Category Archives: Bus Bar

Does 8 x 2 = 6?


This may be a stupid question and a stupid idea, but here goes.

I need to rewire my boat for a bigger trolling motor. Right now there are two #8 positive wires and two #8 negative wires running from battery area to where trolling motor plugs in.

Bus bar

The trolling motor manufacturer says that this model of motor requires #6 wires.

Can I attach the two #8 positive wires to a heavy duty marine buss terminal located by plug in receptacle and then a short piece of #6 to tie into receptacle? And do the same for negative side with another buss?




It is not the best solution but it would certainly work fine. Make certain that you have the correct circuit protection in the positive lead at the battery.

Good luck,


Sled Boat Wiring


I have a basic 16.5 aluminum open river sled with a single engine. Currently the motor runs off one 12v battery with a manual bilge pump switch hooked up.

Koffler River Boat

I would like to add a second battery for:

  • Electric trolling motor
  • Navigation lights, to be installed
  • Bilge pump to this battery
  • Fish finder
In addition I would like to add a 12 volt two bank marine electrical boat battery charger for both batteries. I will add a 6 position fuse panel to the mix.

My questions or concerns:

Any interaction between the 12vdc system and the 120v battery charger to insure that I don’t get a ground fault. Would adding a battery switch assist with this potential problem or will a bus bar be necessary?

Thank you for any input,


Since your system is 12 volt only, I would connect all of your grounds together, including your batteries, at a common bus.

You will only trip a ground fault outlet if there is a fault from ac hot to boat ground and the boat is connected to earth ground.

If this happens it will not matter if you have a battery switch or not.

Hope this helps.


Crestliner Wiring Upgrade


Could you please check out my homemade boat wiring diagram? It’s for my 17.5′ Crestliner Fish Hawk.

As you can see, I added and wired the two Attwood battery selector switches and a fourth battery to the system (Battery 2) and wired all the batteries in parallel. I also added the Blue Seas fuse block, all lighting, fishfinder, trolling motor, etc.

Crestliner boat wiring diagram

Click to Enlarge

My goal here is to be able to run the trolling motor off the extra battery if needed and/or to be able to start the motor with the extra battery if the main died. Also, to run all the lights off the extra battery for night fishing. I’ve yet to turn both switches to “both” because I’m not sure if that would be bad for……..everything.

I’m obviously an amateur at this so was wondering if you think it’s a safe setup assuming all the fuses, connections, wire gauge, etc. are kosher? I haven’t had any trouble yet but I’m a little nervous about burning something up.

I’d really appreciate your input if you can decipher the wiring diagram.




I would make a couple of minor changes:

  • Move the trolling motor circuit protection to the trolling motor side of the switch.
  • Add circuit protection for the Blue Seas fuse panel at its connection location to the boat battery switch.
  • Use the same size boat battery cables for all of your battery link wires. You call out 4/0 for the positive lead (I would assume you mean 4 AWG) but not on any of the other cables.
  • If the trolling motor batteries are near the bow of the boat I would consider adding circuit protection at each end of this wire to help reduce the risk of a catastrophic fault.

Hope this helps,



Thanks for the help Kevin. I’ll do all of that.

Also, I was wrong about the battery cable being 4/0, its actually 1/0 welding cable.
Thanks again,



Most people don’t use welding cable on boats or cars because it is not oil resistant.


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,


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,


Adding navigation lights and outlet

Hi Kevin,

Can you please help me with a fishing boat wiring question?

I need to wire a Mariner 40hp outboard to navigation lights and a 12 volt outlet.

Can you tell me how I wire the engine to the battery, the battery to fuse panel and then to switches and then to the Attwood navigation lights and outlet? Here’s my boat wiring diagram of what I’m trying to accomplish.boat wiring diagram

Yours sincerely


Hi Louis,

To start with, I would add a ground bus like these on our sister boat wiring site. You may already have on incorporated in your fuse panel.

Here are the basics for your marine electrical switches and 12 volt receptacle.


  • Constant power wire from fuse panel to navigation light switch.
  • Switch power wire from navigation light switch to navigation light.
  • Ground wire from navigation light to ground bus.

12v receptacle

  • Constant power wire from fuse panel to 12v receptacle.
  • Ground wire from 12v receptacle to ground bus.

Hope this helps,