Choosing the correct wire
When making a project on protoboard or a custom PCB, here are some considerations for selecting wires that connect that board to other devices:

Current carrying capability (ampacity)
What wire gauge is needed for the current? What voltage drop will the wire impose for the intended distance?

Voltage
What voltage needs to be carried? Is the insulation appropriate?

Temperature
What temperatures will the wires be exposed to? How does that affect their performance?

Flexibility
Does it connect to moving parts? Will vibration be a factor? Stranded wire is more flexible than solid.

Durability
Are the wires going to be pulled on or be routed through an enclosure? Should a grommet be used to prevent sharp edges cutting into the insulation? Is a strain relief needed?

Connectivity
Does the wire need to be disconnected for servicing or convenience? Is the connection something a customer or end user will have access to?

Label / Color
Is the wire color important? Does it need to follow a standard or published document?


All of these things may or may not be a factor in your project, and any of them can define what is &quot;correct&quot; for your application. (There are other things to consider which I've not listed, such as conductor material, weight, etc.)
Because you mentioned &quot;12V 1.8A&quot; I presume the first point is most important to you in this question. The ampacity is the ampere capacity of a particular conductor. There are lots of charts which provide these figures, as well as the math behind them. Generally the values are given for two cases:

Free air (the wire is not in a bundle and has the benefit of air cooling)
Bundle or enclosed (the wire is part of a bundle or otherwise not convection cooled)

Using the first linked chart, you can see that 24 gauge wire is sufficient for 1.8A in both free air (3.5A) and enclosed (2.1A). You have to derate these figures if the ambient temperature is significantly higher.
I frequently salvage wire from standard ethernet (UTP Cat5e) cabling for projects. Be careful though, Cat5/5e can use 26 gauge, too. Check the outer jacket for the wire gauge used.