Ampacity
Ampacity is the current a conductor can carry continuously without its insulation getting hotter than it is rated for. Underneath, it is a temperature limit — the amps are simply how much heating the insulation is allowed to take before it starts to degrade.
Why it is about heat, not just amps
Current through a conductor makes heat, and that heat has to escape into the surroundings. If the surroundings are already warm, or if several loaded conductors are packed together each adding their own heat, the conductor hits its temperature ceiling at a lower current than it would on its own in cool air. That is why ampacity is not a single number stamped on a wire size — it is a starting value that gets adjusted, downward, for the conditions the conductor actually runs in. Adjusting it down is what tradespeople mean by derating.
The starting values for a given conductor and insulation, and the factors you multiply them by, are code-book and manufacturer data — and they move on a code cycle. So this page deliberately carries none of those numbers. What it gives you is the idea, so that when you open your book the figures mean something: a temperature limit, expressed as a current, reduced for heat it cannot easily shed.
Questions people ask
What is ampacity in simple terms?
The current a conductor can carry continuously without its insulation getting hotter than it is rated for. It is really a temperature limit dressed up as a current — the amps are just the amount of heating the insulation can live with.
Why does ampacity depend on more than the wire size?
Because it is about heat, and heat depends on the surroundings. A hot ambient, or several loaded conductors bundled together sharing their warmth, means the conductor reaches its temperature limit at a lower current — so the allowable ampacity is derated.
Where do the actual numbers come from?
From your code book and the manufacturer's data, which is exactly why this page does not print them. The figures change on a code cycle and by product; this explains the idea so the numbers make sense when you look them up.
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