An ampacity is not a fixed property of a conductor. It is a permitted current under a stated set of conditions, and if the conditions change the number changes.
A number copied out of a table without its conditions is the most convincing wrong answer in the trade. It came from a real table, it looks authoritative, and it may not apply to the installation in front of you.
Three things can move the answer. Ambient temperature is one. How many conductors are bundled together is another -- conductors that cannot shed heat because they are grouped do not behave like a single conductor in free air. And the third is the terminations at each end: the usable value can be limited by what the terminals are rated for, no matter what the conductor itself could do.
Check yourself: You find an ampacity in a table for the conductor you are using. Can you use that number?
Answer: Not until you have checked the conditions the table states, the temperature and grouping, and the termination ratings.
The table value is stated for a set of conditions. Change the conditions and the permitted current changes with them, and the terminations can cap the usable value independently of the conductor.
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Written from the owner's poster for this lesson. Nothing added beyond what the poster states.
Written for Texas apprentices as free study material. The Texas apprentice licence is an application, not an exam (Occupations Code 1305.161), and nothing here counts as apprenticeship hours or continuing-education credit. VoltMark is not affiliated with, endorsed by, or sponsored by NFPA, PSI, TDLR, or any state or local licensing authority. Always follow your employer's safety programme and your supervising electrician.
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