Switches, Relays and Fuses
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.
Fuses
This brings us to fuses. A common misconception is that fuses are used to protect the device or equipment. Their main purpose is to protect the wire from getting damaged and starting a fire. Systems on boats should be fused or have a breaker at both ends of the wire. If the maximum amperage the wire is capable of handling is 20 amps, then you want to step down to the next standard size fuse and use 15 amps. This will ensure that the wire never reaches the maximum rated amperage and overheat, the fuse will trip and the power will stop flowing along the wire.
Switches
Switches are used to turn power on or off and can either be momentary or latching. An example of a momentary switch is for a horn. You press the switch and the horn sounds, when you release the switch, the horn stops. A latching switch is what we are more used to. You press the switch and it clicks on and you have to push the switch again to turn the power off.
Switches are similar to fuses as they have rating on them for the maximum amount of current they will handle. Running 30 amps to a switch only rated for 2 amps will cause the switch to fail or overheat. It does not matter if the switch is on the negative or positive side, it needs to be rated for the proper amperage. If you are using wire that can carry 30 amps but you are connecting it to 1 amp LED string, then a 2 amp switch is fine, there is no harm in upsizing your wire.
Relays
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.