Mobile Wiring

Discussion in 'General Technical Questions and Answers' started by KJ4QCV, Nov 11, 2009.

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  1. KJ4QCV

    KJ4QCV Ham Member QRZ Page

    Ok I'm the proud new owner of a Yaesu FT-2800 and a Larsen NMO antenna. The big brown truck should be delivering it today. Now I need to run some wires from the battery to the inside of the cab of my truck, Yaesu says use 12awg wire??? That seems barely enough as 12 awg is rated to 9.3amp and the 2800 can draw 10amp(65watt) on TX. Should I run bigger maybe 10 or 8 awg to a distribution block in the cab then connect the 12 awg that is already attached to the radio with the fuse to the block, and would it be beneficial?
  2. KX0Z

    KX0Z Ham Member QRZ Page

    You can follow Yaesu's suggestion. Although If you want to use larger wire you certainly can, it won't hurt anything. I would run the power wire directly to the battery. Make sure to fuse the + wire close to the battery, as that will protect the wire in case it shorts out to the car chassis.. What guage wire goes to the distribution block in the cab..
  3. KZ4Y

    KZ4Y Ham Member QRZ Page

    Use Heavier gauge wire


    12 Ga wire should be the minimum gauge to use. I use #4 from the battery to inside my vehicle to a distribution point (having lots of capability). I use car stereo connections for easy components available just about anywhere. Then I use 10 Gauge to my radio (an Icom 706) and 12 ga. to other sets. And for easy connect/disconnect I use Anderson Powerpole connectors.

    Gd luck with your setup.

  4. KX0Z

    KX0Z Ham Member QRZ Page

    Thats kind of what I do in my Solara. I have 8 gauge wire running into the car under the passenger seat into a brass distribution block. Then my Icom28H and my 200W car audio amplifier connect into the block.
  5. N3UJM

    N3UJM XML Subscriber QRZ Page

    Stop at the auto parts store and get some plastic wire loom to protect the wires. On my old truck, I made a jones strip "hot buss" under the dash protected by a 30A breaker. I wired the radio into that using the inline fuses and a bendix plug for quick disconnect.
    If I do it again I'll use Anderson PowerPole connectors. I've used them up to 40A with no problem.

  6. AD5MB

    AD5MB Ham Member QRZ Page

  7. K0RGR

    K0RGR Premium Subscriber QRZ Page

    I generally use #10 for short runs, and #8 for longer ones. But, I've certainly used #12 many times with no ill effects. A lot depends on the length of the run.
  8. K9STH

    K9STH Ham Member QRZ Page

    Connect the positive lead (red) directly to the battery using a fuse basically right at the battery connection. Ground the negative lead (black) directly to the body of the vehicle using as short a wire as possible. Do NOT fuse this lead.

    Connecting the negative lead to the body of the vehicle cuts the "IR" drop in the wiring almost in half. Also, if, for some reason, you lose the negative connection between the battery and the chassis of the vehicle you will NOT provide a path for the entire battery load (including starting the vehicle) through the negative lead to the radio.

    The major commercial two-way manufactures have been installing radios as such for many decades. The reason for both leads being long and each having a fuse is just a carry-over from the "CB" days of the Japanese radio manufacturers. In the "goode olde dayes" many tractor units (the "truck" part of "18 wheelers") were positive ground. Therefore, the radios were made so that the chassis was isolated from ground. The same radio could be used in both positive and negative ground vehicles by connecting the appropriate wire to the battery and the body of the vehicle. In a negative ground vehicle the black wire went to the body of the vehicle or the negative terminal of the battery and the red wire to the "hot" or positive terminal of the battery. In a positive ground vehicle the black wire went to the "hot" terminal of the battery and the red wire went to the body of the vehicle or the positive terminal. Basically, this meant that no matter what the polarity of the ground in the vehicle the red wire went to the positive terminal of the battery and the black wire went to the negative terminal of the battery.

    When the Japanese started manufacturing radios for amateur radio use they just continued this practice. Most commercial two-way radios made in the United States have a long red wire (positive) and a fairly short black wire (negative) which is connected to the body of the vehicle. The red lead has a fuse and the black lead does not have a fuse.

    Glen, K9STH
  9. K8ERV

    K8ERV Ham Member QRZ Page

    Sometimes it is easier and more available to run several smaller wires in parallel.

    TOM K8ERV Montrose Colo
  10. N4CR

    N4CR XML Subscriber QRZ Page

    What you are doing is known as Chassis Wiring. According to this, maximum amps for #12 Chassis Wiring is 41 amps.

    From that article:

    The Maximum Amps for Power Transmission uses the 700 circular mils per amp rule, which is very very conservative. The Maximum Amps for Chassis Wiring is also a conservative rating, but is meant for wiring in air, and not in a bundle.

    The larger issue that you need to worry about is voltage drop. According to this page, 15 feet of #12 at 10 amps drops around .25 volts. This is quite acceptable.

    Wire it up.
  11. K9FW

    K9FW XML Subscriber QRZ Page


    12 gauge wire is rated for 20 amps continuous service. 10 gauge is rated for 30 amps continuous service. Why use larger wire than the rig comes with-waste of time and money. For Mobile install the length of wire has no bearing.
  12. K9STH

    K9STH Ham Member QRZ Page


    I seriously disagree with your statement that the length of the wire in a mobile application does not matter.

    The IR (resistance times current) voltage drop can seriously affect the performance of the unit. You are looking at the rating of 12 gauge wire when used in a 120 VAC household wiring system and not in a 12 volt automobile system.

    The resistance of 12 gauge copper wire is 0.001619 ohms per foot minimum. This makes the resistance of a 10 foot long section of 12 gauge wire 0.01619 ohms. The voltage drop in a 10 foot long section of 12 gauge wire with a current of 10 amps which equates to a voltage drop of at least 0.1619 volts. Then you have to take into consideration the resistance in the connectors and fuse. There are at least 4 connectors involved in a single connection to the radio: One at the battery, one at the input to the fuse, one at the output of the fuse, and finally one at the radio. Then there is the resistance of the fuse itself. In the best installation the fuse and connectors are going to add at least another 0.01 ohms (if not considerably more). This increases the resistance of the lead to at least 0.02619 ohms and the voltage drop across that resistance at 10 amps is going to be 0.2619 volts. In a practical installation this voltage drop is going to be considerably more.

    If both leads (positive and negative) are 12 gauge, fused, and 10 feet long the voltage drop then becomes 0.5238 volts, and that is under the best of conditions, the actual voltage drop is probably going to be higher.

    If you are drawing 20 amps (with a higher powered transmitter) then voltage drop in the same 10 feet of wire is going to be doubled and thus becomes 1.0476 volts. Since you are starting with 13 volts average this drops the voltage at the radio to 12 volts.

    If you connect the negative lead to the body of the vehicle the IR drop reduces to almost zero in that lead. There will be a very slight drop but the body of the vehicle has considerably less resistance than a single piece of wire.

    Now when you are starting with 120 volts a drop of 1 volt is not going to be noticed. However, when you are starting with 13 volts that same 1 volt drop is substantial.

    If you were to use 10 gauge wire which has a resistance of 0.001018 ohms per foot. This equates to a resistance of 0.01018 ohms in a 10 foot long section of wire. Add the 0.01 ohms in the fuse/connectors and this becomes a total resistance of 0.02018 ohms. At a current draw of 10 amps this gives a voltage drop of 0.2018 volts for a single connection and 0.4036 volts when 2 wires are used.

    If you were to use 8 gauge wire with a resistance of 0.0006405 ohms per foot then the resistance of 10 feet of wire becomes 0.00605 ohms. Adding in the 0.01 ohms from the fuse/connectors this make a total resistance of 0.01605 ohms. The voltage drop at 10 amps current becomes 0.1605 volts for a single wire and 0.3210 volts if 2 wires are used.

    These figures are based on a "best possible" situation and the "average" installation is going to have more resistance in the wiring and therefore a larger voltage drop on transmit. Since the receiver portion of the unit draws considerably less current the length and gauge of the wire is not going to be that much of a factor. However, when transmitting the voltage drop in the wiring becomes important and, if careful attention is not paid to the actual length of the wiring and the care taken in installing the various connectors, then when transmitting the lower voltage applied to the radio can seriously affect the performance of the unit including a degrading of the quality of the signal.

    If you don't believe this I suggest that you use a digital voltmeter attached between the input voltage to the unit and the body of the vehicle to verify the input voltage to the unit when just receiving and when transmitting. You will definitely see a drop in that voltage reading when transmitting. Just how much the voltage drops is going to depend on both the length of the wire and the gauge of the wire.

    Glen, K9STH
    Last edited: Nov 12, 2009
  13. N3UJM

    N3UJM XML Subscriber QRZ Page

    There hasn't been a good negative ground car since the '70's.
    Plastic interiors, composit body panels, spot welds replaced by adhesive, plastic intake manifolds and whatever other metal they took out to save weight, it's not the faraday cage of old. The screws in and under the dash aren't ground, and if you find a good ground under the carpet, you grinding paint off the screw and metal to make good contact....

    As Jay Leno said about his '58 Buick Roadmaster " No padded dash here...if you crash your head into this baby, they just hose it off and sell it to the next guy:.

    Run both wires to the battery.
  14. N4CR

    N4CR XML Subscriber QRZ Page

    It's most likely that the chassis on even the newest cars provides a better path to ground than #12 wire to the battery. I would always attempt to use the chassis first. Running a ground to the battery is a last resort unless you're wiring a Corvette. Not to mention that a ground fault burning up all of your ground wiring is only a risk if you run a new ground to the battery.

    If you must run a ground wire, be absolutely certain to fuse it at the battery.
  15. KJ4PLT

    KJ4PLT Ham Member QRZ Page

    IDK who told you that but they were wrong. every metal part inside the car that touches the floor pan is a ground. that includes the floor pan itself, the fire wall, seat mounts, just about anything. i have been doing car audio and have never had a problem finding a good ground where i needed it. i cant count the number of car audio installs i have done nor the number of CB's and have never had a grounding problem. the "frame" of the car is grounded to the battery(the body is also grounded because of that), the motor is grounded to the fire wall in most cars. the trans computer, ECM and pretty much any electronic part inside is grounded to the firewall, floor, or even the kick panels.
    Last edited: Nov 12, 2009
  16. N3UJM

    N3UJM XML Subscriber QRZ Page

    Audio is one thing ...RF is another.
    I do communication payloads in mobile platforms for work. Every powered box in the system is brought to a common ground, not one screw on the rack here, another screw on the shell there.... And when I;m done I compare the receiver noise floor on a spectrum analyzer, showing the improvement in common point star grounding.
    That common ground is either a seperate strip or bar mounted next to the battery.

    Do it whatever way you want.
    Last edited: Nov 12, 2009
  17. N8ODF

    N8ODF Premium Subscriber QRZ Page

    Glen is absolutely correct in his response & calculations...a DC plant is much different then AC....use stranded wire, 12 is OK...however I would use can always increase the size of your feeder...& if you a conductor for the ground to the battery....can't hurt...make sure you protect the wire with a grommet or tape where you pass through the firewall...don't know about the spectrum analyzer noise floor thing...doubt if it is very important with your type installation...Good Luck
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