FYI, relay contacts are usually limited to 10A and sometimes as high as 20A. Above that current rating the devices are usually called contactors. Normally closed contacts are not usually available above 100A.
For 300A non-life-safety semi-automatic transfer switches some fire truck manufactures use mechanically interlocked 400A contactors, one coil wired to the on-board generator and the other coil wired to shore power.
For home-built applications stick with a listed commercial transfer switch especially if you need engine start and exercise functions.
