a 15A female straight-blade connector on the other.
For extra protection, Hubbell also makes a ground-fault version (HBL105GF).
