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Planning an install, help with finishing touches...

Discussion in 'Mobile Radio Systems' started by N8MLP, Sep 9, 2019.

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  1. KF5LJW

    KF5LJW Ham Member QRZ Page

    Yep he does reference NEC which has nothing to do whatsoever to do with a Mobile Radio Installation. NEC is for building and home wiring. Not applicable, out of scope, and has no other purpose being there other than taking up space to fill a page sounding important.

    There are some parts of NEC you can get some guidance on most notably is roughly what is the smallest wire you can get away with used with what size fuse. But here is the catch, yes it is safe, but may not work well. Reason is simple, NEC is not a design or How-To document. It is minimum life and safety requirements and does not address performance issues like voltage loss in low voltage systems like 12 volts. If that 12 AWG with a 20 amp fuse, 18 amps when in TX, with say 20-feet of 1-way wire distance is going to loose about 10% voltage and power.

    One thing the NEC or Marine electrical codes would never allow you to do is use Ground (chassis) as a circuit conductor like the Automobile industry. That would be extremely dangerous and a lot of people would be hurt with fatalities, and property damage.
    Last edited: Sep 20, 2019
  2. K0BG

    K0BG Ham Member QRZ Page

    An average 20 to 30 amp ATC fuse has 2 to 3 milliohms of resistance. Test there is a bit of drop, but it is inconsequential. The reason to leave the fuse is to prevent setting up a ground loop. So transceivers are very susceptible, others not. SO unless you have some knowledge of the make an model, it it best to error on the right side.

    I do have one question to ask however. Why do you use such large type? Are. you trying to prove a point? If so, that isn't the way to do it.
  3. KF5LJW

    KF5LJW Ham Member QRZ Page

    Yes 2 to 3 milli-ohms is small. Now add the resistance of the two connections to put the fuse in-line where it should not be to start with. You just added more resistance and noise than necessary. You shot yourself in the foot.

    There is the I GOTTCHA point. The only way to make it a bad loop is if you are ignorant enough to connect to the Negative Battery Term Post as you advocate. Connect at the chassis and you have a dedicated circuit and eliminate a whole lot of problems that have escaped you. When you put the radio Negative Wire in parallel with the battery Bonding Jumper (factory wire ran from battery neg term post to chassis) as you recommend, not only do you risk burning up your coax and radio negative wire, a portion of all the vehicle normal load current are now flowing through your radio. Simple parallel circuit laws. That means your headlights, ignition, stereo, fan, power seats, navigation, you name it are now flowing through your radio. What do you call unwanted voltage and current?


    Like I said it is simple parallel circuit laws which cannot be debated. If you draw the circuit out, it is blatantly obvious what the problem is and why you should never ever connect to the Battery Negative Term Post or use a fuse in the Negative. It is ignorant and dangerous. If you are stupid enough to connect directly to the Negative Battery Term Post, then you had better use a fuse to protect your dumb arse from yourself.

    Now what you could say and recommend as a second alternative, You can run the radio negative to the engine compartment. Locate the factory bonding jumper that runs from the battery Negative Term Post and Chassis. Lift the Jumper from the Chassis, and place the radio Negative on top, NOT BELOW. Essentially same as directly to the Battery Negative Term Post, however does NOT put you in PARALLEL with the Battery Jumper, and still gives you a Dedicated circuit not sharing any load current with anything else. Safe and effective.


    So I can read it.
    Last edited: Sep 22, 2019
  4. AI7PM

    AI7PM Ham Member QRZ Page

    Hummm. I wonder why all of the negative leads from all of the electrical parts in the vehicle aren't negative fused and run to the battery? Asking for a friend.
  5. NG1H

    NG1H XML Subscriber QRZ Page

    1) Cost. Fewer runs and almost half the wire.
    2) Weight. All of the wiring is a material percentage of the vehicle's weight. By doing so you are cutting this weight almost in half.

    No, it isn't a safety issue and no, it doesn't provide less resistance over a fused link.
  6. K0BG

    K0BG Ham Member QRZ Page

    Phil, that is a good question. There is a basic difference between the accessories in a vehicle, and an amateur transceiver—the latter has multiple ground connections. The antenna and its feed line are grounded to the chassis (or should be); the transceiver's body (frame, chassis whatever) is grounded; and so is the power ground. These multiple grounds can, and often do, cause ground loops to occur. Although rare, when they do occur, they manifest themselves as RFI, and are thus very difficult to trace (their origin).

    Internally, transceiver grounding points very with both make and model. So making a pat answer to solving a specific issue, is haphazard at best. I can't speak for others, but I always errored on the side which offered the least chance of creating a ground loop.

    I should add, that vehicle grounds are not haphazard as they used to be. We can all remember (if we're old enough) seeing a vehicle with one headlight much brighter than the other due to multiple ground points. Nowadays, that's not done. Look at any modern vehicle, and you'll see grounding points shared (the headlight grounds for example). Other accessaries are grounded the same way. Wheel sensors for example, all use home-run grounding leads to the same chassis grounding point. The real reason this is done, is the signaling (via B-CAN typically) is digital, and even a slight ground loop can upset the digital data stream.

    I trust this clarifies the issue.
  7. AI7PM

    AI7PM Ham Member QRZ Page

    My comment was somewhat tongue in cheek, but yes.
  8. KF5LJW

    KF5LJW Ham Member QRZ Page

    No sir you have not. You made the biggest nasty ground loop you could possible make and you do not understand what you are saying. If you bond to the battery negative term post, you are asking for big trouble and do not comply with any known electrical codes. You do not understand if you use the battery negative term post, you are forcing all the vehicles load current through your radio. That is a Nasty Ground Loop. You are forcing current to go somewhere it is not intended to go.

    The schematic shows cleanly what is going on. You and your blog is going to get someone hurt, damage their radio, and/or void their vehicle warranty.
  9. W5GX

    W5GX Ham Member QRZ Page


    I'm confused with a few of your posts - see below:

    Another thread:
    This thread:
    In the first post, you describe how to correctly connect near the negative battery terminal - and I would agree with the practice, for the reasons you state.

    However, the second post, would imply that going to the negative terminal as directed in your first post is incorrect and should not be done.

    Is the first post just to point out how to do it correctly, and no comment whether there is a better method?

    Both practices isolate the radio as a parallel ground path for other circuits, but the second would be better to control ground loop problems, right?
  10. NG1H

    NG1H XML Subscriber QRZ Page

    In addition to flip-flopping on issues add not understanding the difference between chassis ground and electrical ground, and posting on the horrible danger of exposure to 12VDC. I think we've got an example of Dunning Kruger. A knowledge, perhaps even correct knowledge, of one subset of the field is thought of as an expert everywhere.

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