Wiring Tips, Tricks and Traps

Everything you ever wanted to know about grounds
(But never asked!)

By Jim Stafford

Here are four great truths about grounds:

  1. All electrical devices must be connected (grounded) electrically to the negative battery post.
  2. All current going into the battery (+) terminal of all electrical devices must exit via the ground terminal.
  3. Heavy wires are able to handle greater amounts of current without heating than thin wires.
  4. There is no such thing as "too heavy wire."
Battery current must flow through each electrical device. In order to accomplish this goal, each electrical device must have a wire or use its case to provide a "ground return" back to your negative battery terminal.

Electrical devices such as headlamps provide a terminal for the ground, while devices such as parking lamps and alternators use their cases to provide the necessary path to ground.

Voltage and Resistance Explained

In order to allow the battery current to operate an electrical device, it must pass through the device much like water passes through a waterwheel. If the water simply remained within the waterwheel and wasn't allowed to exit, the wheel would not turn and no work could be expected from it. Therefore, you can expect that the water entering a waterwheel will be released at the bottom of the wheel when the wheel turns.

Basically, you can equate water flow to current flow and water pressure to battery voltage. The amount of current flow is determined by the resistance of the device and the size of the wires going to the device. Picture water running through a large pipe. This pipe will be able to handle a large volume of water without much pressure loss, but, if you were to force the same volume of water through a small pipe, you will have a large pressure loss. That is, the pressure at the inlet side of the pipe will be much greater than the pressure at the exit. This loss is due to resistance. Resistance to the flow of water and resistance to the flow of battery current is essentially the same thing.

Using the Frame as Ground

Using the frame as an electrical path the the battery (-) terminal is an efficient method of wiring the ground portion of your vehicle. Even though steel is not as efficient as copper when it comes to conducting current, the thickness and size of the frame easily compensates for the loss in efficiency. The main concern when using your frame as a ground is the problem of getting a good low-resistance connection between the battery post and the frame. This method requires the builder to drill and tap the frame or weld on "grounding tabs" at numerous places to allow the attachment of individual ground return wires. In many instances, a ground is made by simply bolting the electrical unit to the frame or to the engine itself.

Central Ground Block

An alternative method of providing a ground return is to run copper wire from each electrical unit back to a central junction block located near the battery. This method is especially effective when the builder desires to keep the frame intact by eliminating grounding points that mar the paint. This is accomplished by mounting a multiple contact terminal strip next to the battery (-) post. A short, heavy (#10) wire is attached between the terminal strip common and the battery (-) post. All electrical components, with the exception of those mounted directly to the engine block, will have a ground wire run back to the terminal strip. The builder must keep in mind that these wires must be fairly heavy (#14 or heavier) to allow proper operation. Remember, the "ground return" wire must carry all the current which was used by the electrical units connected to this wire.

Engine Grounding

The heaviest current use by the electrical system goes to the starter motor. In fact, the current that goes to the starter is so high that it pulls the battery voltage down to about nine volts when cranking the engine. It is imperative that a heavy grounding strap connect the starter motor (engine) to the battery (-) terminal. The best way to make certain that the starter is getting maximum volatage is to connect the grounding strap directly from the battery post to the starter motor mounting bolt. Alternatively, a grounding strap can connect the battery post to the frame and then the frame can be connected to the starter via another heavy strap. Make certain that the connections to the frame are clean and well-secured. Most hard starting and slow cranking problems are due to improper grounding which reduces the voltage available at the starter.

Fiberglass Bodies

As we all know, fiberglass is an insulator. That means that it doesn't conduct battery current. This makes the installation of many electrical items which normally get their ground through the case more difficult. What must be done is to use copper wire to attach each case to the battery (-) post. For example, many gauges use their case as a ground and expect to get grounded when bolted to the instrument panel or dash board. When mounted in a fiberglass dash no "ground return" current can flow and the gauges will not work properly. The best remedy is to purchase a wiring kit (such as Centech's K-30A Gauge Wiring Kit) which provides wires and terminals to connect each gauge case together and then return to the battery (-) post.


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