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Author Topic:   Over-current Protection For Battery Cables
padrefigure posted 02-06-2007 09:39 AM ET (US)   Profile for padrefigure   Send Email to padrefigure  
I moved the batteries from the stern of my 1984 22' Outrage. There was no circuit protection for the (+) leads of the battery going to battery selector switch (also located in the stern) and to the engine. This probably was not a big deal because there is only about 10 feet of wire and it is visible for its entire length. Now that the batteries and the selector switch are in the console, I have about 18 feet of #2AWG lead running through the rigging tunnel to the engine. Should I install an overcurrent protection device (circuit breaker or fuse) to protect against a short in the tunnel? If so, what size and type do you recommend? I assume that the proper location would be between the battery selector switch and the lead going to the engine. (I guess you could fuse both battery (+)'s to protect against faults in the selector switch as well.)
jimh posted 02-06-2007 02:00 PM ET (US)     Profile for jimh  Send Email to jimh     
The proper place for an over-current protection device in a branch circuit is as close to the source of the voltage and current as possible. If adding over-current protection to your vessel's 12-volt battery located in the console, you would put the over-current protection as close to the positive terminal of the battery as possible.

Protecting an engine starting battery from over-current is difficult because the current drawn during normal starting can be vary high, more than 100-amperes. The over-current device has to be tolerant of the normal starting current, yet operate to protect the branch circuit if excessive current is drawn.

If you use a fuse, you run the risk of blowing the fuse and not having a replacement. Oh, you might have one replacement on board, but probably not two. And high-current fuses are expensive. If you use a circuit breaker, you have more flexibility, but the cost will be fairly high.

In automotive applications there is often a fusible link in the primary distribution cable that attaches to the battery. If there is a direct short the link melts and opens the circuit, but it withstands most engine cranking current--for decades--without opening.

I really don't have a good recommendation. As you observed, in most small boat installations there is no over-current protection between the battery and the primary distribution ON-OFF switch, nor is there any in the wiring to the engine's starter motor.

contender posted 02-08-2007 09:26 PM ET (US)     Profile for contender  Send Email to contender     
padrefigure, I do not understand you stated before you moved your battery the cable was exposed with no protection device, now you have protected the cable by placing it in the tunnel and you want to put a protection device on it?? I would just go to a thicker cable to help the flow of the current, thus not putting a big load on your starter or your battery...good luck
padrefigure posted 02-09-2007 08:05 AM ET (US)     Profile for padrefigure  Send Email to padrefigure     
In the tunnel, the cable is protected from physical damage, but not from overcurrent. The fuse or cirucuit breaker senses an overcurrent situation and disconnects power, hopefully before a short circiut can ignite the surrounding fiberglass. Without overcurrent protection, the cable will burn until the weakest link fails (cable, battery, or connector). That's more excitement than I want at sea.
swist posted 02-09-2007 09:03 AM ET (US)     Profile for swist  Send Email to swist     
Interesting thread. Cars have never had overcurrent protection in the starter circuit, but the wiring that could cause such a problem is very limited - the negative lead is usually bolted to chassis ground very near the battery and the positive lead runs to the starter solenoid via a different route, and it's very short and direct in most cases. All other circuits leading to the battery are fused.

But as pointed out, in a boat with switches, batteries, and wiring in all kinds of convoluted configurations, there does seem to be considerably more chance for a dead short across the battery.

And I speak from experience when I say that such an event is to be avoided at all costs. I once accidentally shorted an auto battery with a fairly heavy screwdriver. The screwdriver tip became a molten blob.

Chuck Tribolet posted 02-09-2007 11:16 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Like swist, I've never seen a fusible link on an automotive
starting circuit. They are quite common on the smaller wire
that supplies everything else.


contender posted 02-09-2007 02:59 PM ET (US)     Profile for contender  Send Email to contender     
Both swist and Chuck are correct [about the absence of any fusible link in automotive starting circuits]. [I recommend that you] just go to a larger cable. I ran an over-kill cable just for these reasons: larger cable, less strain on the battery and starter. Never have had any problems and I rigged the boat in 1984. I am still using the same cables. Good luck
jimh posted 02-09-2007 08:10 PM ET (US)     Profile for jimh  Send Email to jimh     
I do not think that the situations of a car and a boat are comparable.

In most cars the starting battery is under the hood and the starting motor is just a few feet away. In some cars, the starter solenoid is located adjacent to the battery. This makes the length of unfused or unswitched conductor limited to a foot or two. The cable between the battery and the solenoid is usually surrounded by the metal chassis of the vehicle, and the cable is retain in some way so that it is not subject to movement relative to the chassis.

In a Boston Whaler boat in which the batteries are located in the console, there will be about ten feet of very large conductor cable from the battery to the engine. The cable lying in the rigging tunnel will be subject to immersion in water and to movement from the boat's motion. Often there are also other cables in the rigging tunnel which might come in contact with the battery cables.

I suspect there are some recommended practices in the ABYC code about this, but unfortunately the ABYC code is a secret and is known only to the people who fork over the money to see it--I don't have a copy and don't plan on forking over money to see it.

But I would say it is very misleading to compare the relatively dry, stable, and steel frame environment of the car with the wet, moving environment of a boat's rigging tunnel. A fiberglass boat is much more combustible than a steel car chassis.

If you locate the batteries ten feet away from the engine, you should have a shut off switch very close to the battery in the console, so that the unswitched leg from the battery to the switch is very short.

The decision of what size cable to use has nothing to do with over-current protection, and recommendations to use the proper cable are really for concern with minimizing the voltage drop in the cables, not with providing over current protection. Just the opposite, really, because if you have some really big cables, say 2-AWG, and they develop a fault in the rigging tunnel, the large diameter of the cables will help carry energy into the fault in the rigging tunnel and create more heat--perhaps enough to start a good fire. Larger cables tend to increase the risk associated with a fault, and so, if anything, they more than ever ought to be protected against over current.

I also suspect that in cars where the battery is located a long way from the engine, such as below the rear seats, there is probably more attention paid to over current protection. There very well might be a fusible link to stop the current of a short circuit which could otherwise start a fire.

contender posted 02-11-2007 05:36 PM ET (US)     Profile for contender  Send Email to contender     
depending on what make of car you have and what year it is determineds were the solenoid is located, (GM has theirs mounted to the starter).. Ran a race(drag) car once we use to mount the battery in the trunk for more weight on the rear tires, by the time we ran the cables in the channel(frame) and to the starter there was a good 7-8 ft of cable length... Jimh have you ever tried to jump a battery with small cables? the cables heat up and if you let them go(keep trying to start)they will burn the plastic coating off the wire not to mention not carry the load. If you use a larger cable the wire can carry the load... It is the same in a house current, try to run a #12 wire to your a/c and see what happens, you will short out your a/c and burn the circuit board. You need to run an #8 or depending on the distance an #6 wire...In my boat I have spent the extra money and purchase the marine type cables that are gas/oil/water resistance/. Like I stated I have ridged my boat in 1984 and use the boat all the time. I have the same battery cables lying in the tunnel that are over size with no problems...good luck
swist posted 02-12-2007 08:50 AM ET (US)     Profile for swist  Send Email to swist     
Hmmm, someone seems to have decided that these are such low-risk situations that they need not be fused, although I do agree that many if not most boat wiring configurations are much more vulnerable than cars.

Also note that, as far as I know, the electric power coming into your house is not fused anywhere upstream of your main breaker. That includes the wiring leading into the main box, the electric meter, and the wiring running up the side of your house and then to the pole.

So we do have protection against small fires, but I guess major ones are just fine!

Chuck Tribolet posted 02-12-2007 09:13 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
House wiring is normally fused at the meter. Beyond that,
it's outside in the air or underground, so little fire risk.

Gophers, that's another story. A buddy of mine had a gopher
chew through the neutral wire leading to is house. That
caused some really strange voltage fluctuations as loads
changed. Start the toaster, some lights dim, some get

But I digress.


contender posted 02-12-2007 01:19 PM ET (US)     Profile for contender  Send Email to contender     
All of my electrical is fused, I'm just stating that a larger cable carries the load more effectively. Place a (I also have one, this is also why I like wing nuts on the battery see earler thread about batteries) battery shut off switch as jimh has stated. swist, is there not a main breaker on the house and then on each circut, both of these are after the meter. And at the electrical pole on the pole there is a fused link also. (Thought, we all live in different areas in the country, all the requirements are different?) I do not have gophers in this neck of the woods so no gopher problem here just hurricanes.
swist posted 02-13-2007 08:00 AM ET (US)     Profile for swist  Send Email to swist     
I live in the Northeast and got my info from an electrician who said that the National Electric Code prohibits a main service entrance from being more than some small number of feet (can't remember how many) inside the house because that would mean an unacceptable amount of unfused cable running through the house. I always took this to mean that there were no fuses between your main breaker and the telephone pole.
Chuck Tribolet posted 02-13-2007 01:23 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
The meter is generally at or very close to the service
entrance. The service entrance has a big breaker, (two if
you have two distribution panels like me). The distribution
panels (with lots of breakers) can be a considerable distance
from the service entrance, in my case at the exact other end
of the house from the service entrance.


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