What size wiring to use for my unit?

Without that information, we are left guessing. The consequence of using undersized (to thin) wiring is voltage drop and potential heating of the wiring. This again leads to other issues, typically called "bugs", where your marine electronics does not work the way you want it to. 

In order to find the maximum draw, you must check the manual. If you can`t find it in the manual, ask the dealer or manufacturer. Here are a few examples from manuals of popular marine electronics so you know what you are looking for:

Minn Kota Ulterra (easy to find - good work there MK)

Motorguide Xi5 (hard to find - what are you thinking MG?)

Lowrance HDS Live (easy to find - good work there Lowrance)

Garmin Echomap Plus 92sv (easy to find - good work there Garmin)

Now you know how to find the maximum draw, then we need to know the total length of wiring. However, before we look at that, let us look at WHY it is important to know both the maximum draw and the total length of wiring:

Voltage drop means that there will be less power (A) getting to your unit then what is leaving the battery, typically leading to problems with the unit not working as it should. Voltage (V) tells you something about how much power (A) can potentially be delivered, so if the voltage drops, power most likely drops as well.

Heat is the result of the voltage drop. As you might remember from school, energy never disappears, it only transforms into another type of energy. Thin cables and high load leads to the wires heating up, as the resistance in the wiring is hindering the load to "flow" through. Kind of like trying to push a thick leader through a tiny loop on your jerkbait. The leader and the loop has to match, or the loop has to be larger than the leader, or your leader will not go through as it should. Electric current need enough "space" in the wires to pass through without resisstance. (In electrical terms, that resistance is called Ohm).

The length of the wiring is important because the resistance in the wiring increases with the length. Kind of like when you walk ten miles and not five, you lose more energy on the long walk. So does electricity, and just like having good shoes can make that ten-mile hike feel less energy-draining then the five mile walk in bad shoes, using larger diameter wiring means the electricity loses less power.

Time to measure, and do it properly. Don`t think that if you have your battery at the back, and want your unit at the front, that the length of the wire will equal the length of you boat. Believe me, the wire will be a lot longer, so do the work and don`t guess. As an example, my boat is 15 feet, the cable from my rear battery to my console panel is 21 feet.  

So now you know why you need to use a minimum size of wires, and you know the length and maximum draw. How can one find that minimum size? That calls for some calculation, and if you feel like testing your skills in math, you can do that manually. For those of us that are a bit more lazy, there are several on-line calculators that do this for us, and one of the better is from BlueSea: http://circuitwizard.bluesea.com/# That calculator is also available as an app on your smartphone, and that is what I`ll use as an example here.

Let us say you have bought a HDS 9 Live, and by looking in the installation manual you have found that this has a maximum draw of 2,9A that we will round up to 3A to keep things simple. (I like "simple".) You have measured in your boat and found that the total length of wiring will be 21 feet.

Open the app, choose "Calculate Wire Size":

That was it, not hard at all as long as one has done ones homework and know the draw and the total length of wiring. As mentioned you can use thicker wiring, that will give even less voltagedrop, and perhaps your next unit will be a 12, and not a 9 like in our example. If you oversize you can update electronics without running new wires every time you go bigger on your unit. Now off to the shop to get what you need, but before you run off take a few minutes to read some info on what fuse to use:

In your manual, there will usually be something on what size fuse to use, but what the manual seldom tells you is that that size is the MINIMUM size to use. I`ll let you in on a little secret:

After reading that you probably think I`m crazy, so allow me to explain the reason for my bold statement.

A fuse will break or trip if the current (A) passing through it supersedes the fuse rating. Therefore, if you use a 5A fuse, and for some reason there is a 6A current going through it, the fuse breaks. That is the SOLE purpose of the fuse, it only protects against rising current. It does not protect against rising voltage, or a fool connecting the negative wire to the positive pole on the battery. Here comes the kicker - it is your unit that draws the current, the battery is not "pushing" out current that there is no need for. The only way to get a higher current flowing then your unit uses, is to short-circuit the negative and positive wire, and that has to happen AFTER the fuse in order for the fuse to break. If that short circuit happens between the fuse and the battery, the fuse will never know it and it won`t break, this is why a fuse should be placed as close to the battery as possible, NOT as close to your unit as possible. 

What you need to protect is the wiring, so it does not overheat in case of a short circuit (or a user connecting equipment that draws a higher current then the wires can take without building up heat). 

Moral: As long as you have taken the time to correctly calculate the wiring needed (which you have), you can use the fuse recommended in the manual, or a larger fuse. Since you now have the BlueSea app, you can use that to see what size fuse you can use on the wiring you have choosen. In our example, the manual says minimum 4A, and the app says 4 to 10 so our calculation was correct and you can use any fuse between 4 and 10A.

Using proper panels in your boat can help keep all the wiring tidy. I for one hate those loose fuses hanging off wires, and I hate loose wires. Keep it simple, use black wires for negative (ground) and red for positive (power), and look into using fuse-panels instead of having a zillion wires going from the back of the boat to the console or front. I highly recommend having at least one panel at the console, and in many cases one up front as well, both controlled by the main breaker in your boat. BlueSea fuse panels are great for such use.