I am going to be putting in some fog lights, and I bought a 30A switch for them. Do I need a relay with a switch that can handle so much current? I mean the relay is 30A/40A, so it's not like it can handle more current. The two fogs together draw roughly 7.5A.

 

In this case a relay would be better and easier

You can run a fused(10amp) 12v wire from battery to relay(pin 30) in engine bay, 16 or 18guage wire, same size wire from relay(pin 87) to fog lights, also jump this 12v wire from pin 30 to relay's Coil, pin 85 or 86

Then run a smaller wire or 18ga from relay's coil, 85 or 86 which ever you didn't use for 12v jumper, to a switch in the cab, switch is also grounded in cab, so switch GROUNDS the relay activating it and the fog lights

The above is what car makers do to avoid running 12volt wires all over, making the potential for shorts to go up with each 12v wire added

If the long ground wire shorts worst that happens is fog light come on, lol

You can use the 30amp switch as the ground switch, doesn't need to be 30amp but a switch is a switch as far as 1/2amp relay is concerned
Relays draw less that 1/2amp to hold them closed(on)

12v 30amp relay means the LOAD pins, 30 and 87, are big enough to carry/pass 30amps


Running two wires into the cab to a 30amp switch, 12v in and 12v out is doable just bigger risk of shorts and more work

 

I must say, running a relay doesn't sound easier than just using a switch. But, I will take your advice and use a relay. I don't know if I'm going to wire it up the way you suggested, but I will use a relay. Thanks, RonD--no wonder you have 13,000 + posts; you're a response machine. Quite helpful.

 

Think of a relay as a switch with a remote control

pins 30 and 87 are the switch, 12v in 12v out

85 and 86 is the remote control, 12v and ground

The more you use relays the more you will like them, especially the one wire part to activate them in vehicles

 

I think I understand well enough what a switch is. I thought it's main point was to allow for devices to draw more current than another control device allows--like a switch. In my case, my switch allows for 30A, so (based on my main understanding of a relay), it can handle the current draw of my lights directly. I could run a fused wire from the battery to the switch and out to the fog lights, without an increased risk of a fire, right? I'd love to read more about this and have a firmer understanding of what I'm working with.

 

Yes

You want a fuse as close to the power source(battery) as practical, this protects the long wire runs

I would also fuse the wire going out to the fog lights, at the switch, the power source when its ON
In theory the fuse at the battery would blow with a short on the wire to the fog lights, but...............that wire and the switch could heat up pretty fast before it blows

I would use 15amp at battery and 10amp at switch, if fog lights are 7.5amp load together, assuming 16 or 18ga wire and 10-foot or less wire runs


Purpose of a relay is to keep higher amp wiring as short and as few as practical
Also cheaper in the long run since you can use smaller gauge wires to operate the relays, which is why car makers use so many relays

 

Okay, I thought the switch would be enough (safely). What would you recommend in my case--switch or relay? I am going to run 14ga wire, just for overkill. The charts say 16ga would be enough, but a little more can't hurt, right? I'm guessing the longest wire run will be in the 13ft range. That's a guess though; it's probably less.