Will this start a fire?

I don't think it will, but a second opinion would help.

I built a bussmann fuse/relay box and now I'm starting to install it.

I am using OEM style switches for all the accessories with a harness that connects to the bed light switch to provide led power and ground. I only need to connect to the source and the relay.

I had the idea I could connect the 5 18 gauge wires to one 10 gauge wire and run it to the fused side of the block. Since this is only power for the switch will this be a problem? Thanks

 

Isn't the point of the box to avoid unions like this? You have one power lead to the box, which distributes the power from there?

 

Depends on the fuse size. If it is sized for the 18awg it will protect the smallest wire. If it is sized for 10awg, then you could damage the 18. Also, when you make up a connection like this, how good is your joint? There is a reason you don't see this method in OEM harnesses.

 

I dont want to use all 5 fused connections for switches. I have other accessories to plug in.

 

The Red wire to Black wires makes my wiring OCD very intensively painful.

 

The point of a fuse is to protect the wire

Wire is rated for amperage, if too many amps go through it, then it will heat up and could cause a fire.

if you fuse that run for the 10g wire, then it is over fused for the 18g wire, and if there is a short on the 18g wire then it could cause a fire before the fuse blows

if you are using the 10g wire to lower resistance and limit voltage drop, and fuse the run for 18g, then it should be safe

 

The red wire looks like it can handle a larger amp load than the black wires, as long as that is correct...
It is fine as long as the total amp load from all of the black wires does not exceed the amp rating of the red wire. Also do not exceed the amp rating of the black wire on the circuit it connects to.

 

This chart is helpful if you need to figure out what wire size to use for an amp load and wire length.

https://www.bluesea.com/resources/1437

 

I think this is just how he wants to power his switches (lights, maybe relay signal?) and then the source power for the loads/devices will go through the fused box.

 

At most I might have 3 switches turned on at a time

 

It will work, depending on the total load that 10ga is suppling. I would have used a terminal bus bar to distribute those. Would this help? There are some smaller and less expensive ones that you can probably find to attach to the side of your relay block.

Blue Sea Systems 2300 150 Amp Common BusBar with 10 screws and a cover

$24 Blue Sea Systems 2300 150 Amp Common BusBar with 10 screws and a cover

https://www.amazon.com/dp/B0091VHLW4

 

Can you draw up what you are trying to do and where each of the components sits. I tried typing up an answer but there are just too many unknowns for me to weigh in.

 

Kinda. I'd argue it's a cost/manufacturing thing. The highest quality custom harnessing we build for race cars uses this splice technique all the time to branch out leads. Granted we do it with the best of parts, but it is proper practice once everything is designed and assembled properly.

I agree here, if your source fuse is sized for an 18 gauge wire and your total normal draw is within that fuse's bound you should be good.

 

The calculation you want to use is the amp load, not really the number of switches. You can have 100 switches as long as the total amp load does not exceed the capacity of the wire.

But also what kmorgan3 states in the post right after this.

 

This is common practice in race car (Formula) harness construction. It is a lightweight way of doing weak power or ground distribution. You are violating the allotted circular mil area of that type of splice when you use 5 18 awg wires, though. Fuse your feed to something small like 2.5A and use 20 awg. Then use a better connection like an f-style crimp:
https://www.corsa-technic.com/item.php?item_id=1018&category_id=64

Calculate your CMA assuming 20 awg mating to a single 10.

 

My dudes, these are relay switch leads, these aren’t drawing that much current. OP just do what everyone said, check out your switch amp ratings, fuse for the weakest collective link in the chain.

Also fyi for everyone else, the Bussman rtmr’s come in different configs, they don’t all have 2 input studs. Idk which one OP has.

 

I came here to add fuel to the fire and mayhem of a controversial topic...

(- my take on the thread header)

I am disappointed that is it just an electrical issue...

 

I don't know if this helps

 

Just so I’m on the same page. You are running the red lead from your Bussman as power for your switches and then switch output goes to your relays?

If so then that’s fine. I ran the same setup when I had previous Bussman panels. Get whatever low amp fuse you can find for the switch power circuit, I believe 2-3amps is the smallest mini fuse.

 

Edit: I could be missing a lot b/c I am thinking about standard automotive relays not a bussman. Please keep that in mind in my response.

Your switches should trigger the relay to latch (that can be a positive or negative trigger - I only use negative triggers but some people do positive // all good whatevs). I struggle with your drawing because it doesn't show the relay powering the harness to your LEDs. The relay handles the load side and the switch handles the trigger side so the only thing that should be connected to the LEDs is the relay, not your switch. Your LEDs should have power from the relay and then ground to the chassis or whatever.

I have a schematic of my winch and lights in my build thread but that is for a negative trigger so the pins on the relay will not be the same if you are using a positive trigger to latch the relay.