Perko 8503DP Medium Duty Battery Selector Switch w/Alternator Field Disconnect w/o Key Lock [8503DP]Medium Duty Battery Selector Switches Diameter: 5 1 4" Depth: 2 5 8" Polycarbonate Body with Brass Terminals Ignition Protected For use with 6 to 32 Volt Marine Electrical Systems Capacity 250 AMPS Continuous, 360 AMPS 5 Mins. Intermittent Terminal Stud Size: 5 16" When used with Non Unitized Alternators, these models will interrupt the field current (reducing the possibility of damage) if turned "OFF" with the engine runnin Specifications: AFD: Yes