A SIMPLE CIRCUIT has:

a source - battery, alternator, PDT (power distribution terminal) or even an existing circuit,

a device being powered - a set of lights, a CB, a stereo, a winch, etc., and

a grounding point - usually the truck frame.

Connect everything with conductors (wire), then insert an on/off switch between the source and the device to control the circuit and Voila! you have a very basic (simple) circuit.

An example of a simple circuit would be:

 A slightly more COMPLEX CIRCUIT would utilize a relay and have two parts:

a "control circuit", and

a "power circuit".

The power circuit is essentially a simple circuit with a relay (an electronic switch) substituted in the place of the manual SPST on/off switch. This portion of the overall circuit is what actually provides power to the device(s).

The control circuit completes the power circuit by energizing the coil inside the relay. It’s important to note that the control circuit can essentially derive it’s power from any source including, but not limited to, the source for the power circuit. Typically an SPST on/off switch is used to manually control the control circuit, but a control circuit could also use the tap from another circuit (existing or new) so that the relay will be energized whenever this other circuit is turned on.

Note that a solenoid is nothing more than a 'large' relay. The difference is that a solenoid can handle a much larger amp load than a standard automotive relay. You'll see solenoids installed on things such as electric winches in order to handle their relatively large power draw.

An illustration of a circuit containing a relay is as follows:

Some control just one circuit (similar to an SPST switch). Others control two different circuits – one at a time (similar to a SPDT switch). Still others control two different circuits simultaneously (similar to DPST and DPDT switches).

These are examples of typical standard automotive relays:

Configurations of various basic relays are illustrated below. In each of these, the control circuit is connected to each relay at point 86. The power circuit is connected to points 30 (power IN) & 87 (power OUT). 87a is also power OUT, if applicable. Note that the schematic diagrams for these basic relays are much like the wiring schematics for switches as shown in Article 1.

A very complex multiple circuit arrangement could use one or more relays, controlled by one or more switches, to power multiple devices. This is where SPDT, DPST and DPDT switches come into play.

Unless you have some rather exotic requirements, most people will want to control one circuit or maybe two circuits simultaneously, and will use one or two relays, controlled by one or two switches. The thing to be careful of when setting up a complex multi-circuit layout utilizing multiple switches and relays, is to analyze the "path logic". Electricity flows thru any open path to ground, even if it backtracks along a path that you think it won't! You need to be careful of how and to what you connect each circuit; in that way you’re not trying to utilize a circuit for power or control that is inadvertently controlled by a different set of relays or switches. You could spend quite a bit of time trying to figure out why that awesome set of off-road lights won’t turn on because you powered the control circuit for the relay from another circuit that is not on when you want it to be. Keep it simple; there’s less chance for errors.

Rather than trying to connect a wire to each prong on the relay (yes, I've seen people do this), I would highly suggest that you install a harness to plug the relay into. If the relay goes bad, then you just unplug it and replace it.

Here's a typical relay harness that is wired into a circuit:

Solar panels - Huh??? I’m really not qualified to comment on these other than that they can't be used at night and therefore are tough to power lights with.

A portable generator mounted in the bed/trunk – effective, but a little extreme. Might be OK for "roughing it" with a TV and satellite dish, but not too practical for powering vehicle accessories. Besides, how do you fill the gas tank and turn it on/off while you’re driving?!?

The Alternator – get your power right from the source! The biggest drawback is that your truck must be running in order for any power to flow. It’s also hard to get to and is in close proximity to the fan, belt, and very hot engine therefore making it difficult to anchor and route a feed wire. Another consideration is voltage/amperage fluctuation due to engine rpm’s.

The Battery – power whenever you want it, and it’s voltage regulated! But, there are a couple of minor problems related to direct battery connection. Mounting and attachment of a feeder cable is awkward due to the size of the terminals, especially if you have a number of accessories to connect. Besides, you'll probably need to drill/cut/modify the nice plastic protective battery cover in order to get at the terminals. If you don’t struggle with either of those issues, then attach away! Before you do so though, check out the next power source.

The Power Distribution Terminal (PDT) – my personal favorite. It’s basically the same as attaching to the battery but in a more convenient location. The PDT is the battery cable connection point on the starter solenoid. The PDT's ultimate advantage is that it has a threaded ¼" lug (at least it does on late model F-150's anyway), which is perfect to use for a mounting point. Crimp an eye lug onto your feed wire, then clamp it under the nut on the PDT. You’re all set. The only limit to the potential draw through the PDT is the capacity of the cable between the starter solenoid and the battery. For most applications, the existing cable will be just fine. Other than an electric winch (which really should have a multi-battery power setup anyway), I can’t think of a single application that this cable would be a limiting factor for.

An existing circuit – is perfect for powering relay "control circuits" and applications with minimal power draws. The drawback of course is that the total connected load is limited by the "amp rating" of the existing circuit. Let’s say, for example, that an existing circuit is rated for 15 amps due to wire size and distance run (the size of the existing fuse is a good indicator because the safety factor is already built in). Let’s also assume that all existing devices connected to that circuit, have a total draw of 12 amps, and then you want to add something that draws 2-3 amps. The circuit MIGHT marginally be OK; it depends on what types of devices are connected.

Devices like motors, which have "surge draws", might be a problem. The initial "in-rush" of power these type of devices have might take the circuit beyond the fuse rating just for an instant, and the fuse will probably "blow". If you hook up some new device(s) which take the connected load beyond the existing circuit’s rating, then eventually something will happen. It might only be something as simple as blowing a fuse, but when that existing circuit powers the headlights and it’s dark out, well…that’s bad. Save yourself some grief, if you need more than a couple of amps - run a new circuit.

There is another way to utilize an existing circuit though. You can just utilize any "unused" wiring. Usually the wiring harness for a particular vehicle is the same from the top-of-the-line model to the base model. Of course the "luxo-versions" have powered everything, but the base and intermediate models typically don’t have some of these electrical amenities. Depending on how the existing circuit is configured, you could use some of them to power some additional devices. This is very tricky though! It's often difficult to determine how existing circuits are controlled and what amp load they are rated for. Obtaining the electrical service manual for your vehicle would be money well spent, but even those schematics may not have everything that you need to properly analyze whether you can use the existing wiring.

All in all, I feel that the PDT is THE perfect power source for most electrical add-ons and I would rather run my own wiring just to be safe.

First, let's look at wire connections and attachments:

To "tap" an existing circuit for a power or control circuit feed, you can:

The "clamshell tap" is fine for low amp draw requirements like relay control circuits that draw about 1/10 an amp +/-. There are different sized clamshell taps, so make sure that it's appropriate for the wire sizes being used, then just squeeze it on with a pair of pliers.

A crimp-on connector is relatively easy and quick to install, but the cheap ones do not hold onto the wire very well. It is important to note that these crimp connectors can only accommodate specific wire sizes. If you use one that's too small, you won't get all of the wire strands into the connector. If you use one that's too big, it'll just pull right off. That wouldn't be very good considering that the connection will probably be subject to vibration or tension. Cheap crimp connectors tend to have a plastic protective covering that will crack when crimped improperly. The better ones have a vinyl or rubber sleeve for protection of the connection. BUT, don't count on these coverings to make the connection weather-resistant! These coverings usually do not extend very far past the connector and water/salt will get in there and corrode the connection. ALWAYS cover these connectors with electrical tape. An important point to remember about bullet and spade connectors is that they use surface contact to transmit electricity. If the connection is not tight then the flow of juice could be interrupted. There are some very good (and costly) crimp connectors that are available, but I personally tend to minimize use of these except for components that will be removed for periodic maintenance.

For power feed taps off of existing wires, I would splice into the existing wire by method 3 or 4. Some people will argue that method 4 (soldering the connection) is a waste of time and could potentially harm the wire, as well as its insulator. If too much heat is used to solder the connection, then the wire could become brittle and may subsequently break due to the movement and vibration which a vehicle will experience. If you decide to use soldered connections, then always use flux to allow the solder to flow without overheating the connection. The real advantage of soldering is that electricity will find a good continuous path thru the splice. By not soldering the connection, the circuit relies on surface contact of the twisted wire to convey the juice. In a high amp draw situation, this "weak link" surface connection will create heat and may cause the conductor insulation to melt and/or burn. As such, I'm partial to soldering the connections for any power feeds.

A fuse tap uses a metal extension to draw power from one leg of a plug-in type fuse. This kind of tap is good for a control circuit feed to a switch inside the cab. You tap the fuse box, run a wire to the switch, then go outside the cab - rather than having to come in and then go back out. You just need to be careful choosing which fuse to tap so that the device you want to control will come on when you want it to. Using a fuse tap as a power circuit feed has the same drawbacks as splicing into an existing circuit. If there's a big amp draw thru the fuse tap, it could fry your fuse block. That’s' BAD! Very, very BAD !!!

To connect two wires together, you can:

The twisted and soldered or non-soldered connections have the same drawbacks/advantages that are listed above for tapping an existing circuit.

Also see tapping an existing circuit for my comments on crimp connectors.

To connect three or more wires together, you can:

Once again, a twisted and soldered or non-soldered connection has the same drawbacks/advantages as listed above for tapping an existing circuit.

A "piggy back" connector has the same benefits/disadvantages as crimp connectors (see tapping an existing circuit).

As we've discussed previously, a buss bar is a great way to distribute power if the things that need to be powered can all be connected to the same source (i.e. battery). These bars have screws or bolts that are used to sandwich the wire under them. Electricity is conveyed by surface transfer, much like twisted splices or plug-together connectors, but the screw posts make it very difficult to pull the connection apart.

To connect a wire under a screw, or on a bolt or threaded stud, you can:

Crimp connectors have the same advantages/disadvantages as listed in the section on tapping an existing circuit. Note that by using a hook, fork or eye lug, or a butt, spade or bullet connector, the final product will look much 'cleaner', more professional and 'finished'.

Bare wire ends sandwiched under a screw or nut, are very susceptible to corrosion so at least coat them with di-electric grease. Also, if just a single strand of the wire touches something it's not supposed to, then a short circuit will occur. Make sure that you only remove as much insulation as necessary and completely wrap it under the screw, bolt or nut.

To ground a circuit, you can:

We've already talked about the advantages/disadvantages of crimp connectors, and bare wire ends so there's no point in doing so again.

I try to keep the ground wire length to a minimum to ensure that the circuit will act as I intend it to.

By far, THE most important issue with ending (grounding) a circuit is to provide a clean grounding point. The mud, dirt, grease, paint and "bees wax" rust inhibitor on our frames and sheet metal tends to disrupt good current flow and will impede circuit grounding. It is very important that you clean this 'stuff' off so that there is good metal-to-metal contact! I've found that using a small circular wire brush attachment on the end of a Dremel tool works great for stripping paint and rust-proofing. Remember that if you use an existing bolt or screw to mount under, to back it out far enough so that you can clean under the head or washer!

As a last step, when you're done fastening the ground wire and tightening the bolt/screw, cover the entire connection with silicone caulk to prevent corrosion.

You will want to use electrical tape to cover any connections that you will need to get at in the future. Heat shrink sleeves are great for splice type connections because these type of connections are permanent and you will probably never need to get at them again anyway.

Electrical tape can be used to cover any connection, but beware - there are many different types and grades. Some brands of PVC tape tend to lose their adhesive qualities if they get wet or if they are subjected to heat. A good quality vinyl tape usually works great for connections that won't be exposed to moisture, but I would recommend using rubber electrical tape for exterior wiring because it seals to itself over time and creates a very weather resistant covering.

You NEED to protect the existing electrical system of the vehicle. If a short occurs in your new circuit, you really don’t want it to 'toast' any existing circuits because then something else won’t work – maybe even something important! Refer to Fuse and Fusible-Link in the definition section of the Article 1.

The draw is the amp load required to operate the device(s) you hook up. The fuse should be rated for approximately 5 to 10% higher than the total actual circuit demand (refer to the first article for determining circuit capacity).

For example let’s say that you’ve got a new circuit which feeds a pair of 100 watt off-road lights. The total demand on the circuit is 200 watts which is 16.67 amps for a 12 volt system (amps = watts/voltage or amps = 200watts/12volts=16.67 amps). A 15 amp fuse would be too small, so you’d have to use the next highest rated fuse that’s available – probably a 20 amp one. A 20 amp fuse is about 20% over the circuit load, but will still work. The key is that you don’t want to select a fuse which is rated too much larger than the circuit rating, or it may never "blow". By melting the fuse, you prevent bad things from happening to other circuits as well as the devices connected to this circuit.

If a short occurs and the fuse doesn’t blow, the 'spike' could trace back through the rest of the electrical system - blow one or more other fuses and even could 'nuke' your vehicle's computers and/or sensors. Damaging these sensitive electronics would be a very costly problem.

Another effect of pulling too much 'juice' through a circuit, could be that your wiring will get fried. Remember, an electrical device will try to pull as much power as it needs in order to operate – whether the circuit is sized for it or not! In the case of a short-circuit, ground has an infinite demand. It will suck ALL of the juice that is available! This unfortunately would be whatever the output of your alternator is. Many portions of a vehicle wiring harness were never designed to accommodate 95+ amps! Therefore, it will start to get hot (due to resistance). When it gets really hot, it’ll melt the insulation on the wiring. Since most of this insulation is plastic, if it gets hot enough it’ll start to burn. Then you’ll have a fire. That's NOT GOOD!

The bottom line is that, for a few measly dollars, put some protection on the new circuit.

Better safe, than really, really sorry!

I also try to make my life easier and install a separate ground wire for anything that I'm adding and try to keep that ground wire as short as possible.

Another thing you should pay attention to with circuit protection is "path logic", especially if you tap an existing circuit. Depending on where the tap is and if there are other intermediate taps, electricity could feed back thru the new tap to complete the existing or new circuit when you least want or intend it to. This is why diodes are sometimes put into circuits. A diode acts like a 'check valve'. Juice can go one way, but the diode will prevent it from going back the other way.

Keep the circuit layout simple and utilize new wiring, and "backfeeding" won't be an issue

This brings up a good point. Pay close attention to the amp rating of all of the components you are using, especially switches and relays. Use a component that's not rated for the demand that you anticipate, and they may get hot-fail-melt-burn. Be careful…

Another thing to keep in mind with circuit protection - just because you use or tap an existing circuit, DO NOT assume that the circuit is protected just because there’s an existing fuse! If a short occurs in the new wiring and you don't put in an intermediate fuse, the short WILL blow the existing fuse and then the existing circuit will be "dead". To prevent this, install another fuse just after the tap. If your new device(s) or wiring does short out, then the new fuse will blow -but the existing circuit should be fine!

The following are illustrations of the proper locations for a fuse(s) in a simple circuit, and a circuit which uses a relay:

You also need to stay away from components that are hot. If you don't, the insulation on the wire could melt and short out the circuit.

Since stock vehicle wiring is routed to avoid things that are hot or components that are moving, if you follow and attach to them, you should have fewer problems.

Just like fuses are protection for the circuit, there is also protection available for your conductors.

Once you cover your wire(s) with the loom, you need to use clamps or wire-ties to support the loom. Do not just lay the loom (or even exposed wires for that matter) on the fender or along the frame and expect them to stay. The easiest thing to do is to utilize existing wiring and/or cables for support. Just wrap cable ties around the new wiring and the existing wiring.

Sometimes you might need to get into or out of the cab with wiring (i.e. for switches). On the late model Ford F-150's, there is a plastic plug in the firewall just about where the brake pedal is.

To run a wire into the cab, you can just drill a hole in this plug and insert a wire though the hole. Make sure that the hole is just large enough for the wire(s); you don't want to make it too big. Once the wire is pulled to wherever you need it, coat the wire-thru-plug intersection with silicone caulk on both the inside and outside. By the way, make sure you clean the plug off first, especially on the outside - caulk doesn't like to stick to a dirty surface.

If the plug warps:

• the caulk could be compromised and the wire-thru-plug connection could leak, or
• the plug will leak where it meets the sheet metal.

In either case, it's something you don't really want to happen at a point that low on the firewall.

There are some other plugs located at various points in the bottom of the cab, and in the lower rear corners of the cab. These can also be used to route wiring through. If you're uncomfortable using the plugs, you could drill holes thru the firewall, BUT, be very careful you don't drill into something important. Look to see what's on the other side first! Then use a grommet or heavy rubber sleeve to protect the wiring as it goes thru the metal firewall or you'll probably end up with a sliced conductor and an electrical short.

Lastly, I need to mention two more items that may seem obvious, but …

• If you are tapping an existing wire, fuse or connection - make sure you have the right one by testing it first! Before you work on it, use a multi-tester or circuit tester light to see if it's "hot". Also test it to make sure that it comes on when you expect it to. Check twice (maybe thrice) then cut/splice/tap.
• If you are unsure of what you are doing, disconnect the battery. I try to run all of the new wiring, mount the switches and relays, and hookup the fuses and devices first. Then I'll do a continuity test on the circuit before I make any control taps or permanent power connections. DO NOT hookup your new circuit to the battery first and then start running wire from there. I guarantee that it'll short out on something for sure, and you won't know until later. Good luck trying to locate the problem.

In the next article I'll cover complex control arrangements for circuits that utilize relays. This is the real reason why I started writing these articles anyway and I'm sure someone will appreciate how unconventional switching setups will allow you to control devices from multiple sources.

"Wiring Made Simple", by Joel Mollis, 4-Wheel & Off-Road, December 1997

"Basic Wiring Diagrams", by Tori Tellem, 4-Wheel & Off-Road, December 1997

"Electrical Mysteries Explained / How Electricity Works", by Trenton McGee, 4x4 Power, February 1998

"Light 'Em Up", by Matt Rawlins, 4x4 Power, October 1998

"Let There Be Light", by Matt Rawlins, 4x4 Power, October 1998

"High-Amp Alternator", by Tori Tellem, 4-Wheel & Off-Road, October 1998

"Electrical System On-the-Trail Repairs", by Tori Tellem, 4-Wheel & Off-Road, November 1998

"Life's Shortcomings", by Cole Quinnell, 4-Wheel & Off-Road, November 1999

8851 W. Freeway

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(800) 423-9696

Springville, CA 93265

(209) 539-7128; [F] (209) 539-7215

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(800) 222-7617; [F] (360) 734-4195

5^th Southern District - Division 17

Western North Carolina

David Stewart, Division Captain

This information is courtesy of the United States Coast Guard Auxiliary and was originally intended for illustrating acceptable methods for wiring boat electrical systems. Since vehicle wiring is very similar, these methods are also applicable.

Small solid conductors may be joined together by a simple connection called the "Western Union Splice". In most cases, the wires will be small enough to twist together with your bare fingers and the sharp end can be crimped in place with a pair of pliers. The sketch shows how to make the splice.

After enough insulation is removed from the wires and they have been cleaned, they are brought into a crossed position. First one wire end is wrapped five or six times around the other which remains in a straight position. Next the other wire is wrapped and the wires are straightened out. At this time excess ends are trimmed and the sharp ends are clamped down. Then the splice is covered with insulating tape. This type of connection is NOT recommended for use with stranded wire conductors because they will easily pull apart unless soldered.

"Western Union Splice"

Wires in a building electrical system are commonly joined by the "Rattail Splice", which can also be used on stranded wire of any size. The splice is made by simply twisting the two wire ends together as shown in the sketch. This type of splice is quick and simple to make, but is low in mechanical strength. To create a more reliable joint, a wire nut must be used. For stranded automotive wire, soldering the connection will usually provide a secure mechanical connection. Remember to fold the connection back along one wire, and wrap the connection with electrical tape.

"Rattail Splice"

When joining a small wire to a larger one, the "Fixture Splice" does provide some amount of mechanical strength, albeit very minimal. Like the "Rattail Splice", the joint is inherently weak. Start the splice by removing insulation from the wires. Next, cross the wire as to form a "Western Union Splice" and wrap the smaller wire around the larger one five or six times. Then bend the straight end into a hook, and continue wrapping the smaller wire around the larger. When this is complete, clamp down the end with a pair of pliers and tape the splice.

"Fixture Splice"

The previous splices were 'open' splices where two wire ends are joined together. Sometimes it may be necessary to splice a wire into a continuous branch wire. To do this the "Knotted Tap Splice" is used. The tap splice is started by removing about one inch of insulation from the branch wire, and then cleaning the conductor. Next remove the insulation from the end of the tap wire and clean it. To start the splice, wrap the tap wire over the branch wire and bring it behind the tap wire as shown in the sketch. Now wrap the tap wire around the branch wire five or six times and clamp down the end. When this is done, tape the splice with electrical tape.

"Knotted Tap Splice"