Chapter 2 of the NEC has a logical progression of five Articles that run from 210 through 230. The first three provide the requirements for branch circuits, feeder circuits, and their calculations. Article 230 provides the requirements for services.

The first three Articles assume the circuits are inside a building. If they aren't inside a building but run on it or attach to it, they must also meet the requirements of Art. 225 [225.1 and Table 225.3].

If you run individual conductors in an open overhead span that's 600V or less, they must be at least 10 AWG copper or 8 AWG aluminum [225.6(A)(1)]. If over 600V, they must be at least 6 AWG copper or 4 AWG aluminum [225.6(A)(2)]. The rationale behind this is the need for a certain minimum physical strength. The conductor must be able to withstand such things as wind, ice, rain, and the weight of birds.

This same rationale is why the NEC lets you use a smaller conductor for festoon lighting. If you have a string of lights suspended by two points, you have festoon lighting [Art. 100]. The conductors for this application can be as small as 12 AWG (the NEC does not specify copper or aluminum), unless the conductors are supported by messenger wires [225.6(B)].