GND Wire question

[RVG8tor]

I bought one of the AeroElectic ground bus bars, which is a 37 pin connector with all of the pins on one side soldered together. The items that need a ground get pinned into the other side of the connector, along with 5 20 AWG wires that go to the airframe ground. Instead of the five 20 AWG wires, they suggest you solder two 14 AWG wired to the back where all the pins are soldered together. Since one side is all connected each item can find a route through the 5 airframe ground wires.

Now the question. The radios use 18 AWG wires for power and ground. It would be nice to use the same ground bus but how does one do this (18 AWG wire does not fit into the D-Sub pin and I believe they are rated at only 5 amps).

Then I had the idea, could you "Y" the 18 AWG ground wire into two 20 AWG wires that each get pinned to the ground connector? This seems to be how the ground bus works, all items that need ground go to the bus (connector) then the ground current is carried by the 5 wires to the airframe.

My idea is to solder a length to 20 AWG wire from the loop end to the 18 AWG wire and cover with heat shrink.

Thanks for any inputs.

[vlittle]

Originally Posted by RVG8tor

I bought one of the AeroElectic ground bus bars, which is a 37 pin connector with all of the pins on one side soldered together. The items that need a ground get pinned into the other side of the connector, along with 5 20 AWG wires that go to the airframe ground. Instead of the five 20 AWG wires, they suggest you solder two 14 AWG wired to the back where all the pins are soldered together. Since one side is all connected each item can find a route through the 5 airframe ground wires.

Now the question. The radios use 18 AWG wires for power and ground. It would be nice to use the same ground bus but how does one do this (18 AWG wire does not fit into the D-Sub pin and I believe they are rated at only 5 amps).

Then I had the idea, could you "Y" the 18 AWG ground wire into two 20 AWG wires that each get pinned to the ground connector? This seems to be how the ground bus works, all items that need ground go to the bus (connector) then the ground current is carried by the 5 wires to the airframe.

My idea is to solder a length to 20 AWG wire from the loop end to the 18 AWG wire and cover with heat shrink.

Thanks for any inputs.

The concept of using a D-sub connector to distribute ground signals is fine (in fact, I use the same technique in some of my products), however, the amount of current per pin is limited to about 3 amps (depending on the manufacturer).

This means that if you need a 10 amp ground (18 AWG), 3 or 4 pins are required. To make matters worse, the physics is such that one or more pins are likely to hog the current and exceed their rating. I'd recommend using these D-Sub type power or ground distribution blocks for lower power devices, and the high power devices be directly grounded to a bus-bar with fast-on tabs.

I sure Bob N. will argue that they will be fine in most applications, and he's usually right. It's just that the connector specs are exceeded.

They have their place and are very useful, but know their limitations.

Vern

[kbehrent]

Originally Posted by RVG8tor

I bought one of the AeroElectic ground bus bars, which is a 37 pin connector with all of the pins on one side soldered together. The items that need a ground get pinned into the other side of the connector, along with 5 20 AWG wires that go to the airframe ground. Instead of the five 20 AWG wires, they suggest you solder two 14 AWG wired to the back where all the pins are soldered together. Since one side is all connected each item can find a route through the 5 airframe ground wires.

Now the question. The radios use 18 AWG wires for power and ground. It would be nice to use the same ground bus but how does one do this (18 AWG wire does not fit into the D-Sub pin and I believe they are rated at only 5 amps).

Can't say I completely understand what your doing or why, but I have the same ground bus bar and mine is bolted to the firewall with the main lug coming through the firewall with the 4awg ground strap connected to it from the engine. All systems requiring a ground are connected individually to one of the 37 tabs. No sharing of ground wires so only the gauge wire needed for that particular device is used. If there is ever a ground wire issue, only that individual system is affected. I have each ground wires labeled using heat shrink labels and I have a map where each ground wire is located on the ground bar.

Personally, I would avoid sharing ground wires.

Just my .02 cents.

[RVG8tor]

I have the same "forest of tabs" ground on my firewall. The thing Bob N. sells is a 37 pin D-Sub connector used for grounds for instrument panel items. The idea is this make a nice close ground bus for all the items in an instrument panel. Looks like I will have to run the wires for the radios all the way to the firewall ground.

Originally Posted by kbehrent

Can't say I completely understand what your doing or why, but I have the same ground bus bar and mine is bolted to the firewall with the main lug coming through the firewall with the 4awg ground strap connected to it from the engine. All systems requiring a ground are connected individually to one of the 37 tabs. No sharing of ground wires so only the gauge wire needed for that particular device is used. If there is ever a ground wire issue, only that individual system is affected. I have each ground wires labeled using heat shrink labels and I have a map where each ground wire is located on the ground bar.

Personally, I would avoid sharing ground wires.

Just my .02 cents.

[bharral]

From the pics, looks like Bob's improvised ground block uses gold-plated screw-machine (standard, not high density) D-Sub contacts, typically good for 7.5A continuous. Most likely, a single such contact would adequately carry the current for any single modern piece of avionics equipment.

While Vern is correct that using two contacts does not guarantee that the current will split evenly, it will nevertheless split, and increase the total current carrying capacity somewhere beyond 7.5A, certainly in excess of 10A.

If some malfunction does cause the current to exceed 10A, the breaker should blow long before there is any failure in the D-sub contacts.

There are special D-sub contacts that can take 18 gauge wire, but you are unlikely to have any laying around. So go ahead and Y the 18 to two 20s. There is a fair chance that the same solution has been implemented on the 'other' end of the 18 gauge wires.

I would use fast-on tabs or screw terminals for high current devices like pitot heat and 100w landing lights, but for typical avionics equipment, Bob's D-sub solution is safe, practical and economical.

Regards,
Blake
RV-4 N72RV

[Norman CYYJ]

The majority of the radios (the heavy current users during transmit) also are grounded through the cases as are most avionics. Unless you are mounting your avionics stack on a plastic panel I would just run the ground wires to the stack and ground the stack to the airframe and not bother running the power grounds to a separate sub D connector. For the audio grounds that is a different matter.
Just my opinion.

[RVG8tor]

I asked this question over at AEC and Bob N. the master behind the instrument gournd bus said to just cut off enough strands of the 18 AWG wire so that the D-Sub pin will go over the wire. This seems logical to me in that a loss of few stands of the wire will not effect it utility all that much.

I am still not sure the route I will take, I only have three 18 AWG ground wires and it will not be that much trouble to run these grounds to the firewall forest of tabs, The ground bus can be used for all of the panel items that need a ground.

Just thought some would like to know this info.

[sf3543]

When removing wires from a stranded wire, you might want to consider Table 11-13 from AC 43.13-1B.

TABLE 11-13. Allowable nicked or broken strands.
Maximum allowable nicked and broken strands
Wire Size Conductor material Number of strands per
conductor

For a #18 AWG, with 19 strands, it is "2 nicked, none broken".

It would probably be better to ground those larger wires to the same grounding point that you are grounding the ground block to. It may not make a difference in your application, but then again, maybe it will.