Tapping into a power and ground??

I was wondering if it is possible (and appropriate!) to tap into both the power and ground leads on my transponder to power and ground my SafeFly GPS and Echo UAT.

If so, what is the most appropriate way to splice into those two wires??

Thanks! Andy

I don't think it is appropriate to do that (current load on undersized wire, etc).

In a previous life I never saw a Boeing installer make a jumper like that. They always ran a new wire.

Agree...

Generally not appropriate. Get power from the main bus with an appropriately rated breaker or fuse (could be in-line) and ground from the main ground point (aka forest of tabs).

mbishop said:

I don't think it is appropriate to do that (current load on undersized wire, etc).

In a previous life I never saw a Boeing installer make a jumper like that. They always ran a new wire.

Click to expand...

It always did strike me as the "easy way out." I crawled under the panel last weekend and just had a difficult time deciphering the "wire harness" in the plane. I couldn't find a grounding block near the firewall, or any clear path of the positive lead once it was on the inside of the firewall. I'm going to have to try harder I guess. Maybe begin on the engine side of the firewall for starters.

So where would be the best place to run the power from? I have very little actual wiring experience (behind the panel anyway) but I think I have a general idea of how it should work. I would presume that you would run a heavy power lead from the battery, through the firewall to some kind of distribution block. From there to (in my case) the circuit breaker panel, on to the switches, and then ultimately to the individual equipment you are powering. Then I would presume you wire your grounds from the equipment to a grounding block on the firewall that passes through and back to the ground terminal on the battery.

If that is the case, would I just be able to run the power lead from the switch that controls my avionics? Possibly using a ring terminal at the switch end of the wire?

I didn't get to take a close look at the back of the circuit breaker/switch panel this weekend so I don't know exactly what I am up against yet. And I hope to be able to find where the avionics are grounded to as well. More crawling around ahead I guess.

Thanks. Andy

Spliced connections are very common. How they are spliced can vary from a terminal block to a simple solder connection. I can't imagine a plane where every power consumer gets a dedicated CB. The key is to understand the current draw. If you transponder draws two amps and is fed by a 24 AWG wire protected by a 5 amp CB, you should have no problem splicing in another 1 AMP device. Use proper butt connectors or solder and protect. Same with the ground. Splicing into a ground that goes to the common ground bus is just as good as going direct to that bus, assuming you know the current load on the wire you are splicing into and don't exceed it's capacity with the added load.

If you don't want to splice, just run the new wire back to the transponder CB and crimp on a ring terminal and add it to the CB output with the other one. You can also run the new ground wire to a location where you can ground to the airframe via a ring terminal and screw. Assuming it is not a device that is sensitive to ground issues.

One thing to consider with CB's is you want some form of isolation for critical devices. For example if you put a cig lighter socket on the same CB as your nav GPS, any failure/short of that cig lighter will pop the CB and take your GPS offline. Often the short condition perpetuates and CB can't be reset until the offending component is removed, which is often impossible during flight.

Just another opinion to consider.

Larry