Vehicle Electrical Systems – Power Distribution

Gearheads love to add accessories to their vehicles. It could be auxiliary lights, a stereo amplifier, or maybe just some utilitarian charging ports. Regardless of the accessory in question power distribution is something many people overlook. Think of it this way, when do you install the fuse panel and wiring in a house: before or after you install the lights? Before, right? Yeah. Same goes for vehicles. While many aftermarket accessories come with their own switch and wires (at least the reputable brands do) after a while the underside of your hood and the raceways of your vehicle look like an electrical rats nest. Since the vast majority of fires in modified vehicles is electrical, a good strong foundation for your electrical system is something that should be done BEFORE any accessories are added.

Electrical Terms & Components

  • Units of Measurement:
    • Volts = the pressure of electrical energy
    • Amps = rate of flow of electrical energy
    • Ohms = resistance of electrical flow
    • Watts = power of electrical energy
  • Components of an Electrical Circuit
    • Wire – Conducts electrical current in a circuit (measured in Gauge)
    • Fuse – consumable link that protects a circuit (measured in Amps)
    • Switch – multi-position component that can complete or interrupt a circuit
    • Relay – electrically controlled switch (usually able to handle higher amps than a normal switch)
    • Battery – stores electrical energy in a electrochemical solution
    • Capacitor – temporarily stores electrical energy in an electrical field

Fuse Boxes

The hub of power distribution in any vehicle is the fuse box. These boxes (pictured above) contain a fuse for each circuit and relays if required. There are usually two fuse boxes on most modern vehicle. One is under the hood and handles most of the major systems like the engine, transmission, and operational systems like turn signals, brake lights, and windshield wipers. The second fix box is for the vehicle’s chassis and covers everything inside the vehicle like HVAC, radio, power windows, interior lighting, and so forth. Historically neither one of these fuse boxes leaves any room for aftermarket accessories. That said, OEM’s (Original Equipment Manufactures) are now starting to add auxiliary switches, relays, and wiring as an option on many popular models. Jeep, Ford, Ram, and Toyota are now all offering auxiliary switch systems on vehicles like the Wrangler, Bronco, Ram 1500, and TRD packaged 4×4’s. Sadly these systems are sometimes still lacking depending on how many accessories you plan on adding. For instance, the auxiliary switch system available in the Jeep Wrangler JL, JLU, and JT Gladiator is only four switches. Might be enough for some, but may be lacking for others.

The two major components to an sPOD are the brain which contains the wiring and relays and the switch panel. In this case the Bantam-X uses a touch-screen switch panel connected via a networking cable. Makes the installation clean and easy.

When looking to add multiple accessories each requiring their own switched electrical supply there are a lot of options in the aftermarket industry. Companies like 4×4 sPOD, Switch Pros, and Painless Wiring all offer systems catered to the automotive industry as well as specific products geared toward the off-road enthusiast. Other companies like BlueSea and Eaton also make a wide variety of electrical components for automotive, marine, and industrial applications that can be of use to off-road enthusiasts. In short (no pun intended) there are a lot of options out there which is why many people are intimidated by vehicle wiring.

Wiring

To get power from the fuse box to the accessory there needs to be a wire. Two actually. One pulls power from the supply side (battery & alternator) and the is a ground (usually to the body or chassis) completing the circuit. The size of the wire depends on the amperage load. The higher the amps the thicker the wire needs to be. The size of the wire also needs to go up the longer the wire gets. This is due to the internal resistance of the wire. A long thin wire carrying too many amps will heat up and melt the insulation off the wire exposing the copper metal core. This can cause a short and, if you’re lucky, blow a fuse. If you’re not lucky it can cause sparks and ignite something flammable. Remember, the number one cause of fires in modified vehicles is electrical, this is why.

One way to mitigate high amp loads on long runs of wires is to use a relay. A relay is basically a remote controlled switch. This means the high amp load to the accessory can be run in a direct fashion from the battery, through a fuse, to a relay, then to the accessory. The relay needs only a faction of an amp to activate, so the more complicated run from the battery through a fuse, through the switch (which is often in the dash requiring a long complicated route), to the relay can be done by a smaller gauge wire which is safer and easier to manage.

Here is a basic chart to help select the proper wiring size for the load and length of the circuit.
A good rule of thumb is to always go one gauge heavier (thicker) – just in case.

That said, sometimes running a high amp load over a relatively long distance is unavoidable. This is when high quality heavy gauge wires are your friend. Running a 4ga, 2ga, or even a 0/1ga wire from the front of your vehicle to a distribution point in the rear of your vehicle is sometimes necessary when looking at secondary house batteries, high amp draw accessories like stereo amps or power inverters, or even other rear mounted accessories like rear winches. Making sure this kind of feed wire is of the proper gauge, and ideally a gauge thicker than minimum recommendations suggest, will help mitigate that wire from getting too hot and melting.

When running thicker gauge wires over long distance there is no shortage of wiring accessories in the car stereo world. Terminals, circuit breakers, large fuses, and power distribution blocks that are often used to wire up car stereo amplifiers can easily be adapted to work in a circuit powering off-road and overland related accessories. There is no difference between how you wire a 300 watt car stereo amplifier and how you wire a 300 watt inverter. Both draw the same voltage and amperage. Both have the same connections for power input. The only difference is their outputs. One outputs noise to a speaker the other outputs 120 volt A/C power.

A car stereo amplified install kit like this one from Crutchfield is a great foundation for a DIY power distribution system or for wiring up a high current accessory like an AC power inverter.

Authors note: While mentioned in this article as an example, the author does not recommend using DC to AC inverters in vehicle builds. While common they are often unneeded since most devices people power off of them (computers, camera battery chargers, etc) all run off DC power and to go from DC to AC back to DC is very inefficient, creates a lot of heat, and adds unnecessary complexity. When in doubt look for devices that run off 12v DC or 5v USB power.

Designing an Accessory Circuit

When you start planning your build and what accessories you plan to add keep the sum total in mind for the finish product. Having a detailed plan for your build and what your end goal is essential when it comes to the electrical system. If all you ever plan on is upgrading one set of lights, such as your OEM fog lights, and adding something like a winch and a fridge – and that’s it – then you’ll have a relatively simple plan that is easy to execute. If, however, you have plans for multiple auxiliary lights, a house battery bank, running solar, a fridge, 12volt van, and the ability to recharge laptops and camera batteries then things get a little more complicated, but a solid plan will help simplify things.

When building a house you always lay the foundation first followed by the plumbing and electrical. The main fuse box is one of the first things to get installed. The same should be true of a vehicle build. If you are planning on adding multiple accessories down the road then it would behove you to install your power distribution system first. While not as sexy and as glamorous as slapping a bunch of lights on the outside of your vehicle, installing something like an sPOD inside your vehicle first will be smarter, safer, and save you a lot of headache down the road. Even if you won’t be adding all the accessories right away, having something like an sPOD installed first will make adding such accessories down the road that much easier. This is why a solid plan is a good idea because it will help prioritize what needs to go in first – sPOD – before you add stuff later – lights.

A well labeled clean sPOD install. See how each circuit is labled?
This makes for easy trouble shooting in the field and prevents a rat’s nest under the hood.

Another good reason to install something like an sPOD into your vehicle is that it gives you a central point for all the switches, fuses, and relays. While most companies provide rudimentary wiring with their accessories they are often about as basic (cheap) as they can. They also end up being very generalized and rarely fit well inside a vehicle. Often wires to switches end up being a lot longer than they need to be in smaller vehicles like Jeeps, and are often too short in larger vehicles like trucks and vans. They also lack mounting points for the fuses and relays so they often get zip-tied or screwed into random locations under the hood. Repeat this a few times with a bunch of accessories and then things get real messy real quick. This can complicate field repairs and, as mentioned before, increase the risk of electrical shorts and potential electrical fires. Centralized systems like an sPOD take all of those fuses and relays and put them into one compact, neat, easy to understand box. Same goes for the switch end. Rather than running wires to individual switches, systems like an sPOD centralize all the switches in one box as well as pre-bundle all the wires into one harness between the switch panel and the relay box. From there trouble shooting becomes a lot easier rather than tracing a bunch of redundant wires leading to multiple fuses and relays tucked god knows where under your hood and/or dash.

Best Practices

Some of the best practices I recommend is taking the time to plan things out as best you can before you install anything. Secondly, when in doubt go with a heavier gauge wire. Conversely, when in doubt, go with a smaller fuse. It might be temping to swap that 5 amp fuse out for a 20 amp fuse, but in reality that can cause a circuit to overload its wiring when it should just pop. Lastly, always run wires in the shortest direct path possible that makes sense. Obviously running wires across your engine is a bad idea, but you want to avoid snaking a main power wire from one side of your vehicle to the other and adding 4-6 feet of unnecessary large gauge wire. The adage “you get what you pay for” is extra true when talking about electrical components. Buying cheaply made bulk terminals might save you a few pennies up front, but over the lifetime of the accessory you’re installing and the lifetime of your vehicle you’re better off paying for quality stuff up front. Not to mention the fit and finish will be better than something hobbled together out of cheaper randomly sourced components. A final best practice is to wrap wires in some sort of protective loom. The corrugated plastic stuff works, but eventually it will become brittle. The new mesh style looms (like those picture in the sPOD photos) are a lot easier to work with and have a much better finished “professional” look when all is said and done.

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