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Multiple ground wires to a single location?

claycookiemonster

claycookiemonster

Well Known Member
I have two grounding blocks (forest of tabs) just behind the instrument panel. Total of 40 tabs available. I also have many, many leads needing grounding. While I believe I have more tabs than leads needing to be grounded, it also seems like I could piggyback several leads onto a single tab to leave room for the future. What limitations might there be to doing this, other than making things crowded around the multi-lead tabs?
 
My professionally-built harness by a well known vendor has 2-3 ground leads to a single tab in many/most cases.
 
I think it’s personal preference up to a certain point. I wouldn’t jam three 16ga wires into a single crimp on a single connector. Also, some avionics devices have multiple, redundant ground pins. Wiring them into a single ground wire defeats the redundancy.

Personally, I stuck to: one ground, one tab. But I ended up using absolutely every tab so expanding my system will be difficult in the future. I’m willing to live with that.
 
Thanks all for your suggestions. FYI, I posed the same question to Stein's, who made my wiring harness. There is a bundle of black ground leads exiting from the harness. The answer from Stein was to go ahead and group the grounds as follows, "Yes, you can combine 2 or 3 wires per terminal as well. AWG22 I would combine 2 or 3 wires, AWG20 I would combine 1 or 2, and AWG18 or larger use one terminal each."
 
I have three grounding positions in my RV8, primarily for convenience. One in the avionics bay for everything avionics, electrical switches, comm. One in the engine bay for all the engine related grounding and sensors that are in the engine bay. I didn't want to run more wires through the firewalls. The last one is in the lower fuselage for other minor lighting because the wire runs are shorter and I was running out of tabs in the forest of tabs
 
Just remember what a ground loop is and how it's formed. All avionics grounds to one forest of tabs = very good idea. Lighting and DC motor circuit grounds = probably not critical as to loops.

Also do a mental FMEA: can I afford to lose X-many items if a single tab works loose/has a crimp failure.
 
Draker said:
Also, some avionics devices have multiple, redundant ground pins. Wiring them into a single ground wire defeats the redundancy.
Click to expand...

Well that explains my GPS 175, wasn't sure why there were 2 power and 2 ground wires goin together must be for pinfail though because it shos them going to the same source
 

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Ruready said:
Well that explains my GPS 175, wasn't sure why there were 2 power and 2 ground wires goin together must be for pinfail though because it shos them going to the same source
Click to expand...
Either that or current-carrying capability of dual wires. I don't think a GPS navigator would draw that much, but a single D-sub pin is rated for a max of what - 8A?
 
Bill Boyd said:
Either that or current-carrying capability of dual wires. I don't think a GPS navigator would draw that much, but a single D-sub pin is rated for a max of what - 8A?
Click to expand...

A regular D-sub pin can handle 5 amps. A high density is only 3.

It's pretty common for Garmin to have 2 power and ground pins for boxes that need more than 5 amps max because one pin can't handle that whole load.
 
IMG_0722.pngClay
You can make more busses for the grounds this way. Out of the aeroconnection book.
IMG_0722.png
 
D-Sub connectors are good for 5A. I like using these, the ground block is physically strong, the pins are rated for 15A, they are gold plated for long life and reliable connections. And the mounting connector screws on.


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34 Pin Grounding Block | Aircraft Spruce

34 Pin Grounding Block Grounding blocks are ideally suited for aircraft applications, where wire is to be grounded to an airframe.
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They are expensive, but if you shop around you can get these for a reasonable price. And you must get a DMC AF8 crimper with the right positioner.

I don't like connecting multiple wires to one pin, unless it's something like a set of lights etc.

Much better to make it more reliable and easier to change out stuff in the future.

Don
 
Desert Rat said:
A regular D-sub pin can handle 5 amps. A high density is only 3.

It's pretty common for Garmin to have 2 power and ground pins for boxes that need more than 5 amps max because one pin can't handle that whole load.
Click to expand...
Learned something today. I always thought they were for redundancy, and that the Garmin units would draw power from either OR both. I’ll have to compare a few of my wiring diagrams with the Garmin manual. I wired several of my avionics using that assumption (one pin from the main bus, one from the backup bus, for example).
 
Draker said:
Learned something today. I always thought they were for redundancy, and that the Garmin units would draw power from either OR both. I’ll have to compare a few of my wiring diagrams with the Garmin manual. I wired several of my avionics using that assumption (one pin from the main bus, one from the backup bus, for example).
Click to expand...
In my experience they will explicitly state in the installation manual when multiple diode-isolated power inputs are supported; if they don't, then there is only one power input. Typically it seems that the TSO CNI products (navigators, transponders, etc) only have a single power input, although they may use more than one connector pin due to current requirements.
 
On a related note, I think it is useful to label grounds. I put numbers on all my ground wires and have a spreadsheet matrix of those wires which maps to the forests of tabs. It took me and my intern a day and a half to do this. When a device stops working first thing you do is check power and grounds. Imagine finding a bad ground at a device but not knowing which of your 80 grounds is at the other end (loose spade connector for example).

Warm regards,
 
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