Derating

Derating is reducing the current you allow a conductor to carry because the conditions around it make it run hotter than its rating assumes. A rating is written for particular conditions; change the conditions and the allowable current comes down.

Why it happens

A conductor’s ampacity is a temperature limit wearing an amps label. Current makes heat and the heat has to escape. If the room is already hot, the conductor starts nearer its limit. If several loaded conductors share a raceway or a bundle, each one is sitting in the others’ heat and none of them can shed its own as easily. Either way the limit arrives at a lower current, so the current has to come down to match.

Devices get derated too: breakers, drives and power supplies all have ratings that assume an ambient temperature and an altitude, and makers publish how much to reduce them outside those.

How the numbers are used

Derating is multiplication. Start from the conductor’s base ampacity for its size and insulation, then multiply by a factor for each condition that applies — one for ambient temperature, one for the number of current-carrying conductors together, and so on. The factors are always one or less, so the answer only ever goes down. Every one of those numbers — the base value and each factor — comes from your code book or the maker, and they change on a code cycle; this site carries none of them.

One worked, so the idea sticks

Two factors, one after the other

Your book gives a base ampacity of 40 A for the conductor. (Practice number.)
The room runs hot: say the book’s temperature factor for it is 0.85. 40 × 0.85 = 34 A.
Several current-carrying conductors share the pipe: say that factor is 0.75. 34 × 0.75 = 25.5 A allowed.

The three numbers above are made up for practice, not taken from any code table. Look up your own — and the order you multiply in does not change the answer.

Questions people ask

Is derating the same as oversizing the wire?

They are two sides of one coin. Derating tells you how much a given conductor may carry in the conditions; if that falls below what the load needs, the fix is usually the next size up — which is what people mean by upsizing for derating.

Does the neutral count as a current-carrying conductor?

Sometimes. It depends on whether it carries only the unbalance between phases or carries current of its own — as it does with some nonlinear loads. The rules for when it counts are in the code book.

Do I derate for voltage drop?

No — voltage drop is a separate check. Derating is about heat in the conductor; voltage drop is about how many volts reach the load. A long run can pass one and fail the other.

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