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Step one: define the load and circuit
Start with what the wire will power. Note the expected current in amps, the voltage, and whether the load is continuous, like lighting or a heater, or occasional. Thicker copper, shown by a lower gauge number, carries more current safely. For example, a homeowner adding a small workshop circuit for a bench grinder and lights should first list every device that could run at once and total their amperage. That total, plus a safety margin, sets the minimum ampacity the wire must handle, before any other choice is made.
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Step two: account for distance and conditions
Next, measure the run from panel to load. Longer runs lose voltage, so a wire adequate for a short distance may need a thicker gauge when stretched across a yard or detached building. Also consider where the wire will live: inside walls, in conduit, buried, or exposed to heat. These conditions change how much current the conductor may carry. In the workshop example, a run of sixty feet to a detached garage could push the choice up a size from what the load alone suggested, which is why distance belongs in the calculation, not as an afterthought.
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Step three: confirm code, insulation and breaker
Finally, match the wire to the breaker and the rules that apply where you live. Wire gauge, breaker rating and insulation type must agree, because the breaker protects the conductor. Check the insulation rating for wet, buried or high-temperature locations, and confirm whether permits or inspection are required. In the workshop example, the electrician would pair the chosen gauge with the correct breaker, specify cable suited to the outbuilding route, and document the plan. Treat any chart as a starting point and let local code and a licensed professional make the final call.