Below is a diagram I created in preparation for a project I am working on to add additional lights to my vehicle. During my research, I found that it would be best to add a relay box to the engine compartment to keep the high current out of the cabin. From there, I looked at the power needed from the lights I will be installing. Each light cube (there are 4) operates on 10V-30V (obviously I will be using 12v) and contains (8) 3W LEDs. With that in mind, I spec'ed the circuit for 100W @ 12V and with the 1.3x margin for cold power-on I settled on a 10A fuse. Given that I will only need a maximum of around 8 feet of "high amperage" wire, and the 10A fuse, I decided to use 14 Ga wire. The switching circuit will use 22 Ga wire as the requirements are small. The switch will tap a fuse slot that is currently not used for ignition keying. Additionally, the switch has an LED that is not pictured in the diagram. A supplemental diagram from the switch manufacturer is provided below. The switch will be wired on pins 3 and 5.

What I am unsure of here, is the use of the 40A auto relay. I don't do much with relays or auto circuitry and looked for smaller relay sizes but they don't seem very common in the auto world. However, the 40A seems excessive. Will this create any problems? Additionally, is there any component here that I am overlooking/not considering?

Switch: Link to Diagram

Diagram: enter image description here

  • \$\begingroup\$ it's just a standard automotive headlamp or horn relay. 40A is a common size. \$\endgroup\$ Feb 21, 2021 at 19:23
  • \$\begingroup\$ Even if the lights are additional, make sure they are automotive rated and intended for the purpose. I've seen this come up several times now where people had lights intended for ATVs and the like installed on a car. \$\endgroup\$
    – K H
    Feb 22, 2021 at 3:58

1 Answer 1


The 40A relay is OK here. That means it's capable of handing up to 40A and will work just fine at ~1A which is what your load requires. The important thing, and you have that covered, is to properly size the fuse for your 14 ga wire. A 10A fuse is fine here.

  • 1
    \$\begingroup\$ Bigger relays typically require higher control current to pull in the contacts. Also, your seat warmer fuse connection is probably capable of high currents. Make sure to use a small fuse to protect your 22 ga. wire as well (2A?). \$\endgroup\$ Feb 21, 2021 at 19:02

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