How do I derate a conductor’s ampacity?

Take the base ampacity you looked up in your own code book, then multiply it by the ambient-temperature correction factor and by the factor for bundling several current-carrying conductors together. The product is the corrected ampacity the conductor can safely carry in those conditions.

Where are the correction factors read from?

From your code book — the ampacity, the temperature correction and the bundling adjustment are all values you read there and type in. This tool holds none of those numbers; it only multiplies the ones you supply, so it stays correct whichever code edition or jurisdiction you work under.

Do I multiply or add the factors?

Multiply. Each factor is a fraction of the base rating, and they compound: a conductor in a hot space that is also bundled loses to both effects at once. So corrected ampacity = base × ambient factor × bundling factor, not base minus a sum.

What if there is only one correction to apply?

Leave the other factor at 1. A factor of 1 means no reduction, so setting the bundling factor to 1 gives you an ambient-only correction, and vice versa. That way the same tool covers a single adjustment or both together.

Conductor derating calculator

Courant admissible de base (selon votre code)

Le courant admissible du tableau pour votre conducteur — vous le lisez, vous le tapez.

Facteur de température ambiante

Facteur de regroupement

— A

Le courant admissible de base et chaque facteur viennent de votre propre code; ceci n’est que leur produit.

Garder ces résultats

Nous envoyons ce seul courriel par notre fournisseur de courriel et ne gardons pas votre adresse.

Every number in this calculation comes from you and your code book; the tool only multiplies them. It does not stand in for the Canadian Electrical Code, the local amendments, or a qualified person's judgement — confirm the base ampacity and each correction factor against the current tables, and have the circuit signed off before it carries load.

Read the base ampacity and the two correction factors from your own tables, type them in, and the tool returns the corrected ampacity. It deliberately ships no lookup values — the arithmetic is the same whatever edition of the code you hold, so the answer never goes stale.

Exemple détaillé

Say your book gives a base ampacity of 30 A, an ambient correction of 0.9 and a bundling factor of 0.75 (both invented here for the arithmetic). The corrected ampacity is 30 × 0.9 × 0.75 ≈ 20.3 A — a reminder that heat and grouping together take a real bite out of what a conductor can carry.

Sur le chantier

Factors multiply; they never add. A 10% cut for heat and a 25% cut for grouping is not a 35% cut. 0.9 × 0.75 = 0.675 — a 32.5% cut. On a 40 A base that is 27 A, not 26. Small on one circuit, and the wrong way on every circuit in the building. (Those two factors are made up for the arithmetic; yours come from your book.)

Three factors, any order. Base × ambient × grouping × whatever else your book applies to the run — the order does not matter, the product does. Type them all; a factor you leave out is a factor of 1.0, which is the same as pretending the condition is not there.

When the corrected figure is under the load. If 27 A comes out and the circuit carries 30, this page does not pick the next size. You go back to your book, read the base ampacity for the next conductor up, type it in the base field with the same factors, and see whether that one clears. The tool multiplies; the book chooses.

Exercez-vous

Quick referencefor correction factors

Courant adm. corrigé: base × factor × factor × …
Factors: multiply — 0.9 and 0.75 together leave 0.675, not 0.65
Order: does not matter; leaving one out does
Every number: base and each factor come from your code book for your run — none live on this page
Rounding: the corrected figure is a ceiling — round it down, never up

Mistakes to catchbefore you trust the number

Applying a factor from a different column of the table than the conductor's insulation rating — the factor you type must be the one for the row you read the ampacity from.
Entering a percentage (80) where the tool wants a factor (0.8); a corrected ampacity larger than the base is the tell.
Leaving a factor at 1 because it 'probably does not apply' — if it is in your book for this run, type it; a 1 means you have decided the correction is not there.

What an inspector looks forthe electrical inspector

On this part of the job

The ampacity you started from, and that it came from the right table and column for this installation method — the base figure is the most common place a derating goes wrong.
The correction for ambient temperature, using the ambient this run actually sees: a boiler room, a roof, an attic and a cold cellar are four different numbers.
The adjustment for the number of current-carrying conductors bundled or in one raceway, counted correctly — which conductors count is itself a rule worth reading twice.
The termination temperature rating, which can cap the result no matter how good the corrected ampacity looks.
That the final corrected figure still carries the load, and that the protective device suits the conductor.

Have this ready

Your code book, open at the ampacity table, the ambient correction factors and the bundling adjustments. The tool multiplies the factors you type in; it does not hold any of them.
The route with the worst ambient identified — that is the one the correction is done for.
The load calculation the conductor was sized for.

Every figure an inspector measures against comes from the code your jurisdiction has adopted, the stamped drawing, or the manufacturer's printed instructions — so none of them are printed here. Look the number up in your own book, type it into the tool, and the arithmetic is yours.

La formule

corrected ampacity = base ampacity × ambient factor × bundling factor

Each correction factor is a proportion of the base rating, and they multiply because the effects stack. A factor of 1 applies no reduction, so you can run either correction on its own by leaving the other at 1.

Questions

How do I derate a conductor’s ampacity?: Take the base ampacity you looked up in your own code book, then multiply it by the ambient-temperature correction factor and by the factor for bundling several current-carrying conductors together. The product is the corrected ampacity the conductor can safely carry in those conditions.
Where are the correction factors read from?: From your code book — the ampacity, the temperature correction and the bundling adjustment are all values you read there and type in. This tool holds none of those numbers; it only multiplies the ones you supply, so it stays correct whichever code edition or jurisdiction you work under.
Do I multiply or add the factors?: Multiply. Each factor is a fraction of the base rating, and they compound: a conductor in a hot space that is also bundled loses to both effects at once. So corrected ampacity = base × ambient factor × bundling factor, not base minus a sum.
What if there is only one correction to apply?: Leave the other factor at 1. A factor of 1 means no reduction, so setting the bundling factor to 1 gives you an ambient-only correction, and vice versa. That way the same tool covers a single adjustment or both together.

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