What is the point of wiring relays in when using an electric fuel pump? Or relays on nitrous switches, etc?

Not sure if this helps but explains relay details

http://www.bcae1.com/relays.htm

Both right. A relay is simply a "remote" switch. Both a switch and a relay ,properly sized, can handle a high amperage load.

Think of it like a starter circuit. It would be tuff to run battery cable in the car to the ign switch and have the ign switch carry the starter load so the starter solenoind acts like the relay in carrying the heavy load circuit to the starter and the ign switch just needs 16 gauge wire to control the winding in the starter solenoid. But all the answers are right in their own way. Ron

I am a very big advocate of relays....

Most Bosche style relays used for fuel pumps, fans and nitrous sold by mail order are on good for 30 amps. Most any aftermarket switch panels uses a minimum of 30 amp switches. ARC solders the switch to the circuit board which helps dissipate heat and therefore they advertise 40 amps. Most any of these 30 amp relays if used for 30 amps continuous will fail. If you know a really good electronic supplier you can get a heavy duty relay of this design rated 40 amps.

A constant duty starter solenoid used by some is a better switch but they are not cheap. A little bosche relay more often than not will fail before a good switch if the same current and voltage is demanded through it.

The reason you want to run relays is just like one of the guys said. If you run wire to your 12Volt bus or starter relay from the battery in trunk that is roughly 15 ft. Then to the switch with small wire minimum 5 ft. Then to the fan, nitrous or fuel pump at the rear of car. You will have a voltage drop between your battery and the load(Fan pump or whatever). This voltage drop is used up energy(HEAT). If you run the wire to a relay and then simply switch the relay on and off you can ussually shorten the distance of wire from the battery to the load. Less voltage drop and more power to the load. The great thing is you can run smaller lighter wire to the switch panel because most of these bosche relays take less than an amp to operate.

Hope this helps or I can bore you to death with Kirchoffs Voltage Law, which has to do with where your voltage is used up in series and parallel circuits.
Ohms Law, every wannabee EE thinks they know this one.
Watts Law, has to do with power, more voltage more current more watts.
And many others that I have tried so hard to forget and fill my brain with gasoline and steel instead.

Leon