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What ampacity means
Ampacity is the maximum current a wire can carry continuously without its insulation getting hotter than it is rated for. It is the single most important safety number in a wiring job, because a wire that carries too much current does not fail loudly at first. It just gets hot, slowly cooks its insulation, and becomes a fire risk inside a wall where nobody can see it. Every cable has an ampacity, and that ampacity changes with the wire size, the conductor material, the insulation temperature rating, and the way the wire is installed.
This estimator gives a simplified ampacity for common copper and aluminium building wire, in free air or inside conduit, so you can compare sizes before you buy. A 12 AWG copper wire, for example, is estimated here at about 25 amps in conduit, while the same size in aluminium is about 20 amps. Those numbers are planning estimates from simplified tables. Real code tables add temperature correction, adjustment for several current-carrying conductors bundled together, and special rules for small conductors, so the number on a real job must come from the applicable code and a licensed electrician.
Why installation method changes the number
Heat is the whole story. A wire in free air sheds heat easily, so it can carry more current than the same wire packed inside a conduit with other warm wires. A wire buried in insulation, run through a hot attic, or bundled with six others loses its ability to cool down, and its safe current drops. That is why this tool asks for installation type instead of giving one number per gauge. Two jobs with the same wire and the same load can have different answers, and the hotter, more crowded job is the one that needs the thicker wire.
A worked example
Suppose you plan a 20-amp circuit and wonder whether 12 AWG copper in conduit is enough. The estimator gives about 25 amps for that combination, and 80 percent of that, about 20 amps, is the usual planning figure for a continuous load. That means 12 AWG copper fits this job on paper, with the continuous-load rule as the binding check rather than the raw ampacity. If the same circuit were aluminium, the estimate drops to about 20 amps in conduit, and 80 percent of that is only 16 amps, so the same load would need 10 AWG aluminium instead.
A second example shows the free-air trap. A wire rated 40 amps in free air may be rated only 35 amps in conduit, and lower still if it runs through insulation or a hot space. People sometimes quote the free-air number for a wire that will live inside a wall, and that is exactly how an overloaded circuit is born. Always pick the installation type that matches where the wire will actually spend its life, and when in doubt, pick the cooler, more conservative number and the thicker wire.
Common mistakes with ampacity
The most dangerous mistake is using the breaker size as the wire size. A 30-amp breaker does not make a 14 AWG wire safe; the wire must be rated for the breaker that protects it, not the other way round. The second mistake is ignoring continuous loads. A load that runs for three hours or more, like a heater or a large charger, is planned at 125 percent of its current, which is the same as using only 80 percent of the wire rating. Skipping that step overloads circuits that look fine on a quick check.
Frequently asked questions
Can I run a wire at its full ampacity all day? Only if the load is not continuous and the installation matches the table. For continuous loads, plan at 80 percent. Does a thicker wire waste energy? No, thicker wire loses less energy as heat, but it costs more and is harder to work with, so the right size is the smallest size that is safely rated, not the largest you can afford. Why is aluminium rated lower? Aluminium has higher resistance than copper, so it heats more at the same current, and it needs larger sizes and proper terminations. If you are choosing between materials for a home circuit, copper is the simpler and more forgiving choice.
Safety first: an estimate, not a code approval
Every electrical tool on this page is an educational estimator only. It is never professional advice and never a substitute for a licensed electrician. Electrical codes differ by country, region and building type, and they change over time. A result that looks fine here can still fail a local inspection, and a result that looks tight here can still be acceptable under a rule this simple calculator does not model, such as temperature correction, grouping, or a special location rule. Before you buy cable, cut a wall, or touch a panel, check your local electrical code and talk to a licensed electrician. Breaker, conduit and box-fill results are estimates, not code-compliance guarantees. If you are unsure about any part of a job, stop and ask a professional. Electricity can injure or kill, and no free online calculator changes that fact.
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