Handy Wire Size Calculator Tool

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FOUNDER of Classic Parker Forum
Feb 16, 2006
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Newbury, MA
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


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Nice catch Dale! Looks to be useful for house/home/misc. wiring as well.
Has anyone tried to download this recently....link doesn't seem to work. Does anyone still have the exe file for this program and could you share it with me.