If you're buying wires by size in inches or millimeters or have stripped and measured uninsulated wires with a gauge tool, use the wire gauge chart to determine the maximum amperage of a wire.

The rule-of-thumb sizes rely on the lowest allowable temperature rating ... but as an example, according to this NEC chart, a 6-gauge copper circuit rated at 90°C (194°F) has an ampacity of 75 amps ...

Nails are generally categorized by their gauge, which is the size of their thickness. This thickness is defined by the diameter of the nail's shaft, rather than the typically wider head.