Size matters with marine wiring

Dave Osmolski


Last spring I installed a new combination sonar (depth/fish finder) and GPS on my boat, which replaced separate units. The new unit has a 7-inch screen, and the older units each had 4-inch screens, so I didn’t reduce the footprint at the helm.

I replaced the old units with the same brand and used the wiring from the old unit to power the new one. Installation went well, and I used my new unit to explore the waters around Pine Island, Florida.

Back home in North Carolina two months later, I wanted to show my next-door neighbor my new toy. We took the boat out on the lake, I pressed the power button and … nothing. A black screen. My neighbor, to say the least, was unimpressed.

Dumbfounded, I wondered what had happened. My old units always worked. The new unit’s power terminals had voltage. Looking through the specifications, I saw that the unit would work at voltages from 9 to 18 VDC. I measured 11.9 volts at the power terminals, but the unit wouldn’t turn on. I rigged up a set of jumpers from the battery terminals to the unit. The unit turned on and ran fine. This was a real puzzle.

Thinking back to the Marine Electronics course, I remembered learning about wiring sizes and voltage drop. Something to do with Ohm’s law. While my boat’s original wiring had been adequate for the original electronics, it was inadequate for the new electronics under certain circumstances.

Boat wiring differs from other wiring

Wiring for boats must withstand high levels of humidity. It must withstand highly corrosive environments. It must withstand high heat in some cases, and in all cases, it must be resilient and able to withstand bouncing, pounding and unusual stretching. Note that some metals work harden, becoming hard, brittle and breaking when repeatedly bent. In a vinyl wire sheath, smaller strands slip by each other and tend to bend less. Standard marine wire has smaller strands, providing more copper and more slip to withstand the extreme motion that boat wiring is subjected to.

Marine wire is tinned, meaning each strand is coated with tin, lead or other solder material to protect the copper wire from the corrosive marine environment. We can “tin” the exposed strands ourselves, but the tinning alloy can solder the strands together, resulting in solid wire that breaks when flexed.

Wiring for boats must be finely stranded, tinned copper sheathed in polyvinyl chloride, or PVC. Although naturally clear, PVC can be dyed in a rainbow of colors. Non-flammable PVC does not support combustion.

Wire size matters

Wire carries electricity from your batteries to all things operated by electricity—from your compass light to your engine’s starter motor.

Obviously, the compass light uses much less electricity than the starter motor. If we compare the flow of electricity to the flow of water, we can see that the pipe (wire) supplying the starter motor needs to be many times larger than the pipe (wire) to the compass light. The consequences of a too-small water pipe are much less than the consequences of a too-small wire. A too-small wire presents resistance, creating heat and voltage drop that can cause some electronics to quit working.

Choose the proper size wire

The American Boat and Yacht Council sets standards for marine wiring. According to ABYC standards, all wire must be at least 16 AWG (American Wire Gauge or cross section). AWG sizes are inverse: the smaller the AWG number, the bigger the wire. A #4 AWG wire is much bigger than a #18.

ABYC standards also state the wire must have the type/style, voltage rating, size and temperature rating written on it. In engine spaces, the wire must be oil resistant and have a temperature rating of at least 75 degrees Celsius (167 degrees Fahrenheit). All wires must be identified, which is normally achieved by color coding. With few exceptions, the leads provided on many electronic devices are less than #16 AWG. That said, just be sure that wires going from your power source to the wires on your device are at least #16 AWG.

Other items on your boat may require wire larger than #16 AWG. To find the right size wire, check the ampacity tables. Ampacity denotes the amperage capacity of a wire that is a given size and length. The wire length and cross section are important in determining the proper size wire. When working on boats, be sure to consult marine DC ampacity tables.

Learn to do it yourself

If you haven’t taken Marine Electronics, talk to your squadron educational officer about presenting it. If that isn’t possible, find a squadron near you that offers this course. You’ll gain valuable knowledge and skills that could save you money on jobs you might otherwise have to farm out to a local marina or marine electrician.

David H. Osmolski of Charlotte Power Squadron/27 has been repairing boats since high school when his first boat, a canvas-covered canoe with cedar ribs, leaked in gallons per minute and required constant repair.

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