Category Archives: Boat Wire

Switched Solar?


My marine electrical includes an Automatic Smart Battery Switch and was wondering how to install a trickle solar charger to it.

Should I bypass the switch, or should it be installed just like a Guest battery charger to the “house” battery? If so, do I still need a charge controller, and where should it be located (between battery and panel, or battery and smart battery switch)?solar panel

Thanks for all of your help.


Hi Greg,

In my opinion, the best location to connect a solar charger is to the house battery, but at the battery switch end of the boat battery cables.

Connect the positive lead of the charger to the battery switch stud with red cable from your house battery and connect the negative lead of the charger to the ground bus. Connecting the charger to the switch will help reduce the amount of clutter on the battery.

If you want the solar charger to charge the starting battery also, turn the switch to either ON (VSR will control the charge) or COMBINE (batteries are mechanically paralleled with the switch).

Hope this helps,


Close Call. Go Small.


I have a 12 volt 55 pound thrust trolling motor on my boat’s marine electrical system.

What gauge of boat wiring is recommended to run 25 feet of wire from the motor to the boat battery?

Thanks,Chalkboard math


Hi Kurt,

Your boat’s trolling motor will draw 15-20 amps.

To maintain a 3% or less voltage drop at 20 amps and a 50 foot total run, you would need to run 4 AWG marine wiring. If you run 6 AWG, your voltage drop will be 4% or less and the wire will weigh and cost 2/3 as much as the 4 AWG. It is common practice to run 6 AWG with your application.

Hope this helps,


Boat Wiring Size


Thank you for your answers to so many electrical questions. I plan to redo my fishing boat wiring and have a couple of questions.

My 1977 18 foot Starcraft has an outboard motor with it’s own circuit and will not be involved in the rewire.

My current marine electrical has a battery in the back of the boat, approximately 16 feet from the fused, marine electrical switch panel.Click to download Kevin's boat wiring size calculator

The old boat wiring is a rat’s nest with each circuit wired individually.

I will be using the standard small boat items: a depth finder, FM radio, bilge pump, navigation lights (bow and stern), gauges and a rear-mounted gas tank.

I would like to run a duplex cable from the rear battery to the switch panel. The total duplex run would be 32 feet and the total amp draw should be no more than 30.

I think I need a 10 AWG duplex cable for the 30 foot run and 16 AWG boat wiring for leads from the switches to the instruments (+) and bus bars (-)?

Does this sound right?

Thank you for any help you can provide.


Hi Tom,

I would recommend using my boat wiring size calculator to answer your wire size questions.

  • Leave the voltage drop percentages in column B and G alone.
  • Enter the current draw of you switch panel devices in column C and H.
  • Enter you total wire length run in inches for each device (switch to device and back to ground) in column D and I
  • Enter your total wire length run in inches for the panel feed (battery to panel and back to battery) in cell 25D – 384 is your guess.
  • Your wire sizes will be in Column E and J.

Let me know if you have any questions,


Mercury Optimax – Boat Battery Wiring

Hello Kevin,

I have a 17 foot fishing boat with a 2004 115 Mercury Optimax and a 2007 9.9 Mercury kicker. I only have a single cranking battery for both motors. Mercury 9.9 Kicker

I was told that my marine electrical setup should have two cranking batteries, one for each motor hooked up to a boat battery switch.

The guy said I could damage the computer in my Optimax if by accident one motor was started while another was running. Would the automatic smart boat battery switch (the one you designed for your sister marine electrical site) prevent that if each motor was hooked up to separate batteries with this switch? How detailed is the marine wiring for this switch?

I really like the idea of this switch. My old boat had a Perko battery switch and I always forgot to switch it back and forth.



Hi Corey,

Your system is very common and I have not heard of people damaging computers in this manner. I have, however, heard of people damaging computers by turning the battery switch to the OFF position while the engine is running.

With our smart battery switch system, when either battery is above 13.7 volts, the batteries are connected together. When it drops below 12.8 volts, the switch opens and the batteries are not connected.

If you had your kicker connected one battery and your main connected to the second battery, while charging, the batteries are connected together and they would not be isolated to prevent the situation that you are describing.

The connection is fairly simple and we supply all of the boat battery wiring required with the Smart VSR Battery Switch System.

Please let me know if you have any questions,


Navigation Light Short

Hey Kevin,Attwood is the world's top manufacturer of navigation lights for marine electrical.

I just bought a 1987 Fisher bass boat with a 1989 110 hp Johnson outboard.

The boat navigation lights had been removed but the light wiring was still running up to the front of the boat. I bought a new Attwood LED navigation light and connected it (pretty sure I did it right) but the navigation light will not work. The white all round anchor light does work when I turn the switch to just anchor lighting and navigation lighting.

I followed the boat wiring for the lights and found that the negative wire was tied into the main negative wire going directly to the boat battery (this was connected). After those two wires meet, it appears the wiring coming from the two is supposed to be connected to a fuse but it was not.

There were two fuses open, a 20 amp and a 10 amp. As soon as I connected it to the 20 amp, the fuse blew. There was a spark when I connected it. I have no clue where to go from here.

Any advice would be greatly appreciated.



Hi Tommy,

It sounds like there is a short in your marine electrical between the fuse and the light.

You can either try to trace down the short or replace the wire. Sometimes it is much easier to replace marine wiring than to find the problem. Make sure to use circuit protection at the power source to protect the wire if a short happens again.

Happy wiring,


Wellcraft Wiring

Hi Kevin,

I’m considering a total redo of the marine electrical on my 1984 23ft Wellcraft Sportsman Walkaround.

I have a 200hp Mercury and the engine harness is fine as is the boat battery wiring and the switch wiring.

What I need to rewire is my navigation lights front and rear, anchor light, the two bilge pumps front and rear (with the option of adding one more in the rear) and docking lights also adding a few underwater lights on the transom. I also have a horn, washdown pump and trim tabs. I have one furuno 620 and two gps setups.Wellcraft Sportsman

What I want to know is if I order a boat wiring harness and switch harness from your sister marine electrical site, EzAcDc, will they cover everything I need? If not any help you could offer would be great.

Thank you


Hi Tom,

Thanks for considering some of the parts on our retail boat wiring site.

Our boat wiring harnesses contain wiring for an automatic bilge pump, a livewell pump, courtesy lights, navigation lights, docking lights, and a boat horn.

When combined with our switch panels, it is simple to connect additional electronics like your Furuno and your GPSs. Additional bilge pump wiring can be run from the pump to the switch panel.

We offer several boat battery switch systems to fit your needs. Our most popular is the smart marine battery switch system. Your 200 HP Merc will work great with this system.

Hope this helps. Thanks again.


Bot Buddies

Dear Kevin,

Last summer, I purchased a new aluminum fishing boat. It’s great, but I screwed up by “cheaping out” at the last minute. I passed on the trolling motor package and – as you’ve probably guessed – now want to add a trolling motor.

As a long time reader, I know that you are a big fan of Motorguide trolling motors and I’ll be buying one in the next couple weeks. My question concerns the boat wiring.

The Motorguide will be bow mounted, but since I didn’t get the trolling package, the only wire at the bow is a 16 gauge pair for the Attwood LED navigation light. But, according to your boat wiring size calculator, the trolling motor should be wired with 6 gauge.

Is there any way to make the smaller wire work? I hate to start rewiring a brand new boat if I can avoid it.



Hi Sean,

Since your boat is new, you may be in luck. A lot of boat builders now use Ancor’s new Nanotech Wire. As you’d guess from the name, it uses nanotechnology to solve problems just like yours.

Simply put, Nanotech Wire matches wire size to power demand by moving copper to where it is most needed – sort of a Viagra for marine electrical.

In the past, running a trolling motor on undersized wire caused voltage drops, heat buildup and embarrassing insurance claims. But now, that same temperature increase signals Nanotech Wire to deploy an army of nanobots who quickly rebuild the harness to match the power demand. While at work, these little guys make the wire looks like a python that swallowed a large rat.

Once your harness is properly sized, the same bots then go to work on its insulation so that it correctly corresponds to standard boat wiring colors.

The patented process is called Nanotech Overload Sensing Heat Induced Tranference and it works great. The only time I’ve seen it fail is when there are so many accessories turned on that there is no extra copper available. If this happens a lot, I usually recommend the addition of a copper reservoir to supply additional material as needed.

Have an excellent day.


Radio Requiem

Hi Kevin,

I have a 19 ft open fisherman and the marine electrical includes a boat battery switch.

Over the winter one of the batteries reversed polarity, and I had the thing hooked up per their diagram. I have removed the bad battery, and have my accessories run to a marine electric fuse block.
My VHF fuse blows as soon as I replace it, and when it blows my GPS cuts off, and will not cut back on for a while. I have wiring coming from the battery red to the positive side, and black to the negative boat bus bar. Everything goes through its separate fuse but none on the fuse block blow.

I have no idea what is wrong. Could it be a bad ground on the motor, or could the radio be blown and have a dead short causing this?

Finally, I’m trying to save up for a new set of fishing rod holders, so if there is any way that I can avoid paying for the fix, it would be great.



It is very possible that there was some internal damage to the radio.

Eliminate all of the boat wiring and connect the radio directly to the boat battery (with an in-line fuse). If it still blows the fuse, the radio is done.

If the radio still works, start tracing the wires that are common between the VHF and the GPS. There may be a bad/burnt connection that resulted from your marine wiring mishap.



The radio still blows fuse when hooked up to battery alone.

Could the short in the radio cause the battery to reverse polarity? By the way I ran the battery down to zero, and have it on charge and it is showing a positive charge now.



The battery should be fine, but I’m afraid that the radio is dead :(


Six into Eight?


According to your marine wire size calculator, I should be using 6AWG tinned boat wiring on my new Motorguide trolling motor.

After running the boat wire, I found that the Marinco trolling motor connector can’t handle any wire larger than 8 gauge. Marinco makes an a adapter, but where the plug is installed, there is no room for it.BEP is one of the world's top suppliers of marine electrical components for boat wiring

Can I splice a short piece of 8 gauge wire on the end of my wiring run to connect to my trolling plug?


Hi Maurice,

The 6 AWG marine wire was required to reduce voltage drop in the system. If the run would have been short, 8 AWG would have been acceptable.

With this in mind, then it is acceptable to run a short section of 8 AWG to a junction block and then connect both sets of wires to the studs. This is a common marine electrical practice.

Good luck,