Story & Photos By Jim Clark, The Hot Rod MD

One of the most frequently overlooked items in an automotive electrical system is the grounding of the system and components.  Grounding is critical because current will not flow unless it has a path for it to return to the source.  In most modern systems the current flows from the positive post on the battery to the device needing power and eventually back to the battery through the negative terminal on the battery.

It doesn’t return via a direct route though; instead it returns through the metal components of the car on its travels back to the negative post.  Every component receiving electrical power has to be connected in some way back to that negative post or the current will not flow.  This is the case for something as high-demand as a starter motor or as low-demand as a light bulb.

Current Flow

Electric current is a flow of electric charge through a medium.  This charge in automotive applications is typically carried by moving electrons in a conductor such as wire.

However, in automotive applications electric current flowing from the positive pole of the battery to the component is not technically accurate.  A flow of positive charges gives the same electric current, and has the same effect in a circuit, as an equal flow of negative charges in the opposite direction.  So, current can be the flow of either positive or negative charges, or both.

When analyzing electrical circuits, the actual direction of current through a specific circuit element is usually unknown.  Consequently, each circuit element is assigned a current variable with an arbitrarily chosen reference direction.  In automotive applications the direction of conventional current is defined arbitrarily to be the direction of the flow of positive charges.

This long-winded technical explanation is included here for those individuals with a lot more knowledge of electronics than this shade-tree mechanic possesses.  All we need to know is that automotive wiring diagrams assume that current flows from the positive post to the device and completes the circuit by returning to the negative post.

Resistance

Voltage is the pressure pushing electrons to flow through a conductor.  Amperage is a measure of electrons flowing through a conductor.  Resistance is anything that impedes that flow.

 

Grounding problems are created by a circuit’s resistance to the flow of electrons through the circuit.  Bad connections or breaks in the circuit (shorts) are the most common causes of increased resistance or lack of current flow.  However, inadequate wire size sufficient to carry the load is the most common error made when wiring a hot rod.

An understanding of how resistance is created in a circuit is necessary in the planning of a circuit or electrical system.  Old cars differ from newer models so each needs to be treated differently.

Older cars came equipped with very few items needing electrical power so the systems consisted of a 6-volt battery, DC generator, voltage regulator, starter motor, distributor/coil ignition system and basic lighting system.  All of the vehicles components were bolted together with very little insulating material separating them so these items did not restrict or interrupt the path to ground.

Newer vehicles, including today’s modern hot rods, have much more insulation between components and utilize a wide variety of both conductive and non-conductive materials.  This factor, combined with the many high-demand electrical components that they are equipped with, necessitates the use of a 12-volt system with jumper wires between the drivetrain and the body or return ground-wires from the component to the negative side of the battery.

Fiberglass-bodied vehicles need a separate grounding system with a ground wire returning from most every component back to the negative post of the battery.

High-demand Components

Older cars with heavy metal frames and steel bodies function well without ground wires returning to the negative post as long as good connections are made between the component and the body or frame.

 

High-demand items like starter motors function best if the source of power like the battery is close by and has a short path from the positive post to the starter motor and back to the negative post.  This is one of the main reasons that the battery is usually placed in the engine compartment.  In early hot rods big engines eat up space in the engine bay necessitating the placement of the battery elsewhere.  Usually the only available alternate space for the battery is in the trunk or at the rear alongside the frame.

This necessitates the use of larger cables from the battery due to the voltage drop created by the resistance to current flow in the longer cable.  Return to the negative post also must travel a longer distance

Wire Selection

There are formulas for calculating resistance (Ohm’s Law), but when wiring a vehicle the average hot rod builder should not need to make these calculations to install proper grounds.  Selecting the correct size of wire supplying power to an application should serve as a good guide when selecting the wire size as a return to ground.

In an electrical system the conductors should not be sized with voltage drops exceeding 3%.  For a 12V system the maximum voltage drop should be less than 12 (V) x 3% = 0.36 (V).

The table below can be used to determine the combination of maximum current through a 12V electrical wire, size (AWG) and length of cable.

Note! Failure to use an adequate size may result in overheating and a fire.  Always protect a wire with a fuse.

1 ft (foot) = 0.3048 m

 

Wire Gauge Design Procedure

Calculate the total length of the wire from the source to the device and back again

Determine the amount of current in the wire

Correct wire gauge is in the intersection of amps and feet

Note! The wire size is required for a 3% voltage drop in 12 Volt circuits.  Oversize the wire if the voltage drop is critical.

 

It is clear that the grounding of the vehicles electrical system is every bit as important as the routing of power to the components.  Failure to provide an uninterrupted path back to the negative post of the battery will cause components to perform poorly or not at all.  Correct wire size and good connections will assure proper operation of the entire system.

 

Joining one side of a terminal strip together with jumper wires can create a common point for connection of grounds.  Use a good crimping tool and the correct crimp connector for the wire size selected.  When joining two wires together in a crimp use the next size larger crimp connector to avoid damaging the wire by crimping the wire too tightly.

 

Jumper strips like those shown here can be use to connect the terminals together instead of a series of jumper wires.  A single buss bar (not shown) with screws to connect the wires to will also work because none of the ground connections need to be insulated from the vehicle.

 

This four-terminal strip was connected with jumper wires because the dividers between the terminals were too high for the jumper strips to bridge.  Wire on the bottom right is the ground coming in from the dashboard area and the one on the bottom left is the heavier gauge ground wire connecting the terminal strip to the frame.

 

Battery cable ends should be crimped instead of soldered because solder makes the stranded wire one solid unit, wicking solder up the cable, which makes them less flexible and more susceptible to breaking strands due to movement of the vehicle.  A local parts house will probably loan one of these high-end crimping tools to you or you can purchase a less expensive style crimper that you strike with a hammer.

 

The positive cable needs to be insulated with shrink tubing where the crimp connection is made.  Shrink tubing is not needed on the ground cable crimp connection but it can add some additional support to the crimp connection.

 

Big engines in small engine bays leave little room for the battery so it usually gets installed in the trunk.  Placing the battery at the rear of the vehicle necessitates the use of larger (1 or 0 gauge) cables for the positive and negative connections.  The accompanying chart illustrates the correlation between cable size and length requirements.

 

The positive cable is run from the battery to the starter providing an uninterrupted connection.  The negative (ground) cable can be connected to the nearest available point on the frame which then serves as a connection between the negative post and the starter motor housing.  Some of the newer vehicles have too many insulating elements between the frame and powertrain, which can make a continuous ground cable necessary.

 

A matching size ground cable needs to be connected between the engine/trans combination and the frame for vehicles with short ground cables from the battery to the frame and those with a ground cable running all the way to the starter.  All connections should be full metal-to-metal contact.

 

A ground wire of at least 10-gauge should be connected between the body and frame of the vehicle because relying on the contact between body mounting bolts and the frame can produce a weak connection.  A good connection can be confirmed by measuring the resistance between the negative post and a point on the body with an Ohmmeter.  The reading should be all zeros as is shown here.