Electrical input requested

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I am now knee deep in my electrical system upgrade, and would appreciate input as to final assembly.  I have gone through the Tech Wiki, as well as helpful articles from Rod at Maine Sail, but am still very much unclear on several matters relating to the final layout within my battery compartment where all the wires, fuses and busbars will join to produce electrical happiness.  I apologize in advance for not including a wiring diagram, and I hope to have explained it in a way below to allow answer to the questions.  Please feel free to jump in with comments, anywhere below, as I learn a lot from this board.


After purchasing my 1990 C34 nearly 3 years ago, I made do with the original house electrical system.   This included 2 group 24 FLA batteries, charged via the original Prestolite—internally regulated—alternator, via the switch on the Nav Station panel.  My original Newmar AC charger charged the 2 batteries.  I did put in a new panel, rewired to the engine, and followed the advice of many members to have essentially trouble-free operation of the boat for the past 2 years.

However, I decided to upgrade the electrical system with the idea of adding an Engel fridge/freezer and (eventually) solar.  I also wanted to be able to live on the anchor a bit more, and maybe use the wheelpilot more when sailing long stretches.  After researching, I decided on upgrading to 3 Firefly AGMs wired in parallel as a house bank.  These fit nicely in the C34 battery compartment.  To make the upgrade I also purchased a Balmar MC-614 external regulator, a Sterling 60Amp charger, a CMI 105amp alternator, and a Victron 712 monitor from Rod at Compass Marine.

Reading through the excellent electrical upgrades in the Tech Wiki—especially Jon's on Della Jean—I started the work of laying out the primary components and wire runs.  All of the old 4 AWG was removed between the batteries, the engine, and the panel.  I am leaving one original 4 AWG positive as a feed from the battery compartment to power the panel.  What I discovered for the original negative to the panel was a 10 AWG wire.

Location of new components

Balmar Regulator:  on the shelf in the aft cabin just aft of the engine

Sterling charger:  under the Nav table

Victron monitor:  on the vertical panel just forward of the Nav table.

Like Jon, I also mounted a new Selector switch (Blues Seas 6008 ("1-OFF-2")) on the outside of the battery compartment.  My thinking was I would use "1" for the house bank and use it for everything.  "2" would be used for the reserve battery when I put it in eventually.  No "Both" because I couldn't see why I would run the reserve and the house banks in parallel.  Coming off the common would be (1) the existing 4 AWG positive to the panel and (2) a new 1 AWG positive to the starter solenoid.  The 4 AWG positive to the panel would go to a new Perko "On/Off" switch to replace the original four position Perko switch.  Since my upgrade does not allow a new panel right now, my thinking was I could simply turn on the new switch at the battery compartment to "1", and then the panel switch to "On" for any DC panel needs.

Orientation of New Batteries

The 3 new Firefly Group 31 batteries are situated side by side in parallel, fore and aft, with the positive terminals oriented aft.  My thinking was to run the positive output of the bank close to the Selector switch.  The negative output would be oriented toward the forward outboard corner of the battery compartment where it would go the Victron shunt mounted on the outboard wall of the compartment up high.  It will then connect aft to a negative busbar on the aft wall of the battery compartment. That negative busbar will pick up the 1 AWG negative to the starter bolt, the 10 AWG negative to the panel, and the 4 AWG to the Sterling charger (as well as the green 6 AWG for the case ground), and others—all fed up through a new hole in the floor, aft and outboard in the bottom of the battery compartment.

Routing new Wires

Because I had already replaced the original water heater with a new Quick model in the hanging locker, I also went with Jon's (and others) idea to run new wires under the galley floor and into three holes already located in the port side of the engine compartment: 1 AWG positive (to starter solenoid bolt), 1 AWG negative to (to starter bolt), and 1 AWG positive to alternator output.  I enlarged these holes with a 1 1/8" hole saw to allow 1" clear tubing to keep the new 1 AWG  dry and off the hull.  I removed the teak and holly floor in the galley and cut a 6" square access hole underneath to assist with this.  In the process I discovered that the subfloor—part of the pan—is over an inch thick and cored with ¾" plywood.  Very sturdy.  These were then routed forward, under the shelf for the water heater, and up through three holes in the bottom of the battery compartment.

When mounting the new Sterling charger the manual said to use 4 AWG for the positive and negative run to the battery compartment (based on the RT distance).  Luckily, I had already cut an access hole in the subfloor under the Nav table for running supply lines and heater hose to the new/relocated water heater.  This allowed me to route the new charger wires to the battery compartment, including the battery temp sensor, using existing glassed-in Catalina conduit over the bilge.  These are also routed up through the bottom of the battery compartment.

When routing the new wires up into the battery compartment, I cut in two new holes.  The original hole is in the port aft corner of the battery compartment.  This is also where the new Selector switch is mounted, and is closest to the positive terminal on the battery bank.  The thought is to run the new 1 AWG wire to the starter and the 4 AWG out this hole.  In the opposite aft corner is a new hole though which to route the various negatives up to a negative busbar.  In the between these two is an intermediate hole through which the 1 AWG Alt output (+) is routed back to the batteries, as well as the Charger + 4 AWG wire.

So here I am with all these wires running into the bottom of the battery compartment, ready for connection to switches, fuses, and batteries.  And if you're still reading this:  God bless you.  This is where I could use some input—except where you probably already felt compelled to comment.


1.  Fusing the positive Alt Output and the Charger output wires.

Given space/clearance considerations in the battery compartment, the best place for me to mount the majority of the fuses/ busbars is on the aft wall of the battery compartment,  Both the negative wires, and the Alt Output and Charger output, are coming up from holes directly below (in the battery compartment floor) and onto that wall. 

Can I use Blue Seas 285-series surface mount circuit breakers for these wires? 



My thinking was to get a 150 amp for the Alt output (105 amp alternator), and the recommended 80 Amp for the charger output (60 Amp Sterling), and then run them to a common power post before going to the battery.  The benefits of using these are (1) they are thinner (with a profile of only 1.25") and (2) the 150 Amp circuit breaker for the Alt Output could act as the "Service Technician Disconnect Switch" recommended by Maine Sail.  The other solution I thought of, a Blue Seas 5196 MRBF surface mount fuse block, is nearly 2.4'' in profile making locating it a little tougher. 


Using ANL fuses presents the same profile problems on that wall, although not as great as the Blue Seas 5196 MRBF surface mount fuse block.

2.  When running the positive back off the battery positive to the new selector switch, I'm thinking of running the 1AWG to a positive busbar located on the inboard wall of the (similar to what Jon did on Della Jean), and then a 1 AWG to the "1" post of the new switch for the house bank.  Its my understanding that I need to fuse the two wires coming off the "common" (for the 1AWG starter and the 4 AWG going to the panel).  If correct:

Can I use a Blue Seas 2151 "dual MRBF terminal fuse block" off the 3/8" stud of the "common" post of the switch—one fuse for the 1AWG starter and 1 fuse for the 4 AWG panel feed?


Or is there a better solution?

3.  As mentioned, although the original electrical had a 4 AWG positive going to the panel (which I am leaving in place), the negative to the panel is only 10 AWG.

Can I leave this as is?  Or do I need to replace that negative with a larger wire?  Again, I'm not changing the panel as part of this phase of the electrical upgrade, other than to change the original Perko Selector Switch from a 4 position to a simple "On/Off" switch.  It seems to have worked for 30 years in this configuration.

4.  Pot pouri:  Feel free to offer any additional thoughts.  I am no electrician but enjoy trying to learn these things.

Thanks in advance for your input.




Blue Seas got back to me to let me know that it was OK to use the 285 series thermal circuit breaker (150amp) to fuse the alt output (+) back at the battery.  Also using their 80 Amp to fuse the new Sterling 60 amp charger there.  Much lower profile than the ANL fuse blocks or MRBF fuse block, so fits on the side of the battery compartment better.  Nice that this can also function as my "Service Technician Disconnect Switch", although mounted in the battery compartment.  I will put a warning placard in the engine compartment to disconnect that switch before working on the engine.

As for the 10 AWG negative and 4 AWG positive going from the panel to the batteries, I decided to toss both in favor of new wires, both 6 AWG.  Neither of the old wires were marine wires, and the connectors needed improvement. The reason the old positive to the panel was 4AWG , I think, is because the starter positive went through the panel switch.  With the starter no longer going through the panel, and not having a windlass or some other high amp drawing motor coming off the panel, a 6 AWG is just fine to handle everything else.  I'm putting a negative busbar above the AC busbar behind the panel, and will connect the new 6 AWG negative wire there.  That can easily pick up the 10 AWG to the panel, as well as the 10 AWG to sub panel I put in for the auto pilot and depth sounder.


Jon W

When sizing your wires, make sure you take into account the amperage they will be carrying under worst condition. Also keep in mind you may be adding devices in the future. For the main distribution panel I used 4AWG positive and negative for that reason.

FWIW - when choosing fuses keep in mind when you're cruising you will want to carry spares onboard. The more fuse commonality you have, the fewer different types of fuses, and fuse holders you need to carry. I carry MRBF, ATC, and glass for the different amperages in the system.

I just responded to your message as well. My apologies again for the delay.
Jon W.
s/v Della Jean
Hull #493, 1987 MK 1, M25XP, 35# Mantus, Std Rig
San Diego, Ca