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Author Topic:   Moving Battery--Wire Size
dscew posted 04-28-2011 09:26 PM ET (US)   Profile for dscew  
I want to move the battery for my '06 Merc 60 HP 4 stroke motor to under the console. The existing wire is 7 feet of #6 guage. Measuring conservatively, it looks like I need 17 feet of wire to replace it. Considering the new length, load, and flexibility, can I use #4 guage, or do I need to go to #2 gauge?
jimh posted 04-28-2011 09:40 PM ET (US)     Profile for jimh  Send Email to jimh     
A rigorous answer to wire size calculation is found at

http://www.boat-project.com/tutorials/wireprimer.htm

In general with a 12-Volt DC power distribution system you will have to calculate the wire size based on consideration of the voltage drop. You will reach the limit of voltage drop before you reach the limit of the wire current carrying capacity (sometimes called ampacity). If the loads on your circuit are sensitive to voltage, compute the maximum voltage drop to be three-percent. If the loads are not sensitive to voltage drop you can use ten-percent as the maximum voltage drop.

contender posted 04-28-2011 10:54 PM ET (US)     Profile for contender  Send Email to contender     
dscew: I would go with the larger wire [that] carries a load better, less stress on the starter, less stress on the battery. Most important to use a good marine wire, make sure you seal both ends. I like to dip them in liquid tape and then use heat shrink. Use one wire with no splits or connections in the middle of the wire. Pick up some good copper lugs for the connections. Do it right do it once. My wires are 1/0 gauge, 17 Whaler.
RMS posted 04-29-2011 08:31 AM ET (US)     Profile for RMS  Send Email to RMS     
I recently moved the battery from the transom to the center console on a 17' Outrage I. The engine is a 120 Johnson (1993) whose service manual suggested I use 1 guage wire due to the length of the cable (18'). Before I ordered the cables from GenuinedealZ (great service BTW) I checked (utilizing a sample of wire) to ensure the heavier cable would be workable under the engine cowling. Bob
dscew posted 04-29-2011 01:06 PM ET (US)     Profile for dscew    
I've emailed back and forth several times this morning with Genuinedealz.com. What great folks, technically oriented and very helpful. They recommend no more than a 5% voltage drop, # 4 is close, but slightly over. I'll go with # 2 to do it 100% right, with tinned and sealed ends. The "round trip" as they decribe it, will be 28 to 30 feet, not the 34 feet I anticipated. I intend to mount the battery box slightly to port inside the console. By the way, the boat is a '74 Katama.
jimh posted 05-01-2011 05:54 PM ET (US)     Profile for jimh  Send Email to jimh     
I have read some reports of off-brand wire, presumably made somewhere in the Far East, being sold here in the USA with erroneous marking of the wire gauge. The bogus wire will be marked with a wire gauge for the next larger size wire. The thickness of the insulation is increased so that the appearance of the wire's size is normal. For example, a bogus wire will be made with 12-AWG conductor and with insulation much thicker than normal to increase the size of the wire to be similar to 10-AWG insulated wire. The wire is marked 10-AWG and is sold as such, usually for a discounted price. Of course the buyer has been cheated, having bought wire with only half as much copper as he thought he was getting. The copper in the wire is probably lousy quality, too. Buyer beware, I guess more today than ever before, due to the proliferation of scammers and cheats facilitated by on-line advertising.
dfmcintyre posted 05-01-2011 10:07 PM ET (US)     Profile for dfmcintyre  Send Email to dfmcintyre     
One tip; avoid splicing into the existing wire where the splice will end up in the cable tunnel. Instead, make one complete run from the battery to the motor. Cost slightly more, but will eliminate potential headaches.

Best - Don

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