Most boats come equipped with fuse blocks bearing just enough fuses to power their factory-installed 12-volt equipment, and you are on your own if you want to add accessories.
Sometimes this leads boat owners to make poor choices.
I have seen some real messes result from splicing a power wire into a hot connection on the back of the instrument panel or to the hot side of a remote trim switch. I’ve even seen bow-mounted depth finders powered by the front running light’s wiring.
Installing additional fuse blocks and doing the job right is a safer and much more reliable way to go.
I admit to being a gadget junkie, and I blame it on having a job that requires me to test all kinds of electronic accessories. Some test subjects stay and some go, but enough stay that I usually end up installing three additional fuse blocks: one near my boat’s factory panel, another to serve the bow area and yet another back near the transom.
This gives me “regional” places where I can check for a blown fuse in seconds rather than having to hunt down and open in-line fuse holders.
I have seen in-line fuse holders installed at the battery, at the accessory and at accessible places anywhere along the power wire. The purpose of a fuse or circuit breaker is to protect as much of the wiring as possible between the battery and the device being powered.
But the proper way to accomplish that with an in-line fuse is to install it in the positive power wire as close to the battery as possible. Then, no matter where a short occurs, power is cut off near the battery and all of the wire except for the few inches between the battery’s positive post and the fuse holder is protected.
I use the engine’s cranking battery to power all my electronics and everything else on the boat except the trolling motor and other high-draw, motorized accessories like anchor lifts, electric downriggers and such that get powered by the No. 1 trolling motor battery (the one that has the trolling motor’s red power cable attached to its positive post).
I connect the high-draw accessories’ positive and negative wires to only the posts on this battery, and install an in-line fuse or circuit breaker in each positive wire as close to the battery as possible.
Powering these high-draw motors with a separate battery lightens the load on the cranking battery and prevents fish finders, radios, radars and other electronics from picking up motor interference.
I wire all my remote fuse blocks to the cranking battery, and protect each one with a fuse in a sealed, marine-grade in-line fuse holder installed at the battery.
Each accessory powered by the fuse panel will be protected by its individual fuse that should be rated according to the recommendation found in its owner’s manual.
I use marine-grade, dual-conductor cable between the battery and the fuse panel. Its outer sheath resists chafing and fatigue, as well as damage from grease, oil, battery acid and sunlight.
Having both conductors in a single sheath also makes snaking the cable through tight spots quicker and easier than trying to run separate wires.
The correct wire size for each cable depends on the length of the wire run and the fuse block’s maximum amp load. A great reference chart can be found at www.marinco.com/page/three-percent-voltage.
The total length of a wire run is the distance from the battery to the fuse panel and back. For instance, when you run both your positive and negative wires 15 feet from the battery to the fuse block it constitutes a total run of 30 feet. It sounds odd, but think of it as letting the electricity make a round trip.
An average fishfinder/GPS combo unit draws about an amp and a half, and calls for a 3-amp fuse. Some large units draw around 3 1/2 amps and call for a 7 1/2-amp fuse.
My biggest power hog is a 500,000-candlepower portable spotlight that draws about 5 amps, and I protect it with a 10-amp fuse.
When I add up the load from everything likely to be powered at the same time from a fuse block, it always seems to total less than 10 amps. Since none of my fuse panels are more than 15 feet from the battery (30 feet in terms of total wire run), the previously mentioned chart tells me I can use 10 AWG wire.
Duplex cable can be routed the length of your boat through existing wire chases, or it can be run under the gunwales and supported by clamps or zip-tied to steering cables or existing wire bundles already supported by clamps.
I like to follow the bow running light’s wiring between the console area and a new bow fuse block, but I stay away from the trolling-motor wiring to avoid picking up interference that could be passed on to the bow fish finder.
Using only marine-grade wire, shrink tubing, fuse blocks, splices and terminal connectors means that I’ll probably only have to do the job once. I cover and seal wire splices and where wires and terminal connectors meet with adhesive-lined heat shrink tubing (some splices and connectors have heat-shrink tubing built in).
I brush liquid electrical tape on connections where heat shrink tubing can’t be used. A layer of liquid electrical tape on the tightened nuts, washers and posts of a fuse block’s main positive and ground connections keeps them corrosion-proof and tight. A dab of dielectric grease (available from auto parts stores) on the fuse blades, and on the push-on connector sleeves and fuse block blades also helps prevent corrosion.
Remember to locate wiring, splices, connections and the fuse blocks themselves high enough in the bilge area that they will never be submerged.
Good places to mount fuse blocks include under the console, behind the bow switch panel or on the sidewall of a bow compartment up forward, and on a transom support or a common bulkhead between a rear compartment and the bilge area aft.
I mount the blocks with stainless steel hardware, and I prefer to through-bolt them with washers and lock nuts rather than just using self-tapping screws. The idea is to mount the fuse blocks where they are protected from the elements, shifting battery boxes and other gear-related collisions but where you can still change a blown fuse without standing on your head out in rough water.