Derating: Ampacity After Real Installation Conditions
Derating is the reduction of a conductor's tabulated ampacity after real installation conditions, usually ambient temperature and current-carrying conductor count, are applied.
A conductor can pass a quick ampacity chart and still fail in the installation. Derating is the part of the sizing sequence that asks how hot the space is and how many current-carrying conductors share the raceway before the result is capped by the equipment terminals.
What derating changes
The ampacity table starts from standard conditions. Derating adjusts that number for heat that is harder to shed in the actual installation.
The usual field inputs are the ambient temperature around the raceway or cable and the count of current-carrying conductors bundled together.
The sequence matters
Read the conductor insulation column first, apply correction and adjustment factors, then compare the result with the terminal temperature cap.
Changing the order can produce a different conductor size. The site calculators show the governing limiter so the result is traceable.
Why splitting the pull can beat upsizing
Four to six current-carrying conductors use an adjustment factor, and larger bundles reduce ampacity further.
Moving circuits into separate raceways can remove a conductor-count penalty. On a long or expensive feeder, that can be cheaper than upsizing every conductor.
Where the Term Decides Something
Four points in the work where the figure is the answer to a question, not vocabulary.
Hot attic feeders
A feeder in a high ambient space may need a larger conductor even when the load is ordinary, because the ambient correction lowers the usable ampacity.
Shared raceways
Several circuits in one raceway can trigger the conductor-count adjustment. That is a thermal calculation, not a physical fill calculation.
Plan-review arithmetic
A reviewer checking a derated pull expects to see the table ampacity, ambient factor, conductor-count factor, terminal cap and final usable ampacity in order.
Conductor upsizing choices
When derating pushes the usable ampacity below design current, the remedy may be a larger conductor, a split raceway, lower ambient exposure or different equipment.
Which Figure to Use
Reading the term correctly in practice.
Start from the insulation column
Use the conductor insulation rating for the correction and adjustment math. THHN normally starts in the 90 C column before the terminal cap is checked.
Multiply the reduction factors
Ambient correction and conductor-count adjustment compound. A 0.91 ambient factor and a 0.80 conductor-count factor leave 72.8 percent before terminal limits.
Keep fill and derating separate
Fill counts occupied raceway area. Derating counts current-carrying conductors and heat. The same pull can pass one check and fail the other.
Apply the terminal cap after reductions
The corrected ampacity cannot exceed the 60 C or 75 C table value allowed by the limiting equipment terminal.
FAQs
Is derating the same as conduit fill?
Which ampacity column is derated?
Does the equipment grounding conductor count for derating?
When does derating force a larger wire?
Calculated against the NEC 2023 edition using 310.15(B)(1), 310.15(C)(1), 110.14(C). Confirm the edition adopted by your AHJ, which may differ and may carry local amendments.
Not covered by this tool
- Free-air ampacity tables and installations outside the site calculators.
- Equipment listing instructions that set a different conductor or raceway condition.
- Project-specific AHJ interpretation of conductor count and ambient conditions.
Data last verified