I have ordered a 24" light bar and 2- 4" windshield lights with out wiring kits. I have all the wiring I need and several rocker switches is why I ordered without, that and it was cheaper. My question is, are the relays necessary, or can I just run an inline 30a fuse for all lighting? Any help will be greatly appreciated.

 

Consider the physical size of the wires and switches needed for each circuit. A relay box can let you use smaller wiring into the cab, and smaller switches that are more easily mounted. But you'll have to consider each circuit independently.