Switches, Relays and Fuses
We will cover the basic understanding of these devices to help understand their uses and functions.
Wire
Not all wire is created equal and when dealing with damp and salty conditions it is important to first select the correct wire. Marine wire is multistrand tinned copper, meaning the wire inside the insulation is comprised of hundreds of small wires to make a single stranded wire, the tin coating helps with corrosion. Solid wires can come loose, corrode or break; and are harder to run in a boats confided spaces.
Wire comes in different thicknesses commonly referred to as the gauge or AWG of the wire. In the U.S. it is a little confusing as the smaller the number the bigger the wire. AWG stands for American Wire Gauge and was implemented in the mid 1800's to designate size of the die the wire was drawn through. A wire pulled through 30 dies is a 30awg and a wire pulled through 10 dies is 10awg, making it a little confusing.
The size of the wire is important because that determines the amperage of the wire. This is the amount of current a wire can safely carry without overheating and damaging the wire, equipment or starting a fire. Instead of trying to do the calculations for ampacity, there are several charts and resources found online to make sure you have the correct size wire for your application.
Inside a relay is a coil that controls a contact. When voltage is applied to the trigger, the contacts close and current is sent out to the device. A starter requires a large amount of amps to turn the engine over, more so for diesel engines since they rely on compression to start.
Normally to carry the amount of amps to turn a starter, you would need large cable and not just wire. From the batteries there is 2/0 to 4/0 cable running to the starter to carry the full amps of the battery. If we were run this from the ignition, we would need to route the large heavy cable to a heavy ignition relay and then back to the starter. To decrease the weight and make running cable easier, we put relays along the wire run. Since relays need only a small amount of amps to function, the wiring can be scaled down from 4/0 to 16awg or smaller.
30 - Is constant 12v input into the relay
85 - Is the ground connection
86 - Is the trigger voltage to close the circuit
87 - Is the output/load to the device.