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Author Topic: Seperate power/ground cables  (Read 2989 times)
KG4NEL
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Posts: 541




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« on: September 24, 2003, 11:50:08 PM »

Just got through running 2 gauge cable through the firewall today...that stuff's interesting to work with Wink

Anyway, while I was working under there I thought of a way I could organize my cables a little better. What about running a single #8 or #10 gauge feed, fused directly at the battery to a distribution block at the firewall. The power cables are already at the firewall, so I'd connect the positives to the block and the grounds to a solid chassis ground. Or, instead of the additional #10 wire I could run the 2 gauge into a T distribution block and route the positive connection from there. Any reason why this wouldn't work? I've always wondered why this is common practice in car audio to just ground the amp and battery side, and not run two seperate cables.



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N2HBX
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« Reply #1 on: September 25, 2003, 08:22:02 AM »

Hello,

The main reason for doing the grounds the way car audio places do it is to eliminate "ground loops" which can induce noise and hum in the amplifier.

Many commercial high power trunk mount radios are also wired this way, with a single heavy hot lead going to the battery and grounded with a short lead in the trunk.

When we're wiring police vehicles, here's how we do it:
Radio (and siren and lighting) hot leads go directly to the battery with appropriate fuses. Usually there is a radio console to mount multiple pieces of equipment. Under the console is a fuse block for accessories (flashlight charger, light controls, etc.), fed by a single #8 or #10 fused line from the battery. This is where their hot leads go.  Everything in the console is grounded to a single point ground in the passenger compartment, usually one of the console mounting bolts. Front mount radios are also grounded to this point. As before, trunk mount radios are grounded with their short leads in the trunk.  We never, never, never connect grounds directly to the battery.  This only invites trouble if for some reason the cable between the negative terminal of the battery and the vehicle frame fails.

So I guess the short answer to your question is, yes, you can run just a single fused line from the battery to your intended distribution point and the ground side of your equipment to a (preferably) single point ground.

Good luck and 73,
Larry, N2HBX
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K0BG
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Posts: 10248


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« Reply #2 on: September 25, 2003, 10:38:27 AM »

Running large gauge wire (positive and negative) is as pointed out, a noise abatement procedure as much as it is to feed power to the rig/amp/etc. You should always run two wires (+ and -). Relying on the chassis for ground return is not a good idea. #2 is overkill unless the runs are very long (over 40 feet). Typically size 6 is enough.

The use of fuses in both leads is mandatory for safety reasons. Having fuses on both ends is also handy especially if more than one rig or accessory is used.

See articles here on e-ham.net, number 4407, 4424, 4425, and 4623.

Alan, KØBG
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AA4PB
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Posts: 15066




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« Reply #3 on: September 25, 2003, 11:38:21 AM »

One thing to watch out for when taking ground directly to the battery is getting alternator whine caused by charging currents flowing in the multiple grounding points. Usually, there is a ground connection at the antenna, and one at the radio mount in addition to the wire run back to the battery negative. With some radios the radio ground wire goes inside the radio and connects to a PC board which takes noise currents right into the circuitry. In my case the whine was eliminated by providing an additional good ground connection to the radio case and mount at the connection between the ground wire coming out of the radio and the ground wire I ran to the battery.

Be sure to fuse the neg connection to the battery (in addition to the pos) in case a bad battery connection causes heavy starter currents to flow through your radio ground wire. It would not be a pretty sight to pull 100 Amps through your coax shield!

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Bob  AA4PB
Garrisonville, VA
KG4NEL
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Posts: 541




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« Reply #4 on: September 26, 2003, 12:20:23 AM »

BTW, I was running the #2 for high power car audio amplifiers, not radios. I've got some extra #10 lying around, so I'll just use that.

I've just always wondered why it's almost standard procedure to run seperate power/ground cables for radios, while in car audio wiring it's almost unheard of - and you don't hear of massive noise problems with that.



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AA4PB
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Posts: 15066




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« Reply #5 on: September 26, 2003, 10:39:19 PM »

I think taking the ground back to the battery is good engineering practice, in general. It would be for the amps as well. It's mostly a matter of not depending on the electrical connection between various painted automotive chassis parts - something you have very little control over. Many (perhaps most) times you could get away with just grounding the rig to the closest point on the vehicle. I like to be on the safe side and run a dedicated ground wire.
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Bob  AA4PB
Garrisonville, VA
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