Getting wired – Proper wiring for your boat

When looking for wiring problems, you might find trouble on the inside where you see trouble on the outside. Note the broken clamp and wire ties that used to secure this covered wire bundle to the bulkhead.

Wires are dirt simple, right?

Wires are the blood vessels of your boat’s electrical system. If they aren’t right, everything that runs on amps and volts is at risk.

We’re talking about boats here, so when we address wire we’re talking about marine-grade wire you can get from your local marine dealer or online at places like West Marine, Boaters World, Cabela’s or Bass Pro Shops.

Marine-grade wiring made by top manufacturers like Ancor or Marinco is stranded wire, and each strand is tinned to resist corrosion.

Since electricity travels along the outer surface of each wire, the many surfaces of stranded wire make a better conductor than a solid wire’s single surface. It’s also more flexible than solid wire, making it easier to snake through wire chases.

Premium marine wire is made to American Wire Gauge (AWG) standards rather than automotive (SAE) standards, which makes it 10 to 12 percent larger and means it can carry a given amount of current with less heat.

And its marine-grade insulation resists damage from UV, gas, oil, acids and other marine hazards.

It’s also available in marine color codes so you can match your existing wiring colors rather than use non-standard colors that could cause confusion during future maintenance.

Perhaps the biggest opportunity to screw up a wiring job comes with choosing a wire size too small for the job at hand. You need a wire thick enough to carry a given amount of power over a given distance, and the more amps you are moving or the longer the wiring run the larger the wire size must be.

This is a common problem with bow-mounted electric trolling motors. Many ship with 10-gauge wire leads factory-installed. These leads work fine as long as they reach your batteries. Unfortunately, these power cables often need to be lengthened to reach the trolling motor batteries, and people who don’t know any better extend the cables using the same 10-gauge wire size.

They think that if the factory used 10-gauge wire then that’s all that is needed. If the leads are attached to a plug that connects to a bow power panel you are OK, as long as the wiring from the panel to the battery system is large enough.

Minn Kota’s website dedicates a table to this problem. It shows that a 12-volt motor with 40 or 45 pounds of thrust drawing 42 amps can get by with 10 AWG wire for a 5-foot run.

But extend the run to 10 feet and you need to move up to 8 AWG wire. If your batteries are at the other end of the boat and you need to extend the cables to 20 or 25 feet, you need to step up to 4 AWG wire to carry the load.

Wire extension length refers to the distance from the batteries to the factory trolling motor leads, not from the batteries to the motor itself.

Symptoms of insufficient wire size include wires hot to the touch while the motor is running and a motor delivering less-than-advertised thrust.

In extreme cases you might see melted insulation on wires. If the insulation melts, and the positive and negative wires touch, an extremely dangerous short could develop.

That much current through a direct short could easily start a fire or injure someone.

Electric downriggers, trim tabs, Power-Pole anchors and conventional anchor lift systems pull less juice than most trolling motors but still require properly sized wiring.

Find out the maximum current draw for the item, and then factor in the length of the wiring run to determine the proper wire size rating.

When wiring a remote fuse panel, you should add up the amp draws from every piece of equipment it powers, and then factor in the distance between the panel and the batteries to determine the wire size needed.

When in doubt, go to the next-larger size.

Two sources for wire gauge-size tables include (use the caps as indicated) and

I recommend using the 3-percent voltage drop information for critical systems rather than the 10-percent drop information, which is for non-critical systems.

I consider all my systems critical.

And when looking at the length of a wiring run using these tables, the distance given is for the complete circuit — the total number of feet from the battery to the electrical device and back.

If you bought your boat new, you are almost guaranteed it has proper wiring throughout. If you bought a used boat with accessories and wiring installed under a shade tree by a previous owner, you are on your own.

When something stops working correctly, I’d include checking wire size as part of trouble-shooting.

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