[szicree]

I'm pretty much going with Bob Nuckoll's advice on this plane but have a question regarding my main ground connection. I've got my battery between my feet but my ground block well up on the upper left of the firewall. I really hate to run a battery cable all the way up there when I could just as easily run a cable straight forward from the battery to the firewall. This would only require about a foot of cable and would then rely on the firewall itself to conduct up to my "forest of tabs". I'd love to hear pros and cons on this. Man, it feels so good to be near the end of the wiring.

 

[gasman]

Don't use just the firewall for a ground.....

Drill through a firewall brace and pass a copper or brass stud through the firewall. Connect the battery to the stud. and connect a cable to go to the ground buss.

On the engine side...
connect a cable (the same size as the one going to the starter) from the stud, to the ENGINE CASE.

If you can find some "DE-OX" made by the UTILCO COMPANY in Cincinnati Ohio, brush all of the surfaces and threads before you connect them.

This stuff is an oxide inhibitor for alum and copper terminations and it will give a clean tight elect. connection. Utility companies use it to go from alum wire to copper.

 

[szicree]

Drill through a firewall brace and pass a copper or brass stud through the firewall. Connect the battery to the stud. and connect a cable to go to the ground buss.

I don't get it. Why would I run a ground cable from battery to firewall, and then from firewall to my ground block (which is on the firewall) when I could just go straight from the battery to the ground block?

 

[gasman]

You need a GOOD ground for the motor case so the starter and the alternator has the best chance to work properly.

You also want the BEST ground to the ground block to avoid resistance in the connections that can cause guages and equipment to not function properly.

I placed my ground block on the firewall stud.

 

[Kevin Horton]

Stainless steel tends to develop a very, very thin protective coating of oxide on its surface, and this oxide is non-conductive. Apparently it can be difficult to get a good quality electrical bond to stainless steel.

 

[gmcjetpilot]

Run the wire, just not a big one

One fix move the ground block?

But better, just run a wire, smaller gauge from battery to your gnd block/tabs or from a ground stud in the firewall you attach the BIG GROUND wire to You don't need #2 wire up there to the ground block, #6 or so. Yes the tabs will get some ground through the firewall as well, but you don't want to rely on it alone.

You of course need to run a BIG GND WIRE to the engine. I know you spent some sweat on the POS wire grounding on the firewall. With the ground it does not matter does it.

The GOOD news is the ground cable can be ONE Piece, from battery to engine, just passing through with a simple grommet; Of course you need a second wire off the battery to your ground block.

The second method and may be better, use a bolt conveniently located in the firewall to ground the battery with a big braided or #2 wire, and on the engine side, a short jumper to the engine off the same bolt.​

The bolt/stud saves you the grommet, gnds firewall and gives you a stud/terminal to run a ground wire from (like to your ground tabs).

You may want another gnd jumper to local aluminum structure. If you look at a car you see from the battery, the big ground to the engine block, than one small gauge wire to a local ground, sometimes a second ground wire running off somewhere.

Corrosion: Yes the firewall is conductive but copper or brass and stainless is not a horrible galvanic pair, but it depends on type of stainless and brass/copper. Some corrosion will occur, but if it's dry, it will be minor. Try to use passivated coated bolts to attach the brass to the stainless. I don't know what stainless the firewall is or what kind of brass or copper you are using. The point don't use the SS firewall only; run that wire from the Bat to your ground block with a #6 or #4. It does not weigh much: 0.25 to 0.33 lbs for 3 feet. Voltage drop is about 0.12v to 0.08v @ 100 amps! I'm guessing you don't have 100 amps.

 

[szicree]

Ahh, I get it. Big wire to post on firewall, small wire to ground buss. I now have a related question. My main buss only carries about 20 amps, and would only need a wire about 5 feet long (at most). Under these conditions I think 10 awg is ok, right?

 

[gmcjetpilot]

It's up to you it will work

Ahh, I get it. Big wire to post on firewall, small wire to ground buss. I now have a related question. My main buss only carries about 20 amps, and would only need a wire about 5 feet long (at most). Under these conditions I think 10 awg is ok, right?

Don't get too cheap or stingy with wire gauge, this is your primary electrical distribution wiring.

5 feet of: #6 is 0.40 lb, #8 is 0.25 lbs, #10 is 0.16 lbs.

5 feet @ 20 amps voltage drop: 0.04, 0.06 and 0.10 volts respectively for #6, #8 and #10.

5 feet or less? Yea I agree it will be less. From center of floor where a typical RV-4 battery is, to anywhere on the firewall on a RV-4 is not far, might be 3 feet? (3 ft makes the weights above 40% less, so #6 is only 0.24 lbs or 3.8 oz's, the #8 is 0.10 lbs or less than 2 oz's. No need to save wire here.)

Its up to you. Ground is as basic (and important) as it gets; you don't want to save the weight here in my opinion. You are looking at saving may be 2 oz's between #6 and #10. Go for the lower voltage drop. I like a good low resistance ground, but #10 will work, but larger dia wires give less voltage drop and heat. Lowering voltage drop is goodness.

Since this feeds all circuits, suggest at least #8 or #6, my personal recommendation, even at 20 amps. They do make crimp terminals in the #6 and #8 size by the way. #6 wire dia (stranded wire itself no installation) is about 0.186 in max (3/16"), add a little insulation , its not that big around. The less voltage drop the better. Now this ignores the metal firewall. However that is the assumption. In engineering you often ignore something that helps you, and size the main path for the full load, not counting on the helper.

On a related note, the reason you hear noise in the audio, is usually on the ground side, being affected by small voltage differences in the ground circuits. This is why a CENTRAL GROUND block for the panel is a nice thing. Its 1/2 the battle of avoiding noise. This really has nothing to do with this main ground wire, but it just shows that minimizing voltage drop is important. Also why not give your central ground tab block a GOOD solid ground. Since this one ground wire affects all circuits, its a good idea to not compromise on size it. Now #10 or #6 will not affect noise but total voltage drop. You're also getting voltage loss on the POS side of the individual circuits; long runs are affected more of course, and you don't want to add to the total voltage drop on the ground side, more than necessary.

 

[szicree]

One more question?

Thanks George and Gasman for the info. I do have another question. It was suggested that I poke a hole in the firewall through a stiffener to mount a stud for the cable/strap. I don't really have a good location that includes a stiffener, so what would you say to just a 5/16 hole right through the lower center firewall with a nice big washer on either side? The cable/strap will be well anchored on both sides, so it's not like it'll be hanging or flopping around. I really appreciate all the advice on this stuff. It can be a little tough balancing the desire for simplicity, functionality, serviceability, safety, aesthetics, etc.

 

[gasman]

I do have another question. It was suggested that I poke a hole in the firewall through a stiffener to mount a stud for the cable/strap. I don't really have a good location that includes a stiffener, so what would you say to just a 5/16 hole right through the lower center firewall with a nice big washer on either side?

That should be no problem. On my six, we have braces at any given location.

Use washers that are of the same materal as the stud that you use and you will be fine. This will give you a good ground to the airframe just incase you ground a light to the frame, it will work. Do not ground any gauges to the frame, they must go back to your ground block.

Try to use De Ox it really is good stuff for connections. It will seal the connection from moisture and air...........

 

[szicree]

Thanks Gas'. Yea, I figured I'd go with brass studs, washers and nuts. I'll try to find som De-Ox.