What all needs a relay?

I am in the process of building a 2003 Mazdaspeed Protege. I am going to be making a completely new harness, engine and body. I am just trying to figure out exactly what i will need a relay for?

I'm sure Zac has done a feature, or two, on this.

One of the first steps is to have a good think of all the circuits you are planning to use - for example, a pure race car may need a lot less than a road car - what the current you expect those circuits to be carrying and how you are planning to switch them.

Going to use a power distribution block/module/unit that uses electronics to switch functions or individual switches, etc?

With the former, different circuits can handle different power levels with some capable of high continuous current levels which removes the fuse and relay requirements.

If you are going 'old school'with individual switches and relays, the switches will have a current rating which are supposed to be good for continuous use but as I have had some burn out/overheat I generally rate them at half that value, even the good quality ones you should be using, and use relays. Even with relays, I generally try and use one rated at twice the current I expect to use - splitting up switched circuits if required. Do not forget to use a decent guage wire for ALL of the relay'd circuit as voltage drop may be an issue - eg, lights. If you do go this route, check out the combined fuse/relay boxes that are commercially available - neater and a great time saver.

For illustration, not necessarily recommended as there are many options out there - note, however, some control panels need a separate distribution device, some have it integrated with the panel.

https://www.demon-tweeks.com/nz/cartek-power-distribution-panel-ctkck-pdp-16/

http://www.pkcgroup.com/de/wiring-systems/komponenten/stromverteilungszentren.html

I have already picked up a fuse box for the body harness and i have a haltech e6x to control the engine and it came with the fuses it will need as well as the relays. I plan on running everything off of switches headlights/taillights, turn signals, hazards, cooling fan, and the ignition.

I went through the forum and couldn't find anything.

Thanks for the input.

G'day Jacob

Gord's answer is brilliant, and exactly what I would have said also. You need to look at the current draw of the device you're switching, to determine if it will require a relay. I'm actually planning a members webinar on body harness and electrical design in the next few weeks, as we have been getting quite a few questions on it.

Thanks again for the input. I'll be keeping my eyes open for the webinar. I will be going back to the wiring fundamentals to re watch the video that shows how to calculate current draw and go from there.