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 do the correction factors come 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

Every number in this calculation comes from you and your code book; the tool only multiplies them. It is not a substitute for the Canadian Electrical Code, your 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.

Worked example

Say your book gives a base ampacity of 30 A, an ambient correction of 0.88 for a warm room, and a bundling factor of 0.80 for several conductors run together. The corrected ampacity is 30 × 0.88 × 0.80 ≈ 21.1 A — a reminder that heat and grouping together take a real bite out of what a conductor can carry.

The formula

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 do the correction factors come 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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