Poor Man's Relay Box

Introduction:
First off, this thread isn't intended to knock on any of the other products on the market. This is simply a challenge to myself to make a great product at an affordable price. Now I would like to share my end product with you for constructive criticism and how I could better the product for future revisions.

My Goals:
My primary goal is to organize the wiring for all my current and future accessories. At the same time I wanted to maintain proper wiring standards, make a reliable products, and a cataloged system for easy maintenance and troubleshooting. All while keeping the expenses under $100.

Purchases:
Relay Box w/ Connectors = $11.90
5 Connector DualBus 100A = $17.99
5x 30A SPDT Relays = $9.69
40A RTT7121A Relay = $3.94
ATC Blade Fuses (size based on your accessory)
8 AWG Wiring (Red and Black ~4')
12 AWG Wiring (Red ~10')
18 AWG Wiring (Black ~4')
18 AWG Wiring (6 assorted colors ~10')
Ring Terminal Crimp Connectors for 10 AWG and 4 AWG Wiring.

The Build:
I started with the Red 10 AWG and made jumpers from the output of the relay sockets (pin 87 on the relay) to one side of the fuse socket. For those who don't know, you always want each circuit to be fused. The most important role of the fuse is to protect the wiring itself from too much current draw. Too much current through an improper sized wire can cause catastrophic results. Be mindful of this, you want to use a big enough wire to handle the amperage but not so big that it becomes impossible to work with. I figured most accessories are under 25A so 12 AWG wiring will safely carry that current up to 20'. For your own personal reference in designing electrical circuits, I've attached the U.S. Coast Guard Wiring Diagram. I strongly advise following this standard.




My next step was to take my 18 AWG assorted colors wiring and connect them to the signal input (pin 86 on the relay) and run the wires out of the box. These will connect to my switches inside the cab. Be sure to document which color wire goes to which relay. I also made 1' jumpers of my red 12 AWG wiring from the output of the fuse socket to outside the box. These wires will connect to your accessories.



Setting the box aside for a bit. Lets work with the DualBus. Using the 12 AWG red & 18 AWG Black wiring I made jumpers from the dual bus and soldered on the proper connectors. Make sure to use as little wiring as possible because there isn't much room once the box is closed up. The idea of the Bus bar is to distribute positive and negative to the relays from the Battery. I also used the ring terminals to connect my 8 AWG wiring, this will go to the battery.

Connect the red wires from the bus bar to pin 30 of the relay sockets and the black wires to pin 85 of the relay sockets. Make sure everything is how you like it, once you start this step it will be very difficult to disassemble.


Close up the bottom portion of the box and you should have 3 pigtails coming out of the box, the pair that connects to the Battery for main power, the 6 output wires that will connect to your accessories, and your 6 signal wires that will connect to your switches inside the cab. Again make sure everything is well documented, I used a label maker to mark everything.


**Side Note: I used some fiber loom on the 3 pigtails to better organize the wires.

Mount the box inside your engine bay, I used the factory location of where the clutch resevoir would be. Run your wires where they need to go. I bought some CH4x4 switches and the cubby replacement panel sold (here) on this forum to control the circuits I also made a simple diagram using photoshop and glued it to the underside of the lid to help me keep track of which relay is connected to which fuse.



So that's all I've got, let me know what you think. What would you have done differently? Hopefully I inspired someone who was on the edge about doing something like this and showed them that it really isn't too bad.

 

@Dalandser

 

Thanks @Speedytech7 - this is the info I've hoped to come across all in one place like this! Awesome writeup OP! Thanks for the info - clean work!

 

Thank you. Hope this is helpful

 

***UPDATE***

I've started work on making a second one very similar to this that I plan to sell. But I'm adding a 100A inline fuse to the 8 AWG wiring that hooks to the battery. That will prevent damage to the relay box from too much current for those that don't take the time to calculate how much amperage their accessories will consume. I'm also applying a braided fiber loom to the signal wires as well as the battery wires. This will clean it up a little bit and also protect the wiring from any jagged surfaces and vibrations.

 

Fuse or circuit breaker? I put a 100A breaker inline on my box.

 

Nice build! Love the bracket! I was looking at a breaker initially but I don't have enough experience with them and I was concerned that over time the breaker would flip for no reason at all or how easily serviced they are. I was also concerned about how water resistant breakers are. In the end I went with an inline fuse.

 

Thanks!

I went with the Bussmann brand circuit breaker because they're weather sealed 100% but yeah I imagine like every circuit breaker it'll eventually go out, but they estimate something like 500 trips before it starts false tripping.

 

what box did you go with? Is that Bussmann as well? I'm curious how you distributed power from the battery. I had a hell of a time trying to figure out how I was going to do that myself until i stumbled across that Blue Seas DualBus

 

It is a Bussmann box, they have an internal positive rail that runs to the fuses and from the fuses you distribute to your outputs or further on to the relays then outputs.

 

No Kidding!? I'm going to have to pick one of those up and check it out for myself. Sounds much more simple than my method. So are circuits fused before the relay? I always thought it was best to fuse after the relay.

 

They're fused before if I'm not mistaken. I haven't heard if before or after the relay makes a difference, basically just a remote control switch ya know

 

I'm curious as to why you chose 8awg as your main power lead? Seems skimpy in comparison to your fuse sizes/device draw.

 

The relay box is only 4' from the battery so I didn't really need a super heavy wire. According to the WireBarn Calculator at 13.8v 100A you will only experience 2% loss at 4.39' with 8 AWG wire, which is well within tolerance of almost any DC powered accessory. I was also concerned about space. There wasn't a whole lot of space inside the box between the jumpers and the DualBus. If I had used 4 AWG, I'm afraid I would have had a really hard time fitting everything.

 

But even going off the Coast Guard chart/same chart Blue Sea consults, that you included in your OP, an 8awg wire is good for 40 amps at short length (and variances in this number by different manufactures...most end up rating between 40-50A dependent on temps,etc.). IF you are running 2x 20amp accessories for example, you've then already hit the limit on what the power lead is rated at. If you are running 2x accessories simultaneously at 20A and 35A for example, your running a 55A draw (potentially), on a 40A rated wire. OR am I just missing how you wired this?

Also, it has been my understanding in regards to fusing and relays, you want your fuse between the power source and the relay. OR at least that is how Bussman panels get wired up. It is power source > internal power buss > fuse > relay, inside of the Bussman panels.

 

I see what your saying and I thank you for your contribution. However I have seen other souces that say an 8 AWG wire can safely handle 100A up to 4' with very little loss. I know the Coast Guard wiring standard didn't include that figure and I'm not sure why not but I am confident 8 AWG would handle the load.

As for the fuse location, I think your right, just did a little research and there are plenty of other articles that back up your claim. That will have to be a change I make for my 2nd box. Thank you for your contribution.

 

Fuses before relay! You always want th fuse as close to the battery as possible

 

That's also how the stock fuse/relay box is wired on our trucks

 

I would do both. Breaker protects your bus and fuses from damage and you can just reset it (if it ever gets tripped) instead of having to replace burned fuses

 

Thanks for the info! I've already started making revisions to my 2nd box based on this info.