mobile: to bat or fuse box?

Discussion in 'Mobile Radio Systems' started by KD0KKV, Jan 5, 2011.

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  1. KD0KKV

    KD0KKV Ham Member QRZ Page

    I'm working towards getting a mobile rig for my truck. I've looked into the different ways to wire the power up. It seems like it would be easiest to wire from my fuse box for the positive line and find a connection that powers off with the key. Would it be better/safer to do it this way, or wire straight to the battery with a fuse at the battery and remember to turn my rig on and off? Also, if I wired to the battery, is there a way to get the power to cut off when the vehicle is off?
  2. W6CD

    W6CD Ham Member QRZ Page

  3. KD0KKV

    KD0KKV Ham Member QRZ Page

    Fantastic read! It looks like I need to wire straight to the battery and use the powerwerx APO3. This is exactly what I wanted. Thanks!
  4. N0AZZ

    N0AZZ Ham Member QRZ Page

    Allways the battery evev back to the early 70's with CB :eek: radios.
  5. KF5BTI

    KF5BTI Ham Member QRZ Page

    Not sure what rigs you are looking at but I have a couple of Yaesu FT-2800 radios that have a battery saver feature built in. All of mine wired direct and if I forget to turn one off after 1 hour of no button/vfo activity the radio turns itself off. Others may have this feature as well but I can only speak for the FT-2800. Another option would be wire to the battery and use a relay to switch the main power on/off.
  6. KD8NPB

    KD8NPB Ham Member QRZ Page

    Anything that has a significant draw, I wire directly to the battery.

    Be sure to place a sufficient fuse within 12" of the connection to the positive terminal.

    Be sure the device is fused too.

    Be sure to use adequate wire for your amperage draw.

    With that being said, 8 AWG stranded wire which is designed for amplifier installation for mobile audio is great for high-current applications and should handle almost all common installations.
  7. W1GUH

    W1GUH Ham Member QRZ Page

    Thanks! I need to re-do my mobile wiring and was contemplating going to 8AWG. Appreciate the info.
  8. K0RGR

    K0RGR Premium Subscriber QRZ Page

    For many years, I've always gone straight to the battery.

    I've tried a lot of other solutions, with mixed success. When redwood trees were just saplings, I wired a few 2 meter rigs to the fuse box. First thing to consider is that these were 1, 5, 10, or 25 watt rigs, max. - never a 50 watt one.

    Second, cars in those days did not have computers. Wiring into the fusebox increases the chance of getting RF into the sensitive car systems, and that's not a happy thing.

    The same thing applies to cigarette lighter plugs, which are just are poorer way of connecting into the fusebox. I still use them for very temporary installations, but that's all. I've seen too many of them melt and/or catch fire due to 'thermal runaway' problems, which can also happen with connections made directly to the fusebox if not done correctly.
  9. WA7PRC

    WA7PRC Ham Member QRZ Page

    Direct to the battery is preferred (with a fuse at/near the battery). If you want to turn the rig off with the ignition switch, you can use a relay to do that for you. You didn't say what rig you'll be using but, the relay contacts need to be rated for the current you'll be pulling. If you mount the relay not inside the passenger compartment, it needs to be environmentally protected (weatherproofed). The relay coil is wired (through a small fuse) to the truck's Accessories circuit.

    About wire size. You want to minimize the voltage drop in the wire to a small percentage of the nominal system voltage. If your limit is 5%, that's 0.69V at 13.8V. Your rig's user manual will tell you how low the voltage can be. Note: your truck's system voltage is ≈13.8V with the engine running. With the engine off, a fully charged battery is around 12.6V. Consider your battery's worst-case terminal voltage (and still being able to start the engine) minus the lowest voltage your rig can reliably tolerate, and that's the allowable voltage drop in your wiring. Example: 12.0V - 11.5V = 0.5V. Many consider it best to run only the positive wire to the battery and pick up your negative return through the vehicle chassis, rather than having your negative lead return to the battery. This cuts the voltage drop in half.

    Now, you need to know what is the smallest size wire you can use versus the length versus the current. There's a nifty spreadsheet calculator that takes the cypherin' out of it.

    For example:
    Wire run = 10'
    Wire size = 12ga
    Current = 20A
    Wire Resistance = 159mΩ
    Voltage Drop = 0.32V
    Temperature Rise = 22°F (12°C)

    Thus, 12ga would meet the requirement in this example. 8ga would be way overkill (but, better than underkill). :D
  10. KD0KKV

    KD0KKV Ham Member QRZ Page

    Ok, looks like a relay will do what I need and is cheaper than the powerwerx jig. Thanks for all the great info everyone. And for those wondering, it's not definite yet but I've got my eye on the Yaesu 8900R.
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