Ampacity Correction Factors in Practice
Correction factors are multiplicative. Two moderate factors can cost you a third of the ampacity, which is usually one or two conductor sizes.
Ambient temperature
Every conductor is rated at a reference ambient. When the actual ambient is higher, the allowable current falls, and the penalty accelerates as the temperature rises.
The relevant temperature is where the raceway runs, not the outdoor weather report — a roof deck or an attic can be dramatically hotter.
Conductor count
Bundling conductors traps heat. Once more than three current-carrying conductors share a raceway, the allowable ampacity is reduced in steps as the count rises.
Note the word current-carrying: a grounding conductor does not count toward this adjustment, although it does count toward conduit fill.
Combine them, then apply the termination cap
Multiply the base ampacity by both factors, then compare the result with the termination-limited ampacity and take the smaller number.
Frequently asked
- Do the factors apply to the base ampacity or to the termination limit?
- They apply to the conductor base ampacity. The termination limit is a separate ceiling that is not adjusted by these factors.
- Does a conduit in sunlight need extra derating?
- Ambient temperature already accounts for the location. Where the raceway is exposed to direct sun, some jurisdictions apply additional rules, so confirm locally.
- What if my ambient exceeds the table range?
- The tables stop at a maximum ambient for each insulation class. Beyond that, the insulation is not suitable and you must change the conductor or the installation.
Related calculators
Educational information only — not professional engineering advice. Reference table values are pending review; see the methodology page.