Negative Bus Bar Question

I would like to install a negative bus bar below the bow deck of my boat under my not yet installed nav light and future fishfinder, light bar, and 12V outlet. I'd prefer to do this in hopes that only fishing positive wires up there in the future would be easier since I won't add these electronics all at once. My question is, if using a bus bar, how do I size the gauge of the wire running from the bar to the battery at the stern? Typical wire sizing is based on current draw plus round trip wiring....in this case, the positive wire would be sized based on the round trip length and current but the negative would need to be much larger to handle simultaneous electronics running at peak current draw correct?

Using a sizing guide, I sized wires for the electronics assuming 5A draw for nav lights (3% V drop) & 5A for fishfinder, 5A for 12V outlet, and 10A for small lightbar (these 3 at 10% voltage drop). I then summed the cross sectional area of each wire and got 7.04 mm. The cross section of 8 AWG is listed as 8.37 mm. Is this an appropriate sizing method? Before even looking at any numbers I was leaning toward 10 AWG because that is a common size of power feed wire to pre-made switch panels. If I use 10% voltage drop for all electronics and sum the current, the guide agrees with 10 AWG.

Another thing I was wondering related to this: could the negative trolling motor battery be increased by one size and act as a ground wire for the bus bar for all electronics plus the TM without any kind of issues like interference? It would simplify it for me as I plan to run the TM wiring up the middle strake all the way to the tip of the bow under the deck while running wiring for everything else either up one strake over or along the side in conduit. This would allow me to size the TM wiring accordingly (guide says 30' at 50A needs 8AWG, assuming 10% V drop)

Why not put the battery up front? The weight is better distributed up there, and it greatly simplifies the circuitry.

You should not run a trolling motor and sonar unit on the same battery. Other electronics should not be affected by it.

Use this ampacity chart below, but BE AWARE that the circuit length listed is the TOTAL length of the run to AND from the battery to the load.

MrGiggles said:

Why not put the battery up front? The weight is better distributed up there, and it greatly simplifies the circuitry.

You should not run a trolling motor and sonar unit on the same battery. Other electronics should not be affected by it.

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I don't really have a good spot up there. My boat has a factory 4' long front deck that is totally filled with foam below it and I don't want to cut into it to place a battery and don't want a battery box on the floor. The past two years I have ran the battery up there though connected with alligator clips to the TM. I have experimented with the battery in the battery location in the stern too and actually thought the boat ran a little better and allowed me to trim down one pin as well. I'm not sure why I don't get a bunch of bow rise....probably helps my motor is only 75 lbs and only have a 3 gal tank though.

If you are using standard Lead-Acid batteries, you can read a lot of comments about guys who were unhappy with a front-placed battery. Seems the bow banging up and down didn't do their batteries any good. Personally, I've had them in both places and haven't noticed any difference.

As far as using, or not using, the trolling motor power for your fish finders, I haven't had any interference doing that yet, either. However, my electronics are not the super high-tech that many guys prefer. If I spent that kind of money on my FF, then I'd run a separate wire also.

Like Rich, I've never had interference issues with my electronics being on the same battery as my TM, but I have had too many instances of my electronics shutting off / rebooting when the TM hit weeds and bogs down, or the battery was getting to ~12.2 volts and I hit the gas

So, now I have a separate "house" battery for all lighting, DF, GPS, USB, bilge pump and gauges and nothing ever drops out and the house battery only needs a "top off" charge from 12.4 volts after a full day of fishing. Night fishing in the rain, I'd expect a bit more of a charge, but my house battery is only a 35Ah U1 battery, a battery size I really like as a house battery because they are inexpensive, save a lot of weight and have a smaller footprint than large batteries.

Your current draw seems really high for your planned installation even in a worst case scenario.

For your nav / running lights you'd be really well served not only for reliability but a power conservation standpoint if you went with LED lighting. Current draw is in the dirt on them. Also an LED lightbar in the 25 to 50 watt range will draw 1/4 to 1/2 of your projected current needs. USB will draw no more than 2A and if you're using a Cig lighter appliance coupling they are (AFAIK) generally fused at 10a but unless you're actually lighting cigs, most anything today that uses LED's will draw well below that.

PS, if you use low current devices from a switch panel, you may not need or want a bus bar to distribute the ground because you can "home run" every individual circuit to its own destination using relatively light gauge wires. I used 12 (bilge pump) and 14 (LED lighting) gauge OFC wire in nylon wire wrapping for abrasion and chew resistance for most everything.

gnappi said:

PS, if you use low current devices from a switch panel, you may not need or want a bus bar to distribute the ground because you can "home run" every individual circuit to its own destination using relatively light gauge wires. I used 12 (bilge pump) and 14 (LED lighting) gauge OFC wire in nylon wire wrapping for abrasion and chew resistance for most everything.

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Yeah, I went super conservative on my current estimates. My anchor light is led while the only nav light I have at the moment is incandescent. The smaller 12-18" light bars that would fit my need are also in that 55W range, so about 5 amp draw. And correct on the 12V outlets...they would be primarily for running led blacklight strips at night and then have a dedicated one for a livewell pump. What are the dimensions of your other battery? My current TM battery is a group 29 and it doesn't even fill up my entire battery box. I'd like to add a second battery at some point anyways as I plan to upgrade to a 25hp outboard with electric start.

My batteries are all an AGM standard U1 size "universal" their size is 8"w x5 1/8"d x 7 1/4" h. and weigh 21 lbs.

gnappi said:

My batteries are all an AGM standard U1 size "universal" their size is 8"w x5 1/8"d x 7 1/4" h. and weigh 21 lbs.

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Thanks gnappi. That's what I'm currently running the few things I have rigged up off of. It's an old U1 from my lawnmower that didnt have the CCA's anymore to start my mowers 20hp engine but seems to handle a few low amperage electronics OK.