Relay Fuse Box for Lights


New member
I'm looking to install an aux fuse box for some lights and seat heaters on my E350. Seems like the best route is to tie it into my starting battery and turn on/off with the ignition, that way the heaters can't be left on and drain either the starting battery or alternatively the house battery.

Anyone have some insight as to how to do this? Is it best to tie the fuse box in directly to the battery and use relays off of that? Or use a single larger relay and tie into something that only has power when the van is on like the radio? How would I find which wire to tie into in that case? Any products out there yall are a fan of for doing this?


Engineer In Residence
I would tie to the battery or positive bus if your vehicle has one. Then use a 30 or 40 amp relay to switch the power for the entire fuse block. There are some universal fuse blocks that also include several relays.

If you have the need to switch the loads remotely and individually then having separate relays has value. For example each load could have its own relay. The relays positive power is supplied the vehicles ignition on bus. Then you would switch the ground to provide manual control if needed.


New member
How much current will your lights/heaters use? I would take power directly from battery, through a marine breaker switch (capable of handling the current), into a Bussmann power distribution block and out through appropriate switch/relay/fuse. Something like the radio circuit would not handle the current.

Marine breaker

Bussmann Power distribution block.

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New member
You send the power from the battery through the correct size wire to the switched side of the relay. You would then take the ingintion key on signal and send that to the coil side of the relay. Those relays are Single Pole Dual Throw (SPDT) which would work but you only really need a SP Single Throw (SPST) for this application.

Personally I like to send the power to the distribution block through a breaker/fuse ie the marine breaker I listed above. That way you can switch the whole panel off completely if needed (regardless of key on/off position) and it's also circuit protected. You can get the relay side of that block bussed so that you could switch all the relays with one signal from the ignition switch. I also like to overbuild these blocks (current capacity wise) so you have room to expand in the future.

Below is an example of a setup I did in my RX-7. Was still in progress in that picture, but you can see the main breaker which then feeds the Bussmann block which houses the relays and fuses. Each individual circuit can be on it's own relay and fuse. That setup powered everything in the car so would obviously be overkill for your application but a scaled down version would work nicely.

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