Derated Conductor Ampacity

A conductor's ampacity is not a property of the conductor. It is a statement about a conductor in a set of conditions — a stated ambient temperature, a stated number of current-carrying conductors bundled together, a stated insulation rating. Change the conditions and the number changes with them. The two corrections that apply almost everywhere are multiplicative: multiply the table figure by the ambient temperature correction, then by the conductor-count adjustment, and what is left is what you may actually load the conductor to. A conductor good for 100 A on paper, run through a hot space that calls for 0.88 and bundled tightly enough to call for 0.80, is good for 70.4 A.

The classic mistake here is applying the factors to the wrong base. The correction and adjustment factors multiply the table ampacity for the conductor's own insulation rating — the 90 °C column for a 90 °C conductor — and only afterwards is the result compared against the terminal temperature limit, which is usually 60 °C or 75 °C. Deriving from the 75 °C column because the terminations are 75 °C throws away the headroom the 90 °C insulation was bought for, and it is the single most common way to over-size a feeder. The mirror mistake is worse: applying no correction at all because the run "is only a few feet in the attic". Ampacity is governed by the worst portion of the run, not the average one.

Correction factors above 1.0 are real — a cold ambient genuinely allows more current — but they are the place to be most careful, since a factor above 1 is also exactly what you get by entering a percentage as a plain number. And note what this arithmetic does not cover: the overcurrent device's own rating, continuous-load factors, the ambient the raceway sits in versus the ambient the conductor sits in, and the terminal limits above. Look every one of them up in your own edition of the code. This page multiplies three numbers correctly; it does not design your feeder.