Building a custom relay box

Came back to Sweden a few weeks ago and have finally cough up with everything.
This “custom relay box” project has been planned for some time, and I finally had the time to complete it.

My Jeep have lots of aux lighting, four in the front and two in the rear and two additional parking lights. I already have everything on relays. Under the hood I have four relays, the two front LED lights are on one relay, the two xenon light are on one relay, my control panel has one and one if for the automatic high beam (my front LED and xenon turns on with my high beams when activated). In the rear of my Jeep I have one relay mounted for the reverse lights.
My new relay box will house five relays and are IP66 rated and will control the front LED, control panel, reverse lights and automatic high beam. The xenons are left alone for now. That will leave me with one relay left for future needs. The new box also has space for 10 fuses.

What’s needed

Wires, connectors, plugs, clipper, box
What’s needed
  • Empty relay box, Bussmann 15303-5-2-4
  • Cable
    • 6mm²
    • 2,5mm²
    • 1,5mm²
  • ISO 280 connectors and plugs
  • Braided cable sleeving
  • Fuses
  • Micro relays
  • Cable shoes
  • Cable tool

The relay box

The relay box is a Bussmann 15303-5-2-4 with a busbar on the fuse side and rated at max 100 amps. The box is available with a separate ground busbar, but it did not suit my needs. Bussmann claims that the box is IP66 rated when using the correct accessories like plugs. A lid is provided that can house even higher than normal relays or micro mount diodes.


ISO 280 connections
ISO 280 connections
There was a lot needed except for the empty relay box, mainly ISO 280 female connectors and plugs. The plugs where available in multiple models. I used two wire plugs and one plug for unused ports. Silicone could replace the plugs, but that would add difficulty to change the wiring at a later time. One problem I had was that Waytekwire used a minimum order quantity on 100 or 50 on some products.
The relays are ISO 280 Micro relays rated ad 35A @ 14VDC, they are 4 connector relays so they do not have a “normally closed”.

Building the relay box

The assembly is not hard, it requires basic knowledge of electricity and normal logical thinking. Make sure you have a clear plan before you start your build. Don’t forget to use the proper size cable, to small cable can be a fire hazard if you try to full too much current through it. I used 6mm² cable for all high current connection and 2,5mm² and 1,5mm² as relay control and relay ground connections. 6mm² will get me roughly 30 amps through 3m of cable safely.

The finished product

After a black wizzard did he’s thing the relay box was finished.

Complete relay box backside
Complete relay box!

The black cables going to the left in the picture is ground wires and one automatic high-beam control. The automatic high-beam works so that one side of the relay coil is controlled by the control panel inside the Jeep, but the other side of the coil is not connected to ground until the high beams are on. This is done by a pull-down connection with the dedicated “automatic high-beam” relay. The “automatic high-beam” relay connects to ground instead of plus. This gives me the same truth table as a logical “AND”. Both sides of the relay coil has to be TRUE for the relay to activate. Depending on how I connect the ground wires I can have 0-4 relays that is high-beam dependent.

All yellow wires are control wires, the one yellow wire going to the left with the ground wires are going to a high-beam wire. The other four yellow control wires are in a braided sleeve and going in to the control panel. The other braided cable is the power for the control panel.

Finished relay box
Finished relay box

Relay 1 is the relay for the aux light control panel.
Relay 2 and 3 general purpose relays and will be used for the front lights and is going to be high-beam dependent.
Relay 4 is for the reverse lights.
Relay 5 is for the automatic high-beams.

Fuse 1-5 are reserved for relays, even tho relay 5 do not need a fuse since it does not provide power.
Fuse 6 are for the constant power to one control panel row.
Fuse 7 are dedicated for the OEM trailer connection.
Fuse 8-10 are currently holders for spare fuses.

Now it’s just to install!
During the install I will make the last connections to the existing wires for the lights and also fuse the OEM trailer connection with the new relay box.

If you want to build your own, check this guy out! He inspired me, even tho my relay box is a bit different.