It may be best at this point to describe the "Under Instrument Panel Accessory Staging". Here are two different builds with the same idea to mount accessories under the instrument panel. One build used square aluminum bracing and the other build used the stock 1/2 inch aluminum tubing used in many places on the airframe. Placing the two braces about 5-1/2 inches apart will give you a perfect place to mount accessories, like a "Hot Box", Strobe Control Box", Fuel Panel and Power Bus' or in the uncommon case below, to mount an encoder box for a transponder. If you use "aluminum rivets" to attach these braces to the airframe, these braces will make an excellent ground source.

Transponder:
This instrument sends off a signal via an antenna mounted on the plane to control towers and major airplanes like commercial and Air Force planes to tell them of your position horizontally. When an encoder is wired to the transponder, the signal also tells them what altitude you are flying at. A pre-wired wiring harness is available to connect the transponder to the encoder (DB25 to DB15), which is highly recommended. Otherwise, the stock harness must be wired to a DB15 plug by the new owner. There are 6 loose lead wires from this harness, 4 we would not use in a Challenger, so just curl them up and tie wrap them together. This leaves an additional two loose wires, a red "HOT" lead and a black, ground. The red lead is wired to the bottom of the on/off bush button, described above. The black is wired directly to the ground bus under the instrument panel. There is also a coaxial cable connector to connect the supplied coaxial cable to a supplied 3" antenna. The coaxial cable is longer than necessary, so you will have to shorten it, affix the coaxial connector to the end and attach it to the antenna in the location you have chosen. (Recommended location of antenna: just in front of the main landing gear on the bottom of the plane. A ground plate must be installed to mount the antenna.)

Fuel Gauge with Capacitance Sender
If using a capacitance gauge sender, there are three wires on the sender unit. The yellow wire is the capacitance gauge signal wire which is wired to the fuel gauge on the instrument panel. The red wire is the +12v power lead which is wired to the fused ON/OFF switch on the instrument panel. The black wire from the capacitance gauge is wired to ground.

Emergency Locator Transmitter (ELT):
This is a battery operated, stand alone circuit not needing to be wired into the planes electrical system. However, the Test/Activate/Override switch is mounted on the instrument panel for emergencies in the event you have an accident and the ELT did not activate automatically. Can also be used to test for proper operation. The ELT, dash mounted, push button switch is connected to the ELT unit in the back of the plane via a supplied telephone cord with RJ-11 connectors at both ends. The only other wire is a coaxial antenna cable from the ELT unit to the antenna, usually mounted in the center gap cover.

Fuse Panel & Power Bus Plate:
An automotive 4 fuse panel was used for the 3, 5 amp fuses in the system with one empty and pre-wired spot in case of an accessory being added later to the system. (Top, Left) Below are two power bus', one for +12v power bus (top) and one for a ground bus (Bottom). The ground bus is attached to the frame. To the right side is a legend of the fuse box and +12v power bus connections. This panel is an inexpensive alternative to buying a "Hot Box". Total Cost: Abt. $12.00

Double Dual Magnum Controller Box:
This one is very simple in its wiring. Wire the red (hot lead) to the +12v power bus and the black wire (ground) to the ground bus. The strobe lights need to be attached to the wing tips in this case, with the wiring cables run inside the front wing spars to the fuselage. Normally, the cables will be fed down thru the front down tubes coming out below the instrument panel, near the rudder pedals and back up to the controller box. You will then shorten the cable by removing the excess wire and connecting the three pin plugs to the cables. The plugs will plug into the control box on the opposite side from the power leads.

Wrapped Wire Cable:
As mentioned earlier, there are two (2), 8 wire, 18 AWG cables now running from the front of the plane to the rear firewall area. The CHT/GHT cable was run along the bottom, right side of the plane and the instrument/power cable was run down the bottom, left side of the plane. This was done to reduce any power bleed over from the power lines to the CHT/GHT lines, causing static interruptions in the EIS readings. It is always the best idea to keep all cables neatly wrapped for esthetics as well as having a chance of hooking onto and breaking a wire. The power cable side has the 8 wire, 18 AWG cable "AND" three additional loose wires running beside it. By wrapping the cables with a wire bundle wrap, both purposes are served.

Radio/Intercom:
At the top of the front seat down tubes, an aluminum sheet was riveted into place and the Intercom unit installed. Here you can note the power lines are run to the top, then down inside the front seat down tubes. At the bottom of the down tube, the power lines were tie wrapped to the seat front & back braces. In the photo above, you can see a red/white twisted wire pair above the wrapped power cables. As the intercom black & white wires were not long enough to reach the on/off switch on the instrument panel, the red & white 18 AWG wires were soldered to the intercom wires to add the needed length. When the radio is installed, the power cords will be directly wired to the power & ground bus as the radio leads already have two (2) 4 amp fuses in-line. The patch cord from the radio to the intercom will be run through the forward, right down tube.

Chassis Ground Bar:
Let's first establish the "Battery or Chassis" ground. The battery ground was established by adding a 3/16" aluminum bar, riveted to the chassis frame with long aluminum rivets. This will make an excellent ground source and is located below the engine firewall at the back of the fuel tank compartment. The Chassis Ground Bar will have the following bolted to it, all bolts "must" have nyloc nuts and washers on the back side.
1. Battery Negative Terminal - 4 AWG wire, 2. Starter Ground - 4 AWG wire, 3. Key West Regulator Ground, 4. Strobe Light Ground, 5. Facet Fuel Pump Ground, 6. EIS Fuel Flow Control Ground.

Starter Solenoid
The starter solenoid should be mounted on or very near the engine firewall as the fairly short 3 foot, 4 AWG wire must reach it from the starter, which is mounted on the engine. Wiring is from left to right.

Master Relay
For those using an electric starter, a Master Relay is suggested to isolate the battery from the (heavy current capacity) #4 wire connecting the battery to the Starter Relay. This would be recommended if your starter solenoid is more than a couple feet away from the battery, like a nose mounted battery. It's a safety consideration that, in the event of a crash, the Master Switch will be turned off, cutting electrical power to the aircraft, so if any broken or frayed wires occur, they will not be energized and possibly start a fire from sparks. Click HERE for additional information.

15 Amp Circuit Breaker:
The 15 amp circuit breaker is used in-line, wired between the starter solenoid and the Key West Regulator. Should the coils short and start putting out too much voltage, a run away condition, the circuit breaker will pop, thus protecting the electrical system. The battery will also stop charging, so you have power for a while until the battery dies.

Key West Regulator:
The Key West regulator is used to provide generated power to the system and recharge the battery from the engine coils. Wiring is as follows:
AC Terminal, Left: Wired to engine "Yellow" wire.
AC Terminal, Right: Wired to engine "Yellow/Black" wire.
Neg. (-): Wired to "Chassis Ground Bar".
POS (+): Wired to "15 amp Circuit Breaker".

Electric Facet Fuel Pump:
The Facet fuel pump is +12v electrically driven and is used as a back-up fuel pump in the event the main Mikuni fuel pump fails. The Facet pump is wired as follows: Red wire to bottom pole of On/Off switch on instrument panel. Black wire is wired directly to the ground bar.

Quick Disconnects On Firewall To Engine & 8 wire, 18 AWG Wire Cables:
It is very important you document what color wires go where in your plane. Therefore, making up a simple wiring diagram and storing it will give you something to refer back to at a later date.

Battery or Power Supply:
The battery "Positive Terminal" is wired with a 4 AWG cable to "Left Large Main Power Terminal" of the Starter Solenoid. The "Negative Terminal" is wired with a 4 AWG cable to the "Chassis Ground Bar" or other suitable common battery ground location, attached to the airframe.