Boat Wiring: Battery Switches and Distribution

5 Jun

Above is what the main cabin ends up looking like when you’re in the middle of a project like this.

In my last installment I covered battery cabling and big circuit protection. Now we’re to the next stops down the line: the battery switches and core distribution.

First, the battery switches. I blogged once before that I like basic battery switches. These big, basic switches from Cole-Hersee have been around for at least sixty years, maybe longer:
My boat already had two that were original equipment and still fine, but the threads were stripped on part of one of the posts and they’d seen a lot of dousings and abuse, so I replaced them, and added a third for the starting battery. Part of this project is to move all the key electrical stuff out of a cockpit compartment, where it might be subject to flooding, and put it in the main cabin. Here are the new switches, right above the trash can, now in the main cabin:

I’ll add some labels at some point, but for now I know the one on the left is Battery 1, the middle is battery 2, and the one on the right is the starting battery. Turn the switch one way and that battery is connected to the main bus for supplying power or to be charged. Switch it the other way and it’s disconnected.

A 1-2-Both-Off switch or a Blue Sea Systems Dual Circuit Plus battery switch doesn’t allow me to do those simple things:

These battery switches are great for switching two battery banks, and combining them, but not three. In practice my Bank 1 and Bank 2 will usually be combined and treated as a single bank, but not always, so I’ve got them on separate switches. The way I’ve got it now I have to remember to connect the starting battery to the main bus to charge it, and disconnect it after I kill the engine so I don’t run it dead. At some point I might add an Automatic Charging Relay or Balmar Digital Duo Charge, but for now things are cheap and simple.

Now to the other side of the bulkhead, in the engine room:

You’ll see the three switches, with the big cables from each battery coming up from below and connecting to the lower post on each switch. Along the top I’ve used a piece of copper bar to connect the three switches, forming a main bus. Using copper bar allows these switches to be spaced tightly together. Trying to connect them as is with cable would be impossible, and connecting each to a separate bus with cable would create a rat’s next.

To substitute copper bar for cable, figure out the cross-sectional area of the cable size you’re trying to emulate, then get copper bar of the same or greater cross section. In my case I wanted the same current carrying capacity as 1/0 cable, which has a cross sectional area of about 54 square millimeters, or 3/32 of an inch. 3/4″ x 1/8″ copper bar has the same cross section, and is the right size and shape to suite my purpose, but I overkilled it slightly and ordered two feet of 3/16″ x 3/4″ copper bar from There’s nothing more exciting than two feet of copper bar arriving in the mail.

To the left of the three switches you’ll see a Blue Sea System’s Power Post Plus, which is connected directly, with a large cable, to the battery side of the battery 1 switch. This is because there are a few things that need to be connected directly to a battery, whether the battery switch is on or off: bilge pumps, and the memory circuit for the stereo. You want the bilge pumps connected directly to the battery so they’re always live. If you don’t have your stereo memory connected, you lose your preset radio stations every time you shut the boat down.

In the photo I haven’t connected any of these things to the Power Post yet, but I will. What I do have connected are the two outputs from the alternator. I thought a dual output alternator was a good idea back in the day: It isn’t. Just get a single output alternator. We have better ways to charge multiple banks now. Anyway, I connect the alternator outputs directly to the battery side of the switches so there’s no risk of somebody switching off the batteries with the engine running and blowing the alternator diodes. (If alternators don’t have somewhere to send their charge, bad things happen.)

I added the Power Post Plus because all these things are too much to stack onto the post of the battery switch: You’ve got the cable to the battery, the alternator outputs, the bilge pump, and the stereo. That’s already too many to fit, and exceeding ABYC standard (you’re only allowed to cram four on a terminal). Plus I know there will be a connection to a battery monitor, at some point, and other things I haven’t thought of, so I just needed more real estate for direct connections to battery 1.

All of these battery connections and copper bar adds up to a lot of exposed, live metal, so I’ll be making a cover out of Starboard that will protect all this from dumb guys in the engine room.

To pan out, we can now see the whole magilla, on the bulkhead forward of my engine:

Mind you this is mid-project and all those wires will get tidied up. In the upper right hand corner are the battery switches. At the lower left and right are the positive and negative main buses, with a few red and black cables connected. In the case of the positive bus, it’s an extension of the bus I made with the copper bar, giving me more big, 5/16-inch posts to connect stuff to.

In the middle is a Blue Sea Systems Safety Hub:

I’m not a shill for Blue Sea Systems: They just seem to be the only manufacturer who makes quality electrical components for small boats. The Safety Hub is meant to be a main distribution panel/fuse block for a much smaller boat, but for me it served as the most compact way to to get four AMI fuse blocks, and some additional negative bus terminals, in a small space. It also has six ATO/automotive fuse slots at the bottom, which I won’t use.

AMI or MIDI fuses seem to be the best way to protect circuits from 30 to 200 amps (glass fuses only go up to 30 Amps), and I have four such circuits I needed to protect. My boat has sort of a convoluted early version of a distributed power system, with four distribution panels spread about the boat, so each of these bigger circuits serves these four distribution points.

In the next installment we’ll cover those…

This article was syndicated from The Adventures of the Vessel Condesa


  1. Henry

    NICE tip with the copper Bar! :)

    I was looking for ideas to extend one pole too short (to low) for some cables and something to replace a very messy rats’ nest cabling on both the minus poles of my two 24V Banks!

    Multiple connectors (more than 6 wires) I haven’t seen anywhere, and the copper bar trick will work great I think.


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