How to derate a conductor properly
The ampacity comes out of your book. Everything that happens to it afterwards is arithmetic — and that is where conductors get undersized.
Sizing a conductor is not one lookup. It is a lookup followed by a chain of adjustments, and then a comparison that most people skip. Getting the lookup right and the chain wrong is the common failure, which is why this tool takes the table value from you and concentrates on everything either side of it.
Step-by-step
- Read the ampacity twice. Once in your conductor insulation's temperature column, and once in the column matching what the terminations are rated for. Both go into the tool, and they do different jobs.
- Enter the ambient. 30°C is the table's own basis and corrects to 1.00.
- Count the current-carrying conductors. Grounds are not among them.
- Read the chain. The tool shows base, then each factor, then the final figure and which constraint governed it.
Why 90°C wire does not give 90°C ampacity
This is the most misunderstood rule in conductor sizing. THHN is rated 90°C and the 90°C column is generous, so it is tempting to size straight from it. You cannot. The conductor lands on a breaker lug listed for 60°C or 75°C, and heat does not respect the difference — run the conductor at its insulation rating and the termination sits at a temperature it was never listed for.
What the 90°C column is genuinely good for is derating. You may start from it, apply your ambient and bundling factors to that larger number, and keep whatever survives — capped at the termination column. In a hot, crowded raceway that is worth real capacity, which is exactly why 90°C wire is worth buying. It is a better starting point, not a bigger finish line.
The cap is a comparison, not another multiplier
Worth stating plainly because the error is easy to make and hard to spot: the termination ampacity is compared against the derated figure and the smaller one wins. It is not multiplied in. Multiplying produces an absurdly small answer that, because it is conservative, can pass review unnoticed while the real conductor gets chosen by guesswork.
Both factors apply, not the worse of the two
Ambient correction and bundling adjustment answer different questions. Ambient asks how hot the air around the raceway is. Bundling asks how much of its own heat the conductor can shed given its neighbours. A conductor in a hot attic and in a full conduit gets both, multiplied. Taking only the worse of the two is a common shortcut and the code does not offer it.
The correction factors are a formula
The published ambient correction table is itself computed from
√((Tc − Ta) ÷ (Tc − 30)), where Tc is the insulation rating.
That reproduces every printed factor to the two decimals the table carries, so the tool derives it
rather than storing it — which also means it answers for ambients between the printed rows
instead of making you round to the nearest one.
What counts as current-carrying
Equipment grounding conductors never count; they carry no current in normal operation. The neutral is the interesting one. Where it carries only the unbalance of a three-phase, four-wire system, it does not count. Where the load is substantially non-linear — drives, electronic ballasts, large numbers of switching supplies — the harmonic current on the neutral is real and continuous, and it does count. Getting that wrong in a panel feeder is how a neutral ends up running hotter than any phase.
What this tool will not do
It contains no code tables. It does not know which edition your jurisdiction has adopted, what local amendments apply, or anything about your job, and it makes no determination of compliance. A licensed professional and the authority having jurisdiction decide that, and where they or the published code differ from the page, they are right.