Something to think about

Discussion in 'Mobile Radio Systems' started by K0BG, Jan 21, 2019.

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  1. K0BG

    K0BG Ham Member QRZ Page

    One of my favorite movie lines, is one made by Ian Malcolm, as played by Jeff Goldblum: “Your scientists were so preoccupied with whether they could, they didn't stop to think if they should.”

    Basically the same thing can be said about installing amateur gear in and on a vehicle, late model or otherwise. To paraphrase… Just because you can, doesn’t mean you should. Perhaps where this scenario rears its ugly head more than any other, is in the power wiring of amateur gear. The major rule is, and always has been—Never use existing vehicle wiring, period! The most egregious wiring faux pas, is using accessory sockets to power amateur radio gear. And here is why you should not!

    While there are exceptions, most are wired with #16 AWG, or the Euro or Japanese equivalent. And most are fused at 10 amps (those fused at 15 amps just add insult to injury). This is where the 120 watt rating comes from. But is isn’t the rating, amperage, or the wire size that is the biggest issue. Rather it is long-term, resistive heating, also known as thermolysis.

    There are at least a dozen manufacturers of accessory socket plugs. All of them rely on a spring-loaded, rounded point contact, whose area is very small. By small, I mean about one square millimeter! This is roughly equivalent to the cross sectional area of solid #18 AWG wire. The resistance of the point contact when new, is typically 4 to 8 milli ohms. As these contacts wear, the junction resistance increases with predicable results—something burns!

    We also have to keep a few other thoughts in mind. First, a 50 watt 2 meter transceiver typically draws about 110 watts. The difference is comprised of final efficiency losses, fan, and other parasitic draws. As a result, you can’t use the wattage rating and power output rating as equivalents.

    Next, we need to remember that vehicle wiring is tightly bundled. Sometimes those bundles contain 50 or more individual conductors. Quite obviously then, we can’t use open-air, AWG ratings as gospel. The National Electrical Code specifications, rate #14 AWG at a scant 5.9 amps! This is slightly higher than that the SAE rating.

    Whatever vehicle you own or lease, is up to you to maintain. It is also yours to wire as you see fit. If you wish to take a chance by incorrectly wiring your amateur radio gear, by all means do so! The same goes for mounting gear, antennas, you name it. But keep in mind, that sooner or later, short cuts will indeed short circuit!
    KU4X, K0UO, KD5BVX and 7 others like this.
  2. US7IGN

    US7IGN Ham Member QRZ Page

    As always, your information is valuable and useful. Thank!
  3. K8MHZ

    K8MHZ Ham Member QRZ Page

    The ratings of conductors in the NEC is based not only upon conductor size, but insulation type. Since automotive wiring is not one of the insulation types the NEC has ratings for, it's ratings mean nothing. Also the scope of the NEC excludes automotive applications.

    That being said.....

    The fact that there are many wires in the cables that the wire turning into a heating element resides in means a good many may be damaged once the first wire gets too hot. That is a bad, bad thing.

    I know many people have used the accessory sockets, which used to be cigarette lighter sockets, with no problems. I have done it myself. But that doesn't make it a good idea, especially with newer vehicles. Wiring harnesses, if available, cost mega bucks nowdays.
    KD5BVX likes this.
  4. AF7IN

    AF7IN Ham Member QRZ Page

    Great post!

    To add to the discussion, there is also the potential issue of line noise, which can cause issues in some circumstances when using accessory circuits rather than wiring directly to the battery.

    And always make sure - if/when you wire your radio (both pos & neg) directly to the battery - that you fuse both the positive and negative leads. This will prevent possible radio/equipment damage should the battery-to-ground connection of your vehicle be compromised (which could cause high currents to seek ground through your radio).

    Alternatively, if you choose to ground to the chassis, omit the negative fuse. Personally I prefer to go direct to the battery in order, so if I have noise issues, that's one less thing to factor in the mix.
  5. N4MU

    N4MU Ham Member QRZ Page

    Okay...Stop the insanity (mine)! I have just finished rewiring my 2011 F-350 diesel for a future HF 100W transceiver. I already have a 50W dual band installed. The wiring was done directly to the battery terminals and is fused (25A) on both pos and neg leads at the battery. Now I read Alan's warning about the current sensor(s) that should not be bypassed! I don't even know whether I have any in the 2011...but I'll find out. My dilemma is what's (bypassing possible sensor's) or downstream from any sensors. Now I'm reading both recommendations. Someone please offer a "bottom line" so I can sleep at night again. BTW, the V/U radio has been in there about 2 years and thousands of miles without incident or blown that any evidence that may be helpful? I need a drink...
  6. K0BG

    K0BG Ham Member QRZ Page

    You do! Connect the power ground lead to the same spot on the chassis as the battery's ground.
    KD5BVX and AI7PM like this.
  7. KF5LJW

    KF5LJW Ham Member QRZ Page

    Complete rubbish. NEC has no such limitations in Table 310.15. The lowest you could possible rate 14 AWG even using piss poor 60 degree insulation is in a 41 core cable bundle is 7.5 amps. SAE is even higher because they use higher temperature insulation, and because you can escape a burning car easier than a house or building. NEC Table 310.15(B)(17) aka Free Air 90 degree insulation is 35 amps. However that Table is rarely used and thus Table 310.15(B)(16) aka 3-current carrying conductors in raceway = 25 amps.

    I am certainly not suggesting anyone use the vehicle wiring harness or 14 AWG. Quite the opposite. run dedicated conductors and KISS using a very good Marine Table using 3% voltage drop is more than good enough and extremely safe. Marine Standards ABYC E-11 are more conservative than NEC and SAE thus safer. Use the current rating on the top as your fuse size, find the 1-way distance and you will find th ewire size. Unless you use a fuse larger than 30 amps and a 1-way distance greater than 10 feet, 10 AWG is about as large as you will ever need.

    Here is an excellent KISS Chart.

    KX4O, KD0CAC and WA4SIX like this.
  8. AB4KA

    AB4KA XML Subscriber QRZ Page

    I've got a 2018 F-150, and it's got the (in my opinion) incredibly stupid "auto stop-start" feature, which shuts the engine off anytime you're stopped with the vehicle in drive and your food on the brake. I've read where wiring your rig directly to the positive and negative terminals on the battery can cause problems with this system such as it shutting off and not restarting. When I installed my 2m/440 rig I ran my positive line to a big fat terminal in the fusebox under the hood where the hot line from the battery attaches to it. I ran my ground to a chassis ground under the hood. Everything seems to work fine and I've had no problems with it.
  9. N4MU

    N4MU Ham Member QRZ Page

    4KA: Interesting. Well, mine's older (2011) and is a 6.7 diesel so I'm not sure what I may have. I'm going to take a serious look today. Never worried too much about it before. Too make things even more interesting I have two batteries (takes a LOT of pizzaze to crank this puppy) so I'll have to try to see where things are connected from each. Trouble is, as with many vehicles, there's not much room under there anymore. BTW, the 350 diesel will be happy to run all day in gear and my foot on brake...hmmm. So how do you keep from having to restart every time you're stopped at a traffic light?
  10. K5DH

    K5DH Ham Member QRZ Page

    To AB4KA: I drive a 2018 Ford Escape (yeah, I know, not the same as an F150, but it is a Ford). FWIW, having my three radios wired directly to the battery (+) and chassis (-) as K0BG suggests has had no ill effects on the "auto stop/start" system. My HF rig (Icom 7000) definitely doesn't like it when the car re-starts after shutting itself off, so when I have the HF rig on, I press the "disable" button on the dash to turn off the stop/start feature. My Escape has a menu setting to completely disable it; you might have that on your F150 also.

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