How to Wire a Fuse Block on a Boat | Tacklebox ’n Tools

The short version

A fuse block gives every device its own fuse from one clean feed. It replaces a mess of in-line fuses on the battery posts with one positive wire, one negative wire and a labeled row of fuses.
Fuse the feed wire within 7 inches of the battery. That's the ABYC rule as Blue Sea Systems explains it. If the wire is fully sheathed in loom or conduit, it can be up to 72 inches.
Size the feed for everything on the block, at 3% drop. Add up the amps of every device, measure the round trip and run the math. A 10 amp load 12 feet away needs 10 AWG.
Use the fuse each device's maker calls for. A Humminbird Helix 7 takes a 3 amp fuse, not whatever was in the drawer.
Bring all the negatives to a negative bus. Then run one negative wire, the same size as the positive feed, back to the battery.
Keep the trolling motor off the block. It draws far more than an accessory block can handle and needs its own breaker.

Why use a fuse block

Most people start by wiring a fish finder straight to the battery with an in-line fuse. That works, and for one device it's fine. Then they add a second graph, a livewell pump, lights and a phone charger. Soon there's a stack of ring terminals on each battery post and fuses hanging in the bilge.

A fuse block cleans that up. One positive feed and one negative wire come up from the battery. Each device gets its own fuse and its own screw on the block. Blue Sea Systems describes its blade fuse blocks as a way to "eliminate the tangle of in-line fuses for electronics."

The rules haven't changed. The Coast Guard and the ABYC standard (the boat wiring standard most builders follow) require circuit protection on every positive wire except the engine starting circuit, according to Blue Sea. A fuse block just makes that easy to do and easy to check.

What you need

A fuse block with a negative bus. It takes standard blade fuses (ATO/ATC). For example, Blue Sea's 12-circuit ST Blade block with negative bus is rated 100 amps for the whole block, 30 amps per circuit and 32 volts DC. Pick one with a cover. Shop our fuses and fuse blocks.
A main fuse and holder for the feed wire, mounted near the battery.
Red and black marine wire for the feed, sized below.
Ring terminals sized to the wire and the studs, with heat shrink. Blue Sea's block uses #10-32 studs for the feeds and #8 screws for the branch circuits.
The right fuse for each device, from its manual.
Loom and cable ties to protect and support the wires along the way.
A multimeter to test your work.

Plan the load

List every device you'll put on the block and how many amps each one draws. The manual or spec sheet will tell you. Here's an example using the presets in our Wire Size Calculator:

Device	Draw	Fuse
Fish finder (Humminbird Helix 7)	0.8 A	3 A, per Humminbird
Forward-facing sonar (Garmin LiveScope Plus)	4.8 A	7.5 A, per Garmin
LED navigation lights (pair)	0.4 A	1 A (125% of the draw, rounded up)
Livewell pump, 800 GPH	4.1 A	7.5 A (125% of the draw, rounded up)
Total	10.1 A

Draws from the makers' specs as listed in our Wire Size Calculator. Helix fuse from Humminbird's fuse chart. Where the maker gives no fuse size, we used Blue Sea's minimum of 125% of the device's current, rounded up to a standard fuse.

Humminbird's chart shows how much fuse sizes vary between models: 1 A for a PiranhaMax, 3 A for a Helix 7, 5 A for a Helix 10, and 7.5 A for a Helix 15 or Apex 19. Always look up your own unit.

Size the feed wire and main fuse

The feed wire

The feed carries everything on the block at once, and it serves electronics, so use 3% voltage drop. Blue Sea counts fuse panel feeds as 3% circuits. The formula ABYC and Ancor use:

CM = 10.75 × amps × round-trip feet ÷ allowed volts dropped

CM is circular mils, a measure of how thick the wire is. For the example above, with the block at the console 12 feet from the battery:

Round trip: 12 × 2 = 24 feet.
Allowed drop: 3% of 12 volts = 0.36 volts.
CM = 10.75 × 10.1 × 24 ÷ 0.36 = 7,238.
12 AWG is 6,530 CM, too thin. It drops about 3.3%.
10 AWG is 10,380 CM. It drops about 2.1%. Use 10 AWG.

Plan for what you'll add later. If this block might grow to 20 amps, the same math says 8 AWG (about 2.6% drop). Running a bigger feed now is much easier than pulling a new one later. Our calculator does this math for you, and our 12-volt wire gauge guide has charts.

The main fuse

Blue Sea's method: the fuse should be at least 125% of the load, and no bigger than what the wire can safely carry. Pick something in between.

Minimum: 10.1 A × 1.25 = 12.6 A, so a 15 A fuse.
Maximum: 10 AWG wire rated 105 °C can carry 60 A outside the engine space, under 33 CFR 183.425.
A 15 to 20 A main fuse fits this setup. Don't go above what the block itself is rated for either.

Where the main fuse goes

As close to the battery as you can. Blue Sea's summary of ABYC: within 7 inches of the battery connection, or up to 72 inches if the wire is fully enclosed in a sheath or conduit. Don't mount fuses or breakers inside the battery box, Blue Sea says, because of corrosion and battery gas.

Install it, step by step

Turn off the battery switch and disconnect the battery's negative cable.
Mount the block in a dry spot you can reach, often under the console. Keep it out of spray and away from the battery compartment.
Run the feed wires. Red and black, the same size, from the battery area to the block. Put them in loom and tie them every so often so they can't rub through on a sharp edge.
Connect the positive feed. Main fuse holder near the battery, then on to the block's positive stud. A forum answer on jetboaters.net suggests connecting it on the load side of the battery switch so the switch turns the block off.
Connect the negative. From the block's negative bus straight to the battery's negative post.
Wire each device. Positive to its own fuse terminal, negative to the negative bus. Crimp ring terminals and seal them with heat shrink.
Install the fuses each device's maker calls for, and label every circuit. Blue Sea's covered block comes with write-on labels.
Reconnect the battery and test. Check for about 12.6 volts at the block with everything off. Turn each device on, one at a time.

Blue Sea lists a torque for its block: 24 inch-pounds on the studs and 18 on the screw terminals. Snug, not cranked.

Fuse or breaker?

Both do the same job: cut the power before a wire overheats. Blue Sea says most blade fuses and most breakers trip at about 130% of their rating. ANL fuses are different and can carry much more before they blow, so they need their own sizing chart.

	Fuse	Breaker
After a fault	Replace it. Carry spares	Reset it
Size and cost	Small and cheap	Bigger and costs more
Doubles as a switch	No	Many do
Best for	Small electronics on a fuse block	Big single loads like trolling motors, and main feeds you want to shut off

Trip points from Blue Sea Systems' guide to choosing circuit protection.

For a trolling motor, use the breaker its maker calls for on its own circuit. See our trolling motor wire and breaker guide, and our breakers.

Mistakes to avoid

No main fuse on the feed. The branch fuses protect the device wires, but nothing protects the feed if it rubs through.
A fuse bigger than the wire can carry, often put in because the small one kept blowing. Find out why it blows instead.
A thin negative. The return wire carries the same current and counts in the voltage drop.
Electronics on the trolling motor battery. Forum members on Bass Resource recommend separate batteries for electronics and a variable-speed trolling motor to keep interference off the screen.
Solid or thin wire. Coast Guard rules require stranded copper, 16 AWG or larger when run on its own.

Questions people ask

Can I wire a fish finder straight to the battery?

Yes. Put an in-line fuse on the positive wire as close to the battery as you can, sized to the maker's spec, like 3 A for a Helix 7. A fuse block is just the neater way to do the same thing for several devices.

Should the fuse block connect to the battery or the battery switch?

Either works if the feed is fused near the battery. Connecting through the switch means turning the switch off kills the accessories too, which stops slow drains while the boat sits.

What size fuse do I need for my fish finder?

The one in its manual. Humminbird's sizes run from 1 A to 7.5 A depending on the model. Don't swap in a bigger fuse.

What size wire runs from the battery to the fuse block?

Add up the amps of everything on the block, measure the round trip and size for 3% drop. For about 10 amps at 12 feet, that's 10 AWG.

Does the negative side need a fuse?

No. Fuses and breakers go on the positive side. The negatives go to the negative bus and back to the battery.

Can I run my trolling motor through the fuse block?

No. A 55 lb motor can pull 50 amps, which is more than a 30 amp blade circuit can handle, and it needs its own breaker per Minn Kota.

Why does my fish finder fuse keep blowing?

Look for a pinched or chafed wire touching ground, a wrong fuse size, or a damaged power cable. Humminbird notes its units draw more current on a low battery, so a weak battery can play a part.