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,