How to Read an Ampacity Chart
An ampacity chart gives you the starting number, not the answer. The answer comes from applying the ambient, conductor-count and termination adjustments to it.
Four steps
- Find the row for the conductor size.
- Pick the column for the insulation temperature rating.
- Apply the ambient temperature correction factor for the installation location.
- Apply the conductor-count adjustment if more than three current-carrying conductors share a raceway.
Then stop and check the termination
The corrected ampacity is only half the story. The device the conductor lands on sets its own ceiling, and for circuits of 100 A or less that ceiling is often the 60°C column.
Take the lower of the two numbers. That is the ampacity you are allowed to use.
Frequently asked
- Which temperature column do I use?
- Start from the conductor insulation rating, then verify the termination is rated for the same temperature. If not, use the lower of the two.
- Do I count the neutral as a current-carrying conductor?
- It depends on the circuit. In many three-phase systems the neutral carries only harmonic current and is not counted; in others it counts. This affects the adjustment factor, so treat it carefully.
- Why is my conductor suddenly not adequate in summer?
- High ambient temperature reduces the correction factor. A raceway on a roof or in an unconditioned space can be far hotter than the air temperature.
Related calculators
Educational information only — not professional engineering advice. Reference table values are pending review; see the methodology page.