Marine Wiring- Determining Wire Size?

Mark Vanderhoof

Mark Vanderhoof

Active member
i have a wiring plan drawn up for my current boat project but i have some questions. here is what i have to wire:

-two AGM batteries
-on board charger
-battery ONE will power; the motor, bilge pump, 6 LED interior lights, bow and stern light, sonar/GPS, DC plug.
-battery TWO will power; transom mount trolling motor and will not be connected to outboard motor.
-each battery will have a perko on/off battery dissconnect switch.
-during hunting season, battery TWO and the trolling motor will be removed from the boat. my backup starting source is a factory pull start on my 50hp outboard.
-i plan to have a neg. bus bar and pos. bus bar for battery ONE connections then on to a 5-way fused switch panel.
-the sonar will not be part of the switch panel.

here are my questions?
-what size wire do i need to go from the battery to the battery switch?(assuming a 4-5' run)
-do i need a main fuse between the battery and the battery switch? if so, what size?
-what size wire do i need to go from the battery switch to the bus bar terminals?(assuming a 1-2' run)
-do i need a fuse in between the battery switch and bus bar?
-what size wire do i need for all accessories? most accessories will be a short run except for the bow light and LED interior lights. id like to use one size wire for all. i was thinking maybe 12-14GA?

i hope this makes sense as i cant make any sense of the wiring charts that ive found online so im in real need of help! thanks alot, mark
 
Copied this from ClassicParker.com

Here's a neat tool you can download to your desktop for use when you need to add some wiring into your boat. An important note: You always need to factor in the TOTAL length of the curcuit ... that means both ways, and not just the 1-way run.

Say I want to add in a cigarette lighter for auxiliary items like a spotlight, for example. I want it to be able to handle 15 amps and it's 20' from the battery connection to the panel where it will be mounted. In the example shown below, you will need to calculate the total circuit length as 40' (and NOT 20') as the current flows "both" ways.

Let's also assume I want this to have around 3% voltage drop, as I'd consider a spotlight a safety item (for example purposes). FYI - the typical voltage drop ranges used are 3% voltage drop for safety/critical items (VHF, running lights, etc.) and 10% max voltage drop for aux items (house lighting or am/fm radio). The dedicate wiring I have in place for my bilge pumps are all rated @ 3% or less too.

Using the calculator, you would need to use #8AWG to give you sufficient ampacity in the wire to handle up to a 15 amp load over that 20' run (40' circuit length) at ~ 3% voltage drop. Note that the total ampacity of that #8 wire is 80 amps ... above that and the wire will start to burn up. So I'd probably would throw a 20-amp fuse into this circuit, for some overhead when starting/energizing a 15-amp device. Tip: Using the ">>" and "<<" keys, you can go up and down in wire size.

Download it here and see below, as it's on the left-hand margin of this page, see: http://www.midcoast.com/~aft/index2.html
 
Mark I'm no expert, but I found a book "The 12 Volt Bible" by Miner Brothern revised by Ed Sherman, it has the info for sizing wiring. It also has info on all aspects of boat electrial systms,batts, panels, wires& wiring, charging, & the rest, A to Z. Hope this helps.
Dennis
 
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