It seems minor, but installing the power cables for a ham radio in a car is a major project if you want it done right. After months of thoughts and tactics, I made good on my plan to install a 25 Amp circuit into my car to power my portable amateur radio equipment. I’ve been needing this capability for a while. It gives me another option for operating away from home, allowing me to drive to any convenient spot and make HF contacts, as well as allowing me to make VHF contacts while on the road. I’ll have radio power for as long as I have gas in the tank.

It took me a while to get around to doing it, but once I started, it was two evenings well-spent. Here’s the details.


  • Circuit must carry 25A at 12V for when I operate 100W on HF. This allows plenty of headroom for additional radios, amplifiers, and antenna tuners, should the need arise. The “cigarette lighter” outlet is an easy go-to for some hams but, fused at 7.5A, my outlet is grossly insufficient for the task.
  • Power cables must be run directly from the battery to the radio on both the 12V and ground leads. This minimizes noise from the ignition, ECU, and ABS systems getting into the radio. Cables should also be run parallel and in close proximity to each other to eliminate differential-mode interference on the lines.
  • Power cables must be protected by fuses as close to the battery as possible, in order to protect the car which costs much, much more than any radio ever could. These fuses need to be in weather-resistant holders in the engine compartment.
  • Circuit must incorporate a relay to disconnect the circuit when the car is not in use to avoid battery drain. This will be mounted inside the cabin for environmental protection.
  • Relay itself must be fuse-protected and powered by the Accessory circuit.
  • All main power cables must be stranded 10AWG copper with heat-resistant insulation and wrapped in a split-loom wiring conduit as an extra shield against heat and abrasion against the car’s body. 10AWG is large enough to take the high current without buildup of heat.
  • Once the cable exits the center console into the cabin space, a pair of thinner 14AWG cables can be used because they are more pliable and can dissipate heat into the free air.
  • Cables terminate to a pair of 30A Anderson Powerpole connectors for universal support of many radio brands and accessories.
  • All cable junctions and terminators must be crimp-style butt-end connectors and ring lugs. This is for physical strength against tugging and road vibrations. They must be covered in heatshrink tubing for moisture resistance. Only the powerpole connectors require solder.
  • Radio body will be located under front passenger seat, provided there is enough physical clearance and airflow.
  • Radio faceplate will be mounted on a gooseneck mount attached to a seat bolt, with separation cable and external speaker strapped to the gooseneck for neatness and snag resistance. These cables will run down to the floorboard and under the seat to the radio body.


After careful research, planning, dreaming, and otherwise misinformed parts purchases, I set into the project with the following knowledge:

  • The 2012 Honda Civic has a 25mm rubber plug on the driver-side kick panel, near the hood release cable, which can be removed to allow split-loom into the cabin from the engine compartment.
  • A simple Add-A-Circuit fuseblock jumper can be used to power the relay. This can be plugged into any convenient fuse position on the interior fuseblock and carries the fuse to protect the original circuit as well as the second fuse to protect the added load. The trick is to locate a fuse position that’s powered only when the key is in the Accessory and Run positions. Make sure the jumper can accept the correct fuse sizes; my Civic uses APM-LP (low profile) fuses; the first jumper I purchased only supported full-height APM.
  • The main inline fuses are 25A ATC fuses. I accidentally ordered MAX form-factor fuse holders and had to scrub the project until I could get the proper form-factor.
  • The metal fuseblock bracket in my car is a convenient place to mount the relay and to ground the cold side of its solenoid to complete the relay circuit. Basically any unpainted metal on a car body is a suitable return path to bolt a ground lug into. Just test continuity to the ground terminal on your battery first.

Project Pictures:

Battery connections and fuses in engine compartment
Inline fuse holders, weather-resistant, with 25A fuses installed
Overview of dash underside with fusebox tap, relay, and split-loom through firewall
Closeup of fuse tap (Add-A-Circuit block), not in its final position yet
Leads to power relay (red/blue cables), split loom, and firewall pass-through at far right.
Split loom running diagonally (upper right) over accelerator sender into the center console
Split loom running behind air handler and into cowl of center console, passenger side
Radio body temporarily stuffed under passenger seat
Gooseneck mount with bracket and temporary speaker, mounted to seat stud
She lives! Radio has full power and mobile contacts are now possible. Separation cable is tied to gooseneck.

Lessons learned:

  • When running split-loom, be sure to first run the cables inside the loom and wrap the loom every 4 inches with electrical tape before trying to snake it through the firewall. I initially ran the empty loom through the firewall and then tried pushing the cables through the loom; they poked out of the split at every curve. Had to pull enough loom out, wrap it, and then push the cables inch-by-inch until they entered the cabin. The loom must be snaked from the cabin, through the firewall hole, into the driver-side fender cavity, and fished up over a ridge into the engine compartment. This is the easiest route.
  • Heat shrink tubing is your friend, but only if you have enough fuel in your lighter and no breeze. A torch or heat gun would be better.
  • Don’t buy the wrong parts; examine the ones you buy before starting.
  • Having a friend or spotter would help tremendously, especially when running the loom and cables.
  • Thick patio pillows are great for when laying upside down in the floorboard handling wires overhead. Bleacher cushions and area rugs are good for knees when kneeling in the doorways.
  • Check your circuit continuity after every crimp. Wiggle the cables to detect bad crimps. Catch problems as you go. Use a cheap voltmeter to test the final circuit before attaching your expensive radio.
  • Measure twice, cut once.
  • Pull enough cable to reach the project end-to-end with enough slack, and then splice in the middle where you need to insert things like relays.
  • Buy a proper one-handed ratcheting crimper with spring release; save your hands. Stamped crimp tools are no match for a good crimper with a machined jaw bit.
  • Anything on a gooseneck will wobble when the car is in motion. Make sure your mounts and faceplate are up to the task.


  • Since my radio won’t be mounted permanently in the car (it’s my only HF radio and theft is a perennial concern), I will build a board/plate for the radio’s temporary mounting bracket to slide under the seat, and will use wing-nuts or other such quick fasteners to secure the radio to the bracket. This will also allow me to tie down the power and faceplate cables to the board to reduce bunching and damage and to ensure there’s enough airflow under the radio body for cooling.
  • Currently, I’ll only use a mag-mount for mobile VHF operation. Will need to obtain a trunk antenna lip-mount and some HF antenna whips for 6m, 10m, 20m and 40m bands and run those through the trunk passthrough into the cabin. Maybe get a tunable antenna or one with a compatible antenna tuner powered by the radio.
  • Use lower-current fuses if I don’t actually need 25A for typical operation. Never hurts to go lower if it will increase safety without compromise. I can probably do fine with 10A~15A fuses if I’m using a single radio and no amplifiers or automatic tuners.
  • If your radio includes functionality to emit a beep when you press a button, turn it on. It will help give you positive feedback when operating while in motion.
  • Add RF ground straps between radio body and seat posts, and install straps between doors, trunk lid, and body for RF grounding.
  • Add ferrite chokes as necessary to minimize EMI/RFI and prevent the radio from interfering with the safe operation of the vehicle’s systems. My accelerator sender may need a ferrule for protection from the power leads that run nearby; don’t want to lose accelerator control if I key up on 100W sideband.

I tested the radio for coverage on VHF and finally made my first simplex QSO on the 2m calling frequency (146.520 MHz FM) in Austin; my cheap handie-talkie just couldn’t push the power to reach out of my neighborhood. Talk about a good maiden run!

Here’s hoping for many successful (and safe) QSOs on the road and on the roadside!


Published by Shawn

He's just this guy, you know?