Wiring Question

I am adding an additional Amp and a couple Tower Speakers to the factory system, with that thinking about possibly adding an additional battery up under the bow seats should it become necessary. My question is if adding a second (house) battery (about a 10'-12' cable run from the factory (starting) battery) what guage wire/cable should be used?


The stereo amps are 320watts and 800watts which would then come off the house battery.

Assuming you have a 100A alt, 6awg wire would be able to handle that amount of current. However, if you ever plan to start the boat off the 2nd battery (in the event the starting battery is dead) you'll want bigger wiring than that. I would use 2awg or 0awg between the 2 batteries.

For the wiring from the 2nd battery to the amps, the worst case (max load) would be 1120W/12V=93A. A 6awg wire could handle 93A, then split to 8awg (800W/12V=67A) for the larger amp and 12awg (320W/12V=27A) for the smaller amp. Those would be minimum recommendations- I would probably use 4awg or 2awg as the main feed, then split to 4awg or 8awg for the 2 amps.

Quinner, I did this exact same mod to my boat. I used a 6 AWG cable from the 2nd Alt (100 AMp) to the "house batt" and put an 80 amp circuit breaker in line. My 2nd batt is powering 2 amps (each are at least 400 watts RMS) with a 2 farad capacitor in parallel. The 2 batteries are isolated from each other on the B+ side. I felt that this was important since every diagram that I came across for a 2 alternator setup used an isolator (expensive) between the two alternators. Since the 2 alternators are not exactly matched for their regulated voltage output, they must be isolated so that they are not fighting each other while charging. For starting emergencies you can tie the two batteries together on the B+ side with a battery disconnect switch and 0 AWG cable

TRBenj wrote: nuttyskier2002 wrote: I used a 6 AWG cable from the 2nd Alt (100 AMp) to the "house batt" and put an 80 amp circuit breaker in line. Is that the only load on the 2nd alternator? If so, why effectively limit a 100A alt to 80A? I see the 80A breaker is necessary to protect the 6awg wire, but why not use a larger wire (4awg)? I assume you placed the breaker at the end of the wire closer to the battery? Quinner's looking to do something a little different since hes considering adding a second battery, but not a second alt.

Yes the amps are the only load on the 2nd alternator and battery. I decided to err on the side of caution with the 80 Amp breaker because I never want to load the 2nd alt to full capacity. The two amps operating at max volume would never pull 80 amps from the alternator unless something is grossly wrong. The breaker is very close to the 2nd alternator.

This system has worked perfectly for me with no charging, starting or sound system problems since its install.

79nautique wrote: what ever you do don't run some skinny ass 4 awg or smaller wire unless you realy are look for the insurance company to buy you a new boat because you will melt the wires and start a fire.

The 4 AWG cable is the charging circuit (from the 2nd alt to the 2nd batt). Right now I do not have my 2nd battery connected as a backup starting battery. If you want to connect the two batteries for emergency starting, 0 AWG or larger with a battery disconnect is the way to go. I stated this in one of my posts above.

Thanks for the thoughts, I am going to take a look at what ga. the factory cables are and survey for the best spot for the second battery tonight and work from there.

The factory wire to the 320w amp is 8ga, I am pulling 4ga to the new 800w amp and my thought was if that 4ga would adequately support a second battery the run I am pulling now could be used as the battery run at a later date should the battery end up in the bow area. Not sure where the second battery would work out best, couple options are up under the bow, under the passenger seat area or in the floor cooler/storage area just in front of the existing battery box. Tim also suggested running the heavy ga. now to a fused dist block and then the amps from there, another good thought. Like the idea of the battery in the bow so the weight has a positive effect on the wake, then again like the floor storage location so the cable runs are not so long.

you have to ask your self one question. do i want to use this second bat to start the boat? if so then the 4 ga isn't going to get it done and if you are then the further away you get the larger the wire needs to be.

Working days next week Chris maybe we could get together Sunday evening or monday night. e-mail your numbers again had to replace the phone and don't have it anymore.

I have posted this before so you may have already read this thread before but I I did a complete stereo install on my 92 SNCB last September and it was an extensive install. As you can see in the pics we used "0" gauge wire for the connection from the battery combiner over to the second battery and also to an added ground block up under the driver's area right below the dash. I agree with 79 that you need at least "0" gauge wire if you are going to use the second battery as a starting battery if necassary. My installer also stated that to do it correctly it must be "0" gauge. I installed the Xantrex battery combiner and I think it is awesome. They are about $100 but are completely automatic with a manual switch you can push over in an emergency that combines both batteries for extra starting power. It has adjustable switches that you can set for example if it gets 11.5 volts from the alternator it automatically combines both batteries for charging and extra juice. As soon as you turn off the engine and no volts are present, it automatically disconnects the batteries so your starting battery is always hot. Again, I love it because everything is completely automatic with nothing to forget regarding switches like Perko that have to be manually switched on/off/1 battery/2 batteries etc. Here are some pics and hope that helped. Boat was extremely dirty in these pics...shame on me!

Thanks!