1
\$\begingroup\$

I am working on an electrical installation in a truck which will operate as a mobile office.

I am wondering if it is possible to only route a 12V positive wire, and use the chassis for ground? Basically to save on cable costs.

Typically a car battery has its negative terminal connected to the chassis. Which to me suggests that the chassis has capacity for high currents. But I'm not sure if that is also valid for continuous load, e.g. at 100A, and whether this would work at longer lengths, e.g. 4-6 meters.

I'm wondering if the chassis will have too much resistance or other problems. The vehicle will be turned off, so I'm assuming noise won't be a problem.

\$\endgroup\$
  • 4
    \$\begingroup\$ You might get better answers on the automotive stack. Anyways, your bottleneck will be the connection to the battery, not the chassis, which has much much more material to conduct through. \$\endgroup\$ – PlasmaHH Oct 25 '16 at 9:08
  • 1
    \$\begingroup\$ Plenty! The starter motor have no separate ground connection than the chassis, even when the battery is located in the trunk in most cars. That's several hundred A. \$\endgroup\$ – winny Oct 25 '16 at 9:14
  • 4
    \$\begingroup\$ I'm working with Mercedes Benz Buses Factory in Turkey. They use chassis as a return path for all electronic devices, because this saves a lot of cabling cost. They don't know (because didn't measure) the continuous current capability of chassis but it should be way more than 100A. \$\endgroup\$ – Rohat Kılıç Oct 25 '16 at 9:20
  • \$\begingroup\$ Will you be feeding those 100A into the chassis at a single point or distributed over several connections? \$\endgroup\$ – JimmyB Oct 25 '16 at 9:32
3
\$\begingroup\$

Because the cross-sectional area of the chassis is so large, it provides an excellent return path for DC currents to the battery, even though steel is not a very good conductor. The steel chassis itself has a lower resistance than the copper cable carrying the positive leg current in the circuit. Also, using the chassis as a DC ground is common and accepted.

Just make sure the connection points between wiring and the chassis remain free of corrosion, and that there is good contact between wires and chassis. Don't rely on simple surface contact between a spade terminal or ring terminal and the chassis. There may be paint or an oily film on the chassis in some places. Remove any paint or other coating on the chassis before making contact. You may also need to use serrated washers under any ring terminals to ensure good contact.

The wires you use must also be adequate for 100A, which means something like AWG 00 cable. This applies to the chassis bonding wire for the battery, too. When you first put your system in service, measure the voltage drop from end to end while 100A is flowing, and check that the wires and connection points do not get hot after a few minutes of high current operation.

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ I'm thinking that the cross-sectional area is great for the parts where current can flow in parallel, but basically those paths have to converge at the bonding point, at which point the cross-sectional area becomes very insufficient for steel material. And automotive practices aren't usually energy efficient. Which might mean that the normal bonding method could have an energy loss of 10-20% (for 3-5 seconds while starting) without problems. I'll try with a single bonding point (stripped,welded) and add more if the voltage drop is too high. Thanks for the input! \$\endgroup\$ – user95482301 Oct 25 '16 at 17:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.