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Author Topic:   Negative Lead Wiring; Trolling Motors
Ablewis posted 07-18-2009 12:11 AM ET (US)   Profile for Ablewis   Send Email to Ablewis  
It's time to re-wire the Montauk and the numerous in-line fused wires feeding directly to the batteries. I have a two battery 12 volt system powering the main motor and a trolling motor. I have a Perko 1-2-0ff-both switch and a dual on-board battery charger. I have ordered a Blue Sea ACR and a blue sea fuse block with negative bus terminal. I found some great diagrams on the Blue Sea web site, but they didn't include the negative leads which puzzled me. I would really appreciate some comments on the revised diagram that I posted at: current=wiring_diagram.jpg

One final question. Should the trolling motor be directly wired to battery #2 or should this go through the fuse box (the wire guage may be too heavy?).

Any suggestions, help would be welcomed!


number9 posted 07-18-2009 03:37 AM ET (US)     Profile for number9  Send Email to number9     
Realize it's a classic you're asking about. I was recently on the BW website [probably means WHALERPARTS.COM] and the current owner's manuals have the [unknown acronym--possibly meant electrical schematic diagrams]. Saved a couple myself in PDF for a general reference for future projects. Figure they must meet current standards and practices.
jimh posted 07-18-2009 08:33 AM ET (US)     Profile for jimh  Send Email to jimh     
In the primary power distribution on a small boat, the negative leads are all wired to a common bus. You often see the negative terminal of the battery used as a common wiring point, but this is not a recommended practice. If too many terminal connectors are placed under one terminal post there will be a tendency for the connections to loosen, particularly in an environment like a small boat where there is a lot of vibration and motion. It is much better to have a negative bus with several binding posts, and to try to limit the number of connections under each post to one--well, OK, maybe two in a pinch.

This primary negative terminal bus will be close to the battery and receive the battery negative lead (or leads if more than one battery), the outboard motor negative lead, and all negative leads to secondary distribution centers, such as the helm area power distribution center. You can find products designed for this which include terminals of different sizes, usually two large terminals and several smaller terminals. These devices are a good way to organize the negative power distribution

The negative distribution is generally never fused or passed through a circuit breaker. About the only exception to this might be on a boat with a metal hull and in which the electrical device makes a direct connection to the hull. For example, if you had a radio whose antenna was connected to the metal hull, you should fuse the negative lead. Let me explain why, briefly: if the primary connection between another high-current load that was bonded to the hull, such as the motor, and the battery negative failed, the current would try to flow on the much smaller connection between the antenna (bonded to hull) and the negative lead of the radio (connected to battery). A high current, such as from trying to start the engine, could damage the antenna, the radio, and its power wiring if it were not fused. But, again, this applies only on boats where the hull is conductive and some part of an low-current electronic device is bonded to the hull. On a fiberglass hull, I cannot think of anyplace where fusing the negative lead of a load would be needed.

The most important consideration in the negative distribution is that there not be a sloppy mess of wires coming to the battery to all be bonded to the negative terminal. Now in my own boat, I am in violation of my own recommendations as I do have up to four terminals on a battery negative terminal post. I use one of the batteries for a common point for the negative bus, and it has leads from:

--the outboard motor
--the other battery
--the secondary negative distribution terminal bus
--its charger

These are held under washer, lock washer, and hex nut that is tightened with a wrench. Never use wing nuts for something like this.

In a perfect world, I will replace that with a high-current bus with four terminals. That will reduce the number of connectors under one fastener at the battery negative post to just one or two.

jimh posted 07-18-2009 08:44 AM ET (US)     Profile for jimh  Send Email to jimh     
Another very important consideration in wiring at the battery or elsewhere is to carefully mark and identify the polarity of each wire. It is common that wires all have black insulation, and it is easy to make a mistake about polarity, particularly at the battery. Mark all the battery connection leads with clear indication of their polarity. I suggest red for positive and yellow for negative. You can use several wraps of vinyl tape of the appropriate color to identify leads. You can also make small labels to further identify the leads if their source or function is not obvious. A few moments spent identifying the leads will prevent a polarity reversal connecting them to the battery. A fully charged battery is a source of extremely high current, and it can do a lot of damage in a very short time if connected with the polarity reversed.
Ablewis posted 07-18-2009 11:15 AM ET (US)     Profile for Ablewis  Send Email to Ablewis     

This was very helpful. I can see where a negative terminal bus bar near the battery would be helpful (both batteries are in the center counsole). I am now realizing that the negative terminal bus bar on the fuse box could serve that function?

The fuse box I will be installing has 12 circuits and I believe I only have 7 accessories (Radio, marine radio, bilge, live well pump, locator/gps, nav lights, counsole light). Without doubling up accessories on the negative terminal bus bar on the fuse block, that leaves 5 connections. Looking at my diagram, couldn't the two ground leads from the on-board charger go to this bus bar and eliminate the ground wire connecting the two negative battery terminals? A ground going from the ACR to the bus bar on the fuse box would then bring the number of connections to the battery terminal down to one. Am I missing someting?

I would think that the same argument for a negative terminal bus applies to a positive as well. I am thinking that the trolling motor would need to also be wired directly to the battery which adds a fourth connection to the battery terminal. Or perhaps that could go through the fuse block as well? I have seen your discussion of using the lock washer and nut as opposed to a wing nut. That makes sense regardless of whether you have 1 or more connections.

Your discussion of the need to fuse a ground on a metal boat makes sense. When I read the Blue Sea instructions for the Automatic Charging Relay, it recommends a 10 to 15 amp fuse on the ground going from the negative terminal bus to the ground on the relay? That seems to add an in-line fuse that I am trying to eliminate with the fuse block and was wondering why it was needed. I assumed it might be there as a precaution against a reverse polarity connection...something that should be nearly eliminated by proper labeling and wire color. I believe my radio currently has a fuse on the negative lead as well. Perhaps neither are needed.

This resource may have already been mentioned on the forum, but I found a great illustrated guide for making marine wiring connections with the proper equipment. I have been using one of those inexpensive all-in-one crimper's for years. See:


cohasett73 posted 07-18-2009 12:19 PM ET (US)     Profile for cohasett73  Send Email to cohasett73     
Check out Steve Salick' power distribution set-up.
May be of interest to you.
Tom from Rubicon
jimh posted 07-18-2009 02:33 PM ET (US)     Profile for jimh  Send Email to jimh     
The negative common bus on a secondary panel would not have sufficient capacity to be used for the primary battery negative distribution. You need a dedicated bus that can handle very high current, around 250-amperes. Something like this Blue Seas BUSSBAR No. 2106:

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