Depends how voltage sensitive your components are. For headlights and radiator fans IMO relays are a must as power output drops fast with voltage loss. For most other things it doesn't matter if it gets 11v instead of 13v.I was planning on putting anything with over 5 amps on a relay in my new design. An engineer at my work said I was being ridiculous, and needlessly adding more complexity than necessary.
After looking at factory fuse boxes in my other trucks, it's seems like only relays are used over 20 amps. The original Wagoneer had one relay for the starter. (it also was a rolling fire hazard).
The way I was designing this was going to need 12-14 relays. I think I'm taking this idea of keeping all high amps firewall forward might be a little stupid. I spent a couple hours trying to find a general consensus on when to use a relay, but couldn't find a good answer. Too many variables. What's the switch rated for? Is it a high amp spike start, then low amp run circuit? How far is the run?
Bearing in mind that I'm using all the 1972 original switches on the dash, what is a general, or rule of thumb amp load to be considered safe without employing a relay?
One of my british vehicles had the headlights running from about 4m of cable through the fuse-box, steering colum switch and then back to the headlights. Relays made an enormous difference.
Using relays can also mean a bundle of smaller trigger wires to your switches instead of big fat ones. Which can help cable routing a lot. I have a table of wire size and max current that I use: But I can't find it right now!