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Catalina/Capri 25/250 Sailor's Forums

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Rewiring your boat

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Voyager
Master Marine Consultant

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USA
5396 Posts

Initially Posted - 09/13/2025 : 11:44:34

Many folks like myself have rewired the 12VDC systems of their boats. With many new owners here, folks try to determine whether and when to upgrade or replace their systems. Your best indicator is when lights start to flicker or fail, outlets go bad, or your VHF and music systems start crapping out, it�s time for a redo.

Catalina did a great job with then current 70s, 80s, and 90s technologies. Crimp connectors, tinned stranded wire, glass fuses (then plastic 2-prong fuses), toggle switches, and shrink tubing. They worked great and lasted for decades.

Because it�s 12V, in order to drive lighting and fan motors with enough wattage, they needed to provide high gauge wiring (10, 12, 14 AWG) to avoid power losses in the cables.
Now with LED lamps that draw around 20% of the power of incandescent lights, current demand is less, so replacement wiring can be made thinner and lighter in most cases.
Four sets of cables can be reduced: deck lights, cabin lighting, navigation lights, and mast lighting.

Wire gauge is calculated by estimating the current requirements given the round trip distance of your circuit. For example, 14AWG wire runs the cabin lights. Starting at the panel, the cable runs along the hull-deck joint on the port side running three to four portside lamps, then it turns around in the Vee berth running along the starboard side hull-deck joint all the way back into the quarterberth under the cockpit.

With LEDs, you can replace this with 16AWG cable with no losses.
The forward red-green navigation lights and the mast lights also run up the port side hull-deck joint, and due to less current, these cables can be made 16AWG.

For radios, VHF, and fans, I would not reduce the wire gauge. For 12VDC outlets, I might consider increasing the wire size (8 or 10AWG) since AC inverters and Bluetooth speakers can pull 10-20A.

New crimp connectors can be purchased with built-in shrink tubing to better waterproof the metal to metal connections. Ratcheting crimp tools make more secure and positive connections between cables and connectors. A friend recommended this self-stick vinyl tape to cover the connectors where shrink tubing didn�t do the complete job.
See : DEI Quick Fix Tape

There are lots of options for replacement of the old plastic cabin lighting including touch switch LED fixtures and LED reading lamps and spots.

New switches with built-in fuses can be purchased for your existing panel, or BlueSea can provide replacement panels.

In my case, I started with the switch panel, the battery, the engine, and solar charger first about ten years ago. I added fuses to all 3 connections to the battery, each on its own circuit. I ran 8 AWG to the outboard, and 10 AWG to the panel. Solar needs 5A so I added a 10A fuse with 14AWG wire.

I completely rewired the panel with new wiring, switches and fuse blocks. Rather than replace the panel, I decided to recondition the components. I added several 12VDC plugs by the panel, and also up near the head.

I can run a 200W inverter for small AC loads when I�m not at the dock.

You can rewire your entire boat over the winter like I did, or you can do one section at a time. It�s a great investment if you plan to keep and use your boat for the next several years, or even decades.

Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain � Milford, CT

Voyager
Master Marine Consultant

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USA
5396 Posts

Response Posted - 09/14/2025 : 09:33:26

According to Don Casey (may God rest his soul) in his Sailboat Electrics Simplified he presents his 3% voltage drop table for wire size versus round trip distance. I�m recapping it here.

Round trip distance in feet across the top and current in Amps down the side, providing the minimum AWG in the cells.
Please forgive the forum formatting.

Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain � Milford, CT

Edited by - Voyager on 09/14/2025 09:35:17

Voyager
Master Marine Consultant

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USA
5396 Posts

Response Posted - 09/14/2025 : 09:35:46

What we can see from the table below is that it�s very easy to undersize your wire. And while our boats are 25 feet from end to end, it�s possible to get a round trip distance over 60 feet since the cabin lights wrap around the boat twice!

In that instance, say you have eight 0.2A LED fixtures in the cabin. Four of them will be less than 30 ft to the panel and the other four will be less than 60 ft round trip. You can use 16AWG to reach almost all of them, and since they are rated for 10-16VDC, the drop at the end of the line should be okay if all 8 lights were running.

Imagine that you installed a bow thruster or anchor winch that required 30A, and round trip distance was 40ft. This would require #4 wire which is HUGE, but necessary.

For the bow red-green navigation lights at 40ft, this requires 0.3A, so 16AWG wire is fine.

So too � use 16AWG for all your mast lights � except perhaps if you have a pair of very bright LED downlights shining on the bow and the cockpit. A 3A downlight might require 14AWG to the mast.

If you plan to add two 12VDC cigar lighter outlets near the vee berth, I�d provide 20A each so you can run a 250W inverter. Two outlets require two separate runs of #6 wire.

Nice thing about USB A-type chargers is they max out around 2.5A. Adding them around the boat usually takes 16AWG, while long runs may need #14 wire.

USB-C type car chargers can put out up to 4A, so for a long run to the vee, you should use #12 or #10 wire.

Obviously, these recommendations are provided as an FYI. You boat may be different in terms of wire run lengths, wiring configurations, fusing, and other factors.

Also, if you turn EVERYTHING ON in the boat, your electrical panel supply from the battery must be gauged and fused correctly to handle the load. Assume you can pull 50A through the panel (assuming you have an anchor winch). At 25 ft round trip, you probably need 4AWG! If no winch, then assume 40A, or #6 wire.

Remember, correctly sized fuses must be provided for each circuit (err on the low side), and each connection to the battery, engine, solar panel, or battery charger must be fused. Circuit breakers are not advised in a marine environment due to failure from corrosion.

Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain � Milford, CT

Edited by - Voyager on 09/14/2025 09:59:41

Voyager
Master Marine Consultant

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USA
5396 Posts

Response Posted - 09/14/2025 : 09:37:36

Wire Size Table
Round trip length 10____20___30___40___60 ft
2A____________16____16___16___14____14 AWG
5A____________16____14___12___10____10 AWG
10A___________14____10___10____8_____6 AWG
15A___________12____10____8____6_____6 AWG
20A___________10_____8____6____6_____4 AWG
30A___________10_____6____4____4_____2 AWG
Abridged from Don Casey

Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain � Milford, CT

Edited by - Voyager on 09/14/2025 09:48:25

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