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Author Topic:   Over-current Protection For Primary Battery Distribution
abwehrly42 posted 05-22-2012 06:32 PM ET (US)   Profile for abwehrly42   Send Email to abwehrly42  
I have moved the cranking battery to the front of my boat, which has a 2001 70hp Johnson (2-stroke). There is a run of ~17' of 4GA cable. I would like to know if I should install a fuse or breaker at the cranking battery, and if so, how to determine the appropriate size?

I found this discussion which discusses the merits of circuit protection in the cranking circuit, but I did not find a clear answer:

jimh posted 05-23-2012 08:50 AM ET (US)     Profile for jimh  Send Email to jimh     
To determine the size of the over-current protection on the circuit that supplies engine cranking current to your outboard motor, you should monitor the current in the circuit. Obtain a clamp-on DC Ammeter with peak recording capability. Monitor the in-rush current and the steady cranking current. Choose over-current protection based on those measurements.

It is my limited observation that most people who have moved their engine cranking battery to the center console from the stern do not provide over-current protection for that circuit.

davej14 posted 05-24-2012 09:12 AM ET (US)     Profile for davej14  Send Email to davej14     
If you are going to fuse the primary distribution cable don't fine tune the fuse selection too much. The last thing you want is a nuisance trip that will shut everything down. If this cable shorts in a fashion that could cause a dangerous situation (heats up) then there will be plenty of current in excess of normal operation to blow a fuse. Size the fuse for a short circuit condition.
abwehrly42 posted 05-27-2012 02:12 PM ET (US)     Profile for abwehrly42  Send Email to abwehrly42     
Thanks for the suggestions. I found athe chart saying that 4GA can carry 160 amps continuously. How should I size a fuseboat or breaker forat short circuit conditions? Twice the rated current?
gusgus posted 05-27-2012 03:41 PM ET (US)     Profile for gusgus  Send Email to gusgus     
Not to be argumentative, but why?

How many new boats use a start circuit breaker? I didn't have one on anything I have, cars included. I do have "fusable links" but not on the start circuit, in the ignition circuit.

I am in the final process's of a complete rewire of my boat, and will not have a circuit breaker in the start circuit. Is this wrong?

abwehrly42 posted 05-27-2012 05:10 PM ET (US)     Profile for abwehrly42  Send Email to abwehrly42     
To be honest, the boat in question is a 16-foot jon boat. The battery cables will be run in a crimp on the side of the boat, passing eight ribs. I am concerned about the potential for the insulation to be compromised, creating a situation where a short could be created by the aluminum hull.
contender posted 05-27-2012 07:26 PM ET (US)     Profile for contender  Send Email to contender     
abwehrly: protect the wires with a plastic spiral-wrap, braided sleeving, or convoluted tubing.
gusgus posted 05-27-2012 07:31 PM ET (US)     Profile for gusgus  Send Email to gusgus     
Is there a way to zip tie them? I will try to explain a method to do that with excellent results in wire chafe protection. If you use 1, 2 or 3 tywraps (side by side) around the ribs to build a surface then a single tywrap under these tywraps and around the wire/s will keep plastic between the wires and ribs. It builds a surface and clamp all in one place. Much cheaper and easier to maintain. I use this method on aircraft, legally!
I hope that makes sense to you.
jimh posted 05-28-2012 08:58 AM ET (US)     Profile for jimh  Send Email to jimh     
Primary power distribution cables should be secured with stainless steel clamps with rubber insulating cushions.
abwehrly42 posted 06-07-2012 07:49 PM ET (US)     Profile for abwehrly42  Send Email to abwehrly42     
I was able to borrow a DC clamp meter with peak recording function.

On my 2001 2-stroke 70hp Johnson outboard, measured at the positive battery cable directly prior to the starter solenoid, I measured the following peak current values (engine was warmed up in 70 degree weather):

Test 1: 238 amps
Test 2: 227 amps

I am questioning whether I should install an expensive breaker, or focus on protecting the battery cables. I will most likely install a battery switch to cut off the current when the boat is not in use.

davej14 posted 06-08-2012 08:34 AM ET (US)     Profile for davej14  Send Email to davej14     
Rather than focus on the circuit protection, you should first evaluate the wire gauge of the 17ft cable run. At 240-Amperes you are seriously undersized with a 4-AWG cable: asp

abwehrly42 posted 06-08-2012 10:39 AM ET (US)     Profile for abwehrly42  Send Email to abwehrly42     
Thank you for the response. I believe the tables you noted are for continuous current loads. Though the peak current was around 230 amps, my guess is that this is only for a fraction of a second, and that the average current draw for the few seconds the starter is engaged is much lower. I don't have any proof, but this is the conclusion I have reached after reading the experiences of a few members of this forum who have moved their batteries to the center console.

I intend to replace the original battery cables with this 17-foot run of 4-AWG cable, so there will not be any splices in the path from battery to outboard.

davej14 posted 06-08-2012 12:05 PM ET (US)     Profile for davej14  Send Email to davej14     
You need to consider the voltage drop and the potential for a short circuit not just the ideal "hit the switch and you are running" scenario. Your boat and your decision.

Considering your wire gauge and the peak current you are measuring I don't believe you will find and over-current protection device that would allow for your peak current and still protect your wire in a worst case scenario. You will probably be just as well off direct wiring from the battery via an on/off switch in close proximity to the battery.

Hope it all works out for you.

davej14 posted 06-08-2012 12:08 PM ET (US)     Profile for davej14  Send Email to davej14     
One other recommendation. You should not run any electronics from the motor end of the cable because they will experience quite a bit of voltage drop when you start your motor. Electronics should be on a separate circuit.
Ridge Runner posted 06-08-2012 10:42 PM ET (US)     Profile for Ridge Runner  Send Email to Ridge Runner     
Blue Sea makes a 200amp marine breaker
gusgus posted 06-09-2012 01:05 AM ET (US)     Profile for gusgus  Send Email to gusgus     
I ran 17-feet of 4-AWG wire to the motor from the console battery location. It all seems to work well so far. Sea trial in two weeks.
abwehrly42 posted 06-09-2012 01:42 PM ET (US)     Profile for abwehrly42  Send Email to abwehrly42     
Gusgus, what size motor do you have? Did you run your wires to the original battery leads on the outboard or did you replace them with one run?
gusgus posted 06-09-2012 03:39 PM ET (US)     Profile for gusgus  Send Email to gusgus     
I replaced the entire wire from battery cutoff switch (now in the console) to the starter, and ground on the motor. I also replaced all the wires from the guages, lights, controls, bilge and trim, all the way, every wire.
Being the anal retentive kind of mechanic that I am, every connection is crimped and adhesive heat shrink protected.
abwehrly42 posted 06-09-2012 04:50 PM ET (US)     Profile for abwehrly42  Send Email to abwehrly42     
Thanks. What size motor are you running?
gusgus posted 06-10-2012 02:29 AM ET (US)     Profile for gusgus  Send Email to gusgus     
Man, I am sorry I missed answering your question. I have an '87 Merc 150 hp, 2 smoker.

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