Battery Cable Size

[markh1f]

Dealer installed 2nd battery on my 26M and used 8 gauge cable between the negative posts of the two batteries. It is a short run (about 20") as the batteries are next to one another but the 8 gauge just looks to tiny to carry the starting load of my Etec 60 if I were to use the 2nd batter to start. Anyway dealer says due to the short run the 8 gauge is fine but I don't really believe him. Anyone have a ABYC or other standard that I can quote to the dealer or is he right? And yes I have searched but did not find anything specific to the short run.
Thanks,
Mark

[mikelinmon]

It is a very short run. That much wire costs about a buck, if you use the right size. Someone saved about 50 cents on you. Just change it out and get well.

[Kelly Hanson East]

Its not like there is a magical gauge for a given current, its a question of what is the acceptable voltage drop, especially in starting circuits, which are only used briefly, and are also tolerant of voltage drop.

From the internet I get the following wire info


WG gauge Diameter Inches Diameter mm Ohms per 1000 ft.
8 0.13 3.26 0.6282


So that AWG 8 2 foot run will have a total resistance of (2 feet)* (0.6300Ohm)/1000 ft = 0.012 Ohm

For a cranking circuit of 200 Amps (a high estimate for most outboard) , that would give a voltage drop of

V= I(current) * (Resistance) = (200 Amps) (0.012 Ohm) = 2.4 Volts

This would be fine for the starter as well as an electronic control unit for a EFI motor.

[Currie]

Yikes, I disagree Steph. While AWG 8 does have a resistance of 0.6282/1000ft, it doesn't mean it can carry an unlimited amp load. 200 amps is way over the rated load for 8 guage wire: 73A for short runs (chassis), 24A amps for longer runs. If the wire size is underrated, it basically becomes a fuse .

http://online.physics.uiuc.edu/courses/ ... _Gauge.pdf

[March]

Isn't the cable to the battery supposed to be provided with a fuse? That fuse will blow before the cable melts out.

I had wondered about this issue myself, since I am in the process of rewiring the Mac, adding an extra house battery and a XANTREX Pathmaker for three banks. The way markhf1 describes it, I imagine that the second battery is added in parallel to the first (engine) one. In which case a switch (Bat1 Bat2 BOTH) is required. I imagine tthis switch having a common ground and three points of contact--in which case the 8 gauge line between the two negative poles off the batteries is superfluous. The common negative ground pole will do its duty, and that is certainly not a 8 gauge wire.

If the configuration is different, it may also be that the negative 8 gauge wire will carry electricity ONLY to compensate for what the first battery is lacking--in which case it shoould be more than sufficient..

Just wondering....

[Currie]

Hmmm, I read the original post to mean that the 8 guage wire between the two batteries *is* the common ground. I'm picturing a battery-switch selecting the positive from battery A and battery B - all with heavy gauge wire. I also imagine battery A has the heavy guage ground wire coming from the enigine connecting to its negative terminal. This ground is then jumpered to Battery B with the 8 guage wire. With battery B selected to start the engine, all 200 amps will have to go thru the AWG-8 wire to complete the starting circuit from battery B. If this is what the configuraiton is, it is wrong. I'd definitely connect the common ground with the appropriate sized heay-guage wire, like an AWG-1. (IMHO)

~Bob

[markh1f]

Thanks to all for the comments, I have replaced the 8 gauge wire with a 4 gauge for my own piece of mind but disagreement with the dealer is part of a larger negotiation about what was wrong with the boat and what I had to do to make it right.

March - there is a battery switch 1-2-both but that only deals with the postitive leads. The gound going to the panel and the engine all come off of the terminal of battery #1, so the only gound path for anything operating off of battery #2 was the 8 gauge wire connected to battery #1.

Bob - you have it right.

Mark

[Currie]

FWIW - Here's another sizing chart -

http://www.e-marine-inc.com/products/al ... ables.html

[Hamin' X]

Since you already replaced the wire, this may be like closing the barn door after the horses are gone, but here goes.

We do not have enough information to answer your question. It is dangerous to make recommendations without all the facts. I tried to search out the current draw of the Etec 60 starter, but only got specs for no load, not the actual starting load. Here is some general info for automotive applications:

• • 4 cylinder gas engine -typically draws up to 160 amps
  • 4 cylinder diesel -typically draws up to 350 amps.
  • 6 cylinder gas engine -typically draws up to 210 amps.
  • 6 cylinder diesel -typically draws up to 450 amps.
  • 8 cylinder gas -typically draws up to 250 amps.
  • 8 cylinder diesel -typically draws up to 650 amps.

The Etec-60 will be considerably lower, especially since it is a 2-stroke. But I think it would be safe to estimate 100 amps, until someone comes up with the actual figure. Running the calculations for this, the voltage drop for your 20" run of 8 AGW wire would be ~1/4V. Starting circuits should be designed for 100% overload, so that means ~1/2V, still low enough for the wire to handle.

Like so many other things in life however, you may be worrying about the wrong thing. Of greater concern to me would be the quality of the connections. Standard crimp connections are not suitable for high current applications and soldering larger gauge wires takes special equipment. The added resistance from poor connections can easily out strip the wire gauge problems. Improperly crimped and sealed connections, with proper strain relief would be my top concern.

For those not conversant with electrical formulae, here is a Voltage Drop Calculator and here is a Wire Gauge Chart.

~Rich

[c130king]

Like so many other things in life however, you may be worrying about the wrong thing. Of greater concern to me would be the quality of the connections. Standard crimp connections are not suitable for high current applications and soldering larger gauge wires takes special equipment. The added resistance from poor connections can easily out strip the wire gauge problems. Improperly crimped and sealed connections, with proper strain relief would be my top concern.

Rich,

This issue confuses me a little. I was under the impression that all negative connections (and positive connections for that matter) to the batteries, swtich, fuse panel, engine starter, and alternator would be via some sort of "post" or "lug" where you slip a connector over and tighten down a nut or something.

I also will be doing this "soon" and do not quite understand...don't think I have any of the "special equipment" you are talking about...and definitely don't have the skills.

Will I need to be doing any soldering?

Thanks,
Jim

[March]

Currie: now that makes sense. I did assume that the original ground (negative) cable for the original starter battery was already in place. No problem there--or else the system would have not worked until now. Thick, battery cables, for both plus and minus, right? But the second battery is the house battery. I can't exactly see it drawing 200 Amps in order to melt a gauge 8 wire--like I said, the fuse that ought to be on its positive lead will blow long before that.

I still wonder: when will the second battery pour 200 amps into the engine? When the first battery is flat and Mark will use either Bat 2 or Both (most likely Both) In which case, the current will go (let's say it "goes" for the sake of the argument) from the house battery into the switch, into the engine and then back into the negative pole through the skinny 8 gauge cable. Or: from the Battery 2 into Battery one (the dreained one), then along the thick positive cable into the engine and back through the thick negative cable into Battery 1. In which case, again, if you measure the amps between the the two negative poles of the two batteries, will you get 200 Amps, or even 100 as Rich is suggesting? I kind of doubt it--the Amps will be needed to turn the engine, not to melt the cable--or is that dumb to assume?

I dunno. I am no electrical engineer, and I would also apreciate a full explanation. Sure, better safe and sorry and replacing a 8 gauge with a 4 gauge will do nothing but good. But I was puzzling over the Xantrex separator for a long time: the poles for the plus side on the Pathmaker are heavy and they DEMAND battery cables. But the negative or ground point does not. It's a little screw that looks perfectly happy with a 8 (if not 12) gauge wire. What the....?

Surely, the Xantrex separator functions like a three-point switch with a common ground to BOTH batetries? Why doesn't it have a heavy-duty ground point, I wonder? The negatives of my original two batteries were connected by an equally skinny wire. The only heavy-duty wires were between the engine battery and the engine: one negative, one positive. Also, heavy cables from the positive poles to the three way switch (the one I replaced with the Xantrex separator)

[Hamin' X]

The ground on the Xantrex separator is only for powering the separator. It is expected that your other grounds will be common and of sufficient capability for each use.

~Rich

[March]

Aha! That makes sense now. Thanks, Rich. How about the other questions? Wouldn't the battery fuse blow before the 8 gauge cable melts?

[Hamin' X]

Jim,

For ordinary wiring, soldering of 12 AWG, or smaller is not usually a problem, as long as proper crimping and soldering technique is used to avoid "cold solder joints". For larger gauge wires, one should really use soldering equipment that has a temperature controlled element. Most off-the-shelf soldering equipment will not maintain a constant temperature into the core of the wire. The result is a good looking connection on the outside and a failure waiting to happen on the inside.

As far as crimping goes, high current connections require die crimpers, not the $1.298 ones available at Wally-World. This insures uniform compression of the connector and and wire and is what is used on OEM battery cables. Can you get by with less? Sure, for a while. Sooner or later, vibration or corrosion will degrade the connection. There are too many variables to predict how long it will last.

Best for 12 gauge and down is crimp, then solder. All connections require a solid mechanical connection. It is the mechanical part that does the heavy lifting, solder just keeps it in place.

~Rich

[Currie]

Hi March,

What Rich said on the isolator/separator It allows flow from the alternator to go to one or both batteries without allowing flow *between* batteries. Little if any current will actually flow out the isolator's ground connection.

I'm assuming that Markh1f's setup is about like mine. I have a KISS system for now with a starter battery, house battery and switch, but no combiner/isolator. I keep them separate, but wish to have the house battery available for backup, in case I run the starter battery down. Here's my simple schematic...



You can see the ground jumper between the two batteries. If the backup battery is selected, all cranking amps have to run thru the ground shunt between the two batteries. That's why I'd spend a couple of bucks and make sure it up to the task.

Cheers,

~Bob