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Author Topic:   Dual battery setup leads to wiring questions
La Samba posted 04-13-2009 02:50 PM ET (US)   Profile for La Samba   Send Email to La Samba  
A few days back I made my first post with a now embarrasing question involving my mistaken notion that my 12V deep cycle marine battery was 24 volts due to its label which was really indicating that it was a “series” 24 size. Obviously a basic fact to most and now to me as well thanks to CW. I’m a beginner but I’m learning.

My spring project is to install a seond battery on my 17’ 1992 Montauk. I purchased a second deep cycle battery as well as a Blue Sea battery combiner kit which includes a battery switch (on/off or both which combines batteries in parallel for emergency start situations when start battery is run down) and an isolator/integrator that manages the charge received by both batteries to charge them equally while the motor is running and to isolate the house battery for running accessories when the motor is not running.

I have also bought and read the “12Volt Bible for Boats” I skipped work on a beautiful spring day and took apart the existing battery switch and began wiring up the new components. I had some confusion about the wiring of switch and integrator but was helped out of it by a service guy at the marina who was nice enough to give me advice. My main point of confusion is trying to understand what feeds the console accessories with power coming from the battery.

My console includes the engine gauges and ignition/throttle. Accessories include a GPS, marine radio bilge pump, and soon a small amp to power an Ipod. Nav lights – bow and stern.

In the original one battery set up there was a violet 10ga wire that connected directly to the battery and into a black plastic box mounted on the starboard side of the boat. Inside the box this one wire was connected to a 20 amp fuse I think. It is a small black rectangle with two contact screws – violet wire in and violet wire out. It was badly corroded so I forced it apart and found a thin metal plate (bendable) that was fixed to one terminal and touching the other. I’m guessing it is a fuse where excess heat indicating a problem heats the plate enough to make it bend so it breaks contact with the other terminal. It was labeled as 20amp. Anyway, the violet wire in question goes to the console via the bilge tunnel and looks as best as I can tell to connect to the accessories switch. It looks like wires then run from the accessory switch to power the nav light switch.

Does this mean that this wire is probably the main feed for the accessories and nav lights? It was originally connected directly to the battery so if it was powering accessories wouldn’t they get power even if the battery switch was in the off position? I know from using the boat last year that when the battery was set to off everything was dead . There is also a bundle of cables/wires going from the engine to the ignition/throttle. Is it likely that this contains a power connection only between the ignition and motor? Or is it likely that this is the main feed of power to accessories and lights as well as to the ignition? If this is the case, what is the violet wire likely to be doing? I found in a reference article that OMC color code for starter circuit wiring and accessories circuit wiring is violet.

So far, I bought a small power bus that has one feed in and connections for 4 circuits with plug in fuses. I have connected a 4ga wire from it to the connector on the new battery switch identified as going to the house panel and then connected the violet wire to the bus with a 20 amp fuse. The violet wire then feeds back to the console. My bilge pump was also directly connected to the battery with an inline fuse so I removed the inline fuse and connected the wire to the bus also with a 5 amp fuse. Wires from the bow and stern lights also are in the area but I’m assuming the wires I see already are coming from the console and are feeding the lights so they should stay as is. Both my nav light switch and accessories switch have a push button reset above them which I assume is the equivalent of a breaker protecting those circuits.

I would appreciate any feedback on all of this. Am I completely over my head at this point? A simple schematic of the original boat wiring would surely help. Does anyone know of a source for this? I’ve spent considerable time searching on this site and the internet to no avail.


La Samba

jimh posted 04-13-2009 08:40 PM ET (US)     Profile for jimh  Send Email to jimh     
Based on the description in your narrative, I infer that the device which is connected to the 10-AWG violet conductor is probably a circuit breaker, albeit a somewhat corroded circuit breaker.

Typically the main feed from the primary battery distribution to a secondary battery distribution panel should be fused or have a circuit breaker. The fuse or circuit breaker should be chosen to match the maximum current drain that is anticipated in the sub-panel. If the panel is fed with 10-AWG wire, you can generally figure the current capacity is at least 30-amperes.

It is difficult to teach basic electrical concepts in a question and answer exchange in a forum. Since you have purchased a well-written book on the subject of small boat electrical wiring, I am certain that you will gain a better understanding of basic electrical principles from reading that book.

To answer your question, a switch that is connected in another circuit does not influence the current flow in a different circuit. If the feed to the sub-panel was connected directly to the battery terminal, then, of course, it will remain connected to the battery terminal no matter what the position of a switch that is inserted into another circuit.

Some circuit breakers are intended to be routinely operated as control device for the branch circuit they are feeding, however, not all circuit breakers are designed this way. Some circuit breakers are intended to only operate as current protection devices and should not be used as routine ON-OFF control devices. Check with the manufacturer of the circuit breaker to determine what its rating and application are.

Chuck Tribolet posted 04-13-2009 09:13 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
I suspect that La Samba's corroded "fuse" is a thermal,
self-resetting, circuit breaker.

The ABYC does not require that the wire from battery to
distribution strip be fused if it's less than (IIRC) 18".
That would typically be the case when both battery and
distribution strip are inside the console (as is the case
with my '97ish Montauk, which came wired that way).


La Samba posted 04-14-2009 12:25 PM ET (US)     Profile for La Samba  Send Email to La Samba     
JimH and Chuck,

Thanks for your response. At this point I'm pretty sure that the violet 10ga. wire in question is in fact the main feed to the lights and accessories at the console. My batteries are located at the stern so I should still be fusing the feed as it is way over the 18" length limit Chuck mentioned. As I described, the device that Chuck identified as a thermal self-resetting breaker was badly corroded and I should replace it.

What is the appropriate replacement? I can't seem to find a stand alone single fuse block in the 20 -30 amp range. I saw a fuse block ANL 35-300 AMPS 104214 on the West marine site that looks like the right type of device for this application but it is intended for higher amp situations 35-300.

Is a simple in line fuse satisfactory? I also previously bought a Blue Sea 6 circuit fuse block that utilizes ATC type fuses. Could I still use this just for the one wire?

As always, it seems each response gets me a little closer to successfully completing this job.

La Samba

jimh posted 04-14-2009 04:25 PM ET (US)     Profile for jimh  Send Email to jimh     
The older OEM Boston Whaler solution was to mount a toggle-type circuit breaker in a small plastic enclosure. Typically they used an E-T-A circuit breaker. Here is a lead to some more information:

You can also use a wiring accessory from Blue Seas. They have some high-current circuit breakers mounted on a small panel which are intended for this purpose. I think the current Boston Whaler wiring uses them.

La Samba posted 04-14-2009 05:21 PM ET (US)     Profile for La Samba  Send Email to La Samba     
Thanks Jim,
Its all becoming more clear now.

I had seen the Blue Sea breaker at my local marine shop and thought it looked like the right device for the job but they only stocked the higher amp versions. I didn't realize it was available as low as 25 amp. The device I removed which I don't think was original equipment was rated at 20 amp.

I'm guessing I should go with 25 amp. As I described before, the feeder line is 10ga. How do I arrive at the proper breaker size? Is it a matter of adding up the loads I'm feeding? (GPS, Marine Radio, Nav lights, bilge pump and small marine stereo amp) or is it a function of wire gauge?

Thanks again,
La Samba

jimh posted 04-14-2009 09:29 PM ET (US)     Profile for jimh  Send Email to jimh     
Let's look at the question of the rating of the primary distribution circuit that feeds a sub-panel.

We have a circuit fed from the primary battery distribution to a sub-panel with a wire of 10-AWG. What is the proper size of the fuse for this circuit?

The first consideration is that we must not protect the circuit from over-current at a higher rating than the maximum safe capacity of the wire. So we look to see what is the maximum rating. For a reference we turn to:

This shows us the current capacity is a function of the wire insulation characteristics. We can have either a 30, 35, or 40-ampere capacity from a 10-AWG wire.

Depending on the grade of wire, we can set the current protection between 30 and 40-amperes.

We also have to look at the loads in the sub-panel. Add up all the loads in the sub-panel. Let us say you have six circuits with loads fused at 10, 5, 5, 5, 5, and 1 ampere. This means the maximum load on the sub-panel could be a total of 31-amperes. It is unlikely that every load in the sub-panel will hit maximum current simultaneously. You should be safe with a 30-ampere fuse in the primary distribution to the sub-panel. That would be a reasonable level to set the current protection.

Over-current devices do no blow immediately when their rating is exceeded. The time to trip to an open circuit depends on the amount of the overload and the duration of the overload. It also depends on the sensitivity of the device. Usually a fuse will be more sensitive than a circuit breaker.

La Samba posted 04-16-2009 07:49 AM ET (US)     Profile for La Samba  Send Email to La Samba     

Thanks Jim that is very helpful. This info should allow me to wrap up this project successfully.
La Samba
jimh posted 04-16-2009 09:13 AM ET (US)     Profile for jimh  Send Email to jimh     
We should also consider another factor in designing our small boat electrical system and selecting a wire to feed the secondary distribution from the primary distribution: voltage drop. Although a particular size wire can handle a certain current, when the maximum rated current flows through that wire we should also consider the voltage drop that will occur. The voltage drop is determined by three factors:

--the current flowing in the conductor
--the size of the conductor
--the length of the conductor

Let us examine the situation of a 10-AWG wire being used to feed a sub-panel where we could have a load of 30-amperes. What will the voltage drop be in the wire? To determine this we need to know the resistance of the wire per foot and the length of the wire. We again turn to

From the table we find that a 10-AWG conductor has a resistance of


To consider the voltage drop we need the length of the wire between the primary distribution panel and the secondary distribution panel. For a small boat like a MONTAUK 17 we can figure that the distance between the battery switch (primary distribution) and the console panel (secondary distribution) will be about 10-feet. However, we have to account for the positive and negative leads, so we double that to 20-feet. Now we find the resistance in the wire:

0.9989-ohms/1,000-feet X 20-feet = 0.019978-ohms

To find the voltage drop, we multiply by the current flow, 30-amperes:

0.019978-ohms X 30-amperes = 0.59934-volt

We compare the drop in voltage with the nominal voltage, 12.0-volts, to find the percentage of voltage drop:

0.59934-volts / 12-volts = 0.049945 x 100 = 4.9945-percent

We conclude that using a 10-AWG conductor to feed a secondary panel that is 10-feet away from the primary panel will result in about a 5-percent voltage drop in 12-volt system at maximum rated current flow of 30-amperes.

jimh posted 04-16-2009 09:19 AM ET (US)     Profile for jimh  Send Email to jimh     
The marine wire seller ANCOR has reduced many of these calculations to a tabular form. You can use the table to look up some pre-calculated wire sizes for particular lengths and currents. See:

That page will link to some tables and also to a calculator which will help you select a wire size needed to achieve a certain voltage drop in a circuit of a particular current, length, and operating voltage.

jimh posted 04-16-2009 08:05 PM ET (US)     Profile for jimh  Send Email to jimh     
ANCOR notes that in any electrical distribution system in which the voltage is below 50-volts, you can generally ignore the current rating of the conductor as a limiting factor, and, instead, focus only on the voltage drop as the design parameter. Since just about all small boats operate with a 12-volt electrical system, then we can ignore the current carrying capacity of a conductor as a limiting factor and instead just compute the voltage drop as the limiting factor.

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