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Author Topic:   Wire Conductor Size for Long Battery Cable Runs
Barnacle Bob posted 07-31-2007 05:02 PM ET (US)   Profile for Barnacle Bob   Send Email to Barnacle Bob  
[The author informs us he is] considering moving the battery under the console and possible two batteries with a [P]erko switch on a [19]62 Nauset with a console from a 1990 17-footer with a 1993 88SPL [E]vinrude. Existing cable need replacing. [T]hought this might clean up the stern area. What gauge wire would be used?
ericlhorne posted 08-01-2007 10:47 AM ET (US)     Profile for ericlhorne  Send Email to ericlhorne     
I would go with 6-AWG or 4-AWG cable. The heavier the gauge, the less voltage loss and better conductivity. Hope this helps.
jandrewg posted 08-01-2007 12:45 PM ET (US)     Profile for jandrewg  Send Email to jandrewg     
Ancor marine makes first quality marine wiring, as well as other marine electrical products.

Check out their web site, Under "Technical Information" there's a wire calculator. I use it all the time for reference. Remember to count the length of the cable's round trip, that is, from the battery switch to the motor and back again. There is a terrific amount of voltage loss at 12 volts, so err on the large side; the few dollars extra is well worth it.


Dave Sutton posted 08-10-2007 08:54 PM ET (US)     Profile for Dave Sutton  Send Email to Dave Sutton     
Just did the same job on a 17 Montauk. A 6-AWG conductor works fine. Motor spins great on the starter, and that's the heavy load.

The newer Montauk console you have is not designed to support a battery on the internal floor of the console, and the consoles wil be damaged in the long run if you install a battery without a modification to let the battery weight sit on the boats deck proper. Best bet is to flip the console up (I know it's a pain to disconnect the steering and cables, but... you need to...) and then to cut a rectangular hole in the console floor large enough to let a battery box (or 2) drop thru. Reinstall the console, drop the battery box(s) in, and they will sit on the boat deck, not the console floor. The boxes are waterproof, so the batteries will stay dry. Plus, dropping the batteries down to the deck will give you a little more room in the console for all the other crap you'll stuff there.

Capt. Dave

jimh posted 08-10-2007 09:18 PM ET (US)     Profile for jimh  Send Email to jimh     
When you increase the length of the cables between the voltage source and the load, you have to increase the size of the conductors in order to prevent too much voltage drop from occurring in the distribution wiring between source and load. In the case of a starter motor the current draw is very high, probably over 100-amperes, so the potential (to make a bad pun) is there for significant voltage drop in the conductor.

Since you are about to triple the length of the conductor, you will have to completely discard the original wiring and replace it with a conductor of triple the size.

jimh posted 08-11-2007 09:13 AM ET (US)     Profile for jimh  Send Email to jimh     
The ANCOR website unfortunately uses frames as method of presentation of its contents which makes it difficult to give pointers to resources contained there. The wire calculator is located at

To visit the main website you will have to jump back to their top page and reset the frame interface.

The wire calculator is a very handy accessory for investigating problems such as this.

contender posted 08-11-2007 11:00 PM ET (US)     Profile for contender  Send Email to contender     
Maybe its me or maybe i'm crazy but I have 1/0 battery cables on my 17. The battery is under the console and the cable runs through the tunnel to the engine. I rigged the boat back in 1985, and I'm still using the same battery cables, and have never had a problem...good luck
Casco Bay Outrage posted 08-12-2007 08:06 AM ET (US)     Profile for Casco Bay Outrage  Send Email to Casco Bay Outrage     
I would recommend the biggest cable (smallest number) you can get.

I found 2 gauge marine battery wire at (sold by the foot) for my classic Montauk.

Don't be tempted to skimp on this since is so key to keeping your boat running reliable. Go for tinned battery wire, good battery connectors and crimps.


jimh posted 08-12-2007 09:16 AM ET (US)     Profile for jimh  Send Email to jimh     
There is no penalty for using a wire conductor which is larger than it needs to be other than the extra cost, the extra weight, and the extra stiffness of the cable, and in the case of an outboard motor which needs to be able to move around, the extra size and stiffness could be a problem.

There are two approaches to extending the battery cables of the outboard motor: splice in a new extension and re-use the old cables, or discard the old cables and get a new, one-piece, splice-free cable. We explore each solution.


If you wish to extend the battery cables by splicing in a new conductor, the new conductor will need to have very low voltage drop. The existing battery cables were probably sized to be about the smallest conductors which would provide proper voltage drop for their length, so the extension cables need to be very large size conductors in order that they have almost no voltage drop. Because a spliced cable will have extra connections, these connections are also potential sources of additional voltage drop, and one must be very careful that the splice and its connections are very well made so as not to introduce any additional voltage drop into the circuit. The location of the splice must also be considered. If it is below deck and in an area where water can pool and stand, the splice must be absolutely waterproof.


Discarding the original battery cables and replacing them with new cables of the proper conductor size for the total length of the circuit eliminates all of the splice and connector problems. If the conductor is run in a tunnel there will not be any spliced connections which may be in standing water. Generally this results in a smaller size conductor being used than if you splice an extension onto the original cable. Although you will have to purchase more cable, the cost will be about the same because the price of the smaller cable will be less per foot. The smaller cable will be more flexible, which is a benefit for use with an outboard motor.


If the extended conductor runs through a tunnel or conduit, consideration ought to be given to using a fuse to protect it. In most cases engine starting circuits are not fused because the peak current is very high and fuses which can tolerate such a current are very expensive, bulky, and difficult to find. Inserting a fuse also adds more connections to the circuit, and these are also sources of potential voltage drop. Most installation do not use a fuse. Locating a primary battery disconnect switch near the battery is a reasonable alternative. This means you will also have to relocate the primary battery distribution switch to be closer to the battery.

sternorama posted 08-16-2007 12:56 AM ET (US)     Profile for sternorama  Send Email to sternorama     
Another reason to not scrimp on battery cables: the 12v potential is present in the cables at all times. A breakdown in the insulation in the area of motion (near the cowling) is all it takes to start a fire. The repetitive motion applied to a poor quality or undersized cable may result in broken conductor strands, which would increase the resistance of the conductor, and contribute to breaking down of the insulation as local temperature would increase.

The only boat fire I have ever experienced was on the trailer, due to ths type of failure.

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