Chassis Grounding Standards

What is Chassis Grounding?

In any electrical product, the chassis is considered to be the metal structure that makes up the frame that all the major parts of the product attach to. In an automotive context, that is normally the steel frame or unibody that makes up the main structural element of the vehicle. Electrically, the chassis has zero voltage potential and is connected directly to the negative terminal of the battery. Any other components of the vehicle are attached to the chassis by bolts, welds, or some other mechanical attachment. Often, the other components are also electrically bonded to the chassis by using wires or straps. You may have seen grounding straps attaching the hood, for example, to the rest of the body, as shown in the image below:

Hood Grounding Strap

Hood Grounding Strap

All electrical power sources, and electrical loads on the vehicle are grounded to the chassis. In most vehicles, the only power sources are the battery and the alternator. More advanced systems may also have solar panels, inverter/chargers, or generators. Some sources produce DC voltage, others produce AC voltage. In any case, all power sources are normally referenced to the chassis ground.

Why is Chassis Grounding Important?

  1. Safety: All power sources must be grounded to the chassis on which they are installed. For example, let’s look at a theoretical dangerous situation. A 120V generator is installed on your work truck and it is chassis grounded, but the aftermarket body of the truck is not chassis grounded, or its chassis ground has corroded and broken. A 120V cable’s insulation wears through, and the 120V line voltage contacts the aftermarket metal body. The body becomes energized to the 120V potential relative to the chassis. No breakers would trip in this situation, because there is no path for the current to flow back to the generator. Now if a person touches the bodywork with one hand, and a chassis-grounded component such as the cab, with the other, their body now completes the circuit. They would be electrocuted.
  2. Convenience: DC Circuits are made up of a feed conductor to the load, and a return conductor from the load. The worst-case electrical resistance of iron steel is such that a 6″ wide bus of 10 gauge has roughly the same resistance as a #0 AWG copper wire. A common steel framed truck or steel unibody vehicle chassis has far less resistivity than any conductor that can be reasonably run for a given load. Since you already have a piece of steel running the length of the vehicle that can handle several hundreds of amps of current, it just makes sense to use it as the return conductor
  3. Cost: The chassis is already there, copper is expensive. It makes good financial sense to use the chassis as a conductor.
  4. Weight: If the chassis wasn’t used as ground, we would have double the copper wires in vehicles. Copper is heavy!
  5. Current Paths: If you have a typical DC load and you run a separate, isolated ground cable to it from the battery, you are effectively placing that cable in parallel with the main bonding cable between the battery negative terminal and the engine block. The engine block serves as the ground for the alternator and the starter. If for any reason the bond cable between the battery and the engine block failed or became high impedance, the cable you ran to your load would now have to carry all the current to and from the alternator and starter. This could easily ruin the device you have installed, at best, or the ground wire you installed could start a fire.

Why do some devices tell me to run the ground cable to the negative battery terminal?

This is a misguided instruction. Often product manufacturers get calls from their customers saying their devices are not working as intended. For example, a winch not pulling strongly enough, or a radio system producing alternator noise. These problems are caused by the battery to engine, or engine to chassis ground cable having higher than normal impedance. This can be caused by corrosion of the copper cable, corrosion of the ring terminal, corrosion between the ring terminal and the metal to which it is attached, or simply a loose attachment. By running a ground cable directly from the device in question to the battery, the user is bypassing the problem, which often does solve the poor performance. However, this is an incorrect and dangerous fix. As the corrosion or failure of the main battery ground cables gets worse (it will), an increasing amount of the alternator and starter current will travel through the user’s wire. This wire may not be sized for that load, may overheat and catch fire.

All ground wires should always go to chassis ground, near the device, and not directly to the battery. This is true for every size of load, no matter what any manufacturer might tell you. It’s just as true for a 200mA LED light bulb circuit as it is for an 800 Amp liftgate pump circuit. At the same time, the ground cable between the chassis and battery should be verified to be of sufficient size to carry the current to the load. Sometimes for large loads such as winches and lift gates, the oem ground cables are insufficiently sized and need to be upgraded.

The only exception to this rule is if you have a product that is completely electrically isolated internally, such that no parts of the case, connections, or mounting make contact with the ground cable for the device. Additionally, all connections coming or going from such device are also isolated. Fully isolated devices are rare and highly specialized.

What makes a good ground?

Epic Energy has the following grounding standards:

  1. Attachment area cleaned of all paint, grease, coatings, etc.  Must be shiny bare metal.  We grind the metal using a paint-removal abrasive disc, then wipe it using brake cleaner then a final wipe with isopropyl alcohol. 
  2. Attachment area must be main unibody or main steel chassis frame members. Not brackets, chassis cross-beams, other other auxiliary chassis parts.
  3. Attachment metal shall be at least #10 AWG
  4. Mechanical attachment using Stainless Steel threaded fasteners with serrated hex flange-lock nuts sized according to hardware chart below
  5. Hole in attachment metal needs to be the same size as the bolt with tight tolerance. Any gaps between the stud and the surrounding metal are places where moisture can cause corrosion and eventual ground failure.
  6. Threaded fastener is coated with copper-based anti-oxidant grease. This is used to take up any clearance between the threaded fastener and the attachment material, to prevent moisture contamination leading to corrosion.
  7. Fastener is assembled onto attachment material with one serrated hex flange-lock nut, and torqued to spec against the attachment material to form a stud.
  8. The ring terminal is crimped onto the cable and sealed with heatshrink.  Heatshrink is lined with dielectric adhesive which melts and flows when heated.
  9. The ring terminal’s hole size must match the bolt size – no larger, and modifications such as drilling the hole larger are not acceptable
  10. The ring terminal is attached and torqued using a second serrated hex flange-lock nut
  11. The cable must be oriented downwards from the attachment point in order to form a “drip loop” allowing water to drip away from the connection area
  12. The ground stud and connection is covered with a silicone rubber sealant boot, sized appropriately for the bolt and the wire
  13. The area inside the boot is filled with dielectric corrosion-inhibiting grease
  14. The wire itself is protected by convoluted tubing or some other abrasion protection and supported to prevent physical damage
  15. The connection area is painted with a rust-preventing spray paint, on both sides

 

Ground Hardware Spec Chart

Ground Wire Size Load Amperage Bolt Spec Bolt Torque Hole Drill Size
#14 AWG 15 Amps #8 – 32 Bolt 20 lb-in 11/64″
#12 AWG 20 Amps #10 – 32 Bolt 30 lb-in 13/64″
#8 AWG 50 Amps 1/4″ – 20 75 lb-in 1/4″
#4 AWG 100 Amps 5/16″ – 18 130 lb-in 5/16″
#2/0 AWG 250 Amps 3/8″ – 16 240 lb-in 3/8″
#4/0 AWG 500 Amps 1/2″ – 13 45 lb-ft 1/2″

 

An example ground cable installation:
in progress ground connection  Completed ground Connection

Please feel free to comment if you have any questions about grounding, or with any suggestions on how to improve this article!

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