In this last installment of our basic electrical series I’m going to put a little closure on this whole matter of amperage. If you’ve read the previous articles discussing amperage you should have a good understanding now of things like voltage, electrical resistance and its causes, and how to measure amperage.

Here are the articles, in case you need refresher:

Marine-grade wire is more expensive than standard SAE automotive wire, but it's much better suited for boats, with more copper conductor in a given gauge size, and better resistance to chafe and heat. Doug Logan photo.

You should also now understand that any electrical appliance that’s not getting the amount of amperage it’s designed to run with is not going to perform as designed. Don’t overcomplicate this matter; it’s that simple, really! So, besides having a power supply (your battery or batteries) that has to deliver adequate amperage, you need to make sure you have provided an electrical circuit with enough "ampacity” to get all that needed current to your baitwell pump, navigation lights, or stereo. Wire sizing and good solid electrical connections are the keys to success here.

As for the selection of the proper type of wire for use on your boat, there are several things you need to be aware of.

First of all, there's a pretty significant difference between automotive and marine-grade wire. One of the primary differences is in the actual amount of copper used for a given gauge size. For example, 14-gauge wire using SAE (Society of Automotive Engineers) grade wire will have less copper in it than a 14 AWG (American Wire Gauge) marine-grade wire. The actual variance here will be different depending on the gauge number, but typically we’re talking about somewhere around 10-15% less copper in the SAE wire. In your car, that makes little or no real difference for several reasons. One, the engine in your car is typically running when you're in it, so the engine’s alternator is producing plenty of voltage to run things. Also, wire-run distance in a car is typically not that long compared to wire runs on your boat in many cases. And on your boat you're more likely to be in a discharging state as you sit at anchor running your electrical appliances. The bottom line here? More copper reduces electrical resistance and therefore enables the delivery of more amperage for a given gauge size.

The other difference between auto and marine-grade wire can involve the insulation that's used. Besides oil and fuel resistance, we're also concerned with properties related to chafe resistance and thermal rating. Typically, the marine-grade wire will be superior than the automotive counterpart in these areas as well.

Tinned Wire vs. Untinned

I get a lot of questions regarding tinned versus untinned wire, with many people believing that tinned wire is the only choice viable in the marine world.

Let there be no doubt, the tinning has no impact on amperage-carrying capability, it’s there to help mitigate corrosion migrating along a conductor under the insulation jacket via capillary action. There are tens of thousands of boats plying the waters of the world using untinned wiring that provides good service, and this has been true for decades!

Untinned copper wire (left) is used in boats with good results, but tinned wire (right) is better able to resist corrosion, both at terminals and in capillary action along the wire.

Is tinned better? Sure. But is it a game changer? I would argue no, as long as you keep the ends of your wiring away from exposure to water, which is generally considered good practice anyhow.

So, what are the things we need to consider as we decide on what size wire we need to use for a new accessory installation? Here’s the line-up of questions:

• How much amperage do we need?

• What is the system operating voltage?

• How long is the wire run, or total circuit length as measured from the power source to the load or appliance and back to the source of power?

• What level of voltage drop is acceptable, 3% or 10%? (See Electrical Resistance on Boats: Keep That Voltage Drop in Check.)

Additionally, and dependent upon some of your usage habits, you may want to consider how long you intend to operate a given appliance under normal circumstances. The idea here is that if you're running an appliance continually versus intermittently, you may generate some additional heat in the power supply circuit to the appliance. In general, this should not be a problem unless your boat’s wiring is in large bundles of multiple conductors and routed through wire tubes tightly packed with power cables supplying the electrical loads on your boat.

So, what's the best way to figure out what wire gauge to use in a given circuit? I use the Circuit Wizard app found at the Blue Sea Systems website. The data entry page from the site looks like this:

Blue Sea Systems' Circuit Wizard app is available online and for iOS and Android.

This program is now available in both an iOS and Android version as well as the web version shown in the screen shot above.

Follow the guidelines outlined here and you should never have any issues with selecting the correct size wiring to satisfy your boat circuits’ appetite for amps.