Ampacity and derating
Corrects for ambient and bundling, then applies the termination temperature limit — the rule that makes a 90 °C conductor a 60 °C conductor.
The wire is not the weakest part
A 90 °C conductor does not give you 90 °C ampacity. What limits you is whatever the conductor is bolted to, because the terminal is what overheats.
This is NEC 110.14(C), and it is the single most commonly skipped rule in conductor sizing. Pick THHN, read the 90 °C column, size the wire, and the design is illegal — not because the insulation cannot take the heat, but because the lug at each end cannot.
The thresholds are simple. Circuits rated 100 A or less, or terminals marked for 14 AWG through 1 AWG, are limited to the 60 °C column unless the equipment is listed and identified for higher. Above 100 A, or larger than 1 AWG, the limit is 75 °C.
The nuance most summaries miss: most modern equipment is listed and marked for 75 °C. So the 60 °C floor is not automatic — it is what you get when the marking is unknown. Checking the label is often worth a conductor size, which is why this tool asks instead of assuming.
The insulation rating still matters
It decides which column the derating arithmetic starts from, and a higher rating survives a hot ambient better. It simply cannot raise the final answer above the termination cap. So a 90 °C conductor is still the right choice in a hot attic; you just cannot bill it at 90 °C.
Corrections multiply, the cap does not
Ambient correction and bundling adjustment are different physical effects — hot air outside the raceway, conductors heating each other inside it — so they multiply. A conductor in a 45 °C attic bundled with nine others takes both hits.
The termination limit is not a factor at all. It is a ceiling, and the answer is the lower of the derated figure and the termination column figure. Either can bind, and the tool says which one did.
What is not in this tool, and why
The ampacity table and the correction factor tables are copyrighted, so they are not here. You enter the base figures from your own copy. That is the same decision as the BS 7671 circuit check: where the authoritative number lives in a book you already own, asking for it beats shipping a copy that cannot be licensed and goes stale every code cycle.
The ambient correction factor is computed rather than looked up, from the formula the published table is itself calculated from. It reproduces every published factor to within half of the last digit — except below the 30 °C base, where the formula is optimistic and is deliberately not used. See the FAQ.
Questions electricians actually ask
I picked the 90 °C column. Why is the answer lower?
Because the conductor is not the limiting part. NEC 110.14(C) limits you to the temperature the terminations are rated for, and the terminal is what overheats — not the wire.
Circuits rated 100 A or less, or terminals marked for 14 AWG through 1 AWG, are limited to the 60 °C column unless the equipment is listed and identified for higher. Above 100 A, or larger than 1 AWG, the limit is 75 °C. Your 90 °C insulation still decides which column the derating starts from — it just cannot raise the final answer past the termination limit.
Sizing a conductor on its own 90 °C rating produces a design that fails inspection even though the wire itself is perfectly happy. Almost no calculator applies this.
The equipment label says 75 °C. Does that change things?
Yes, often by a whole conductor size, and it is worth looking. Most modern equipment is listed and marked for 75 °C, so the 60 °C floor is not automatic on smaller circuits — it is the answer when the marking is absent or unknown.
This tool asks rather than assuming, and defaults to the conservative reading if you leave it unstated. Guessing 75 °C where the equipment is really 60 °C undersizes the conductor, which is the failure worth avoiding.
Why do I have to type the ampacity in myself?
Because the ampacity table is copyrighted and cannot be republished here. The same is true of the correction factor tables.
You already own the authoritative copy, so the tool asks for the figure rather than shipping one that could not be licensed and would go out of date every three years anyway. It is the same arrangement as the BS 7671 circuit check on this site: where the number lives in a book the practitioner already has, asking beats copying.
What the tool does supply is the arithmetic — the corrections, the adjustment, and the termination cap — which is where the mistakes actually happen.
Where does the ambient correction factor come from, if not the table?
From the formula the table is calculated from: the factor is the square root of the ratio of temperature headroom, √((Tc − Ta) ÷ (Tc − 30)). Computing it is not the same as reproducing the table.
It was checked against the published factors at every combination that matters — 60, 75 and 90 °C conductors at ambients from 40 to 70 °C — and agrees with all of them to within half of the last published digit.
One deliberate exception. Below the 30 °C base the formula is optimistic: it produces 1.14 for a 60 °C conductor at 21 °C where the table allows 1.08. That would hand out ampacity the code does not permit, so uprating is not computed at all. The factor is held at 1.00 and the tool says so. Read the uprate off your own table if you need it.
Which conductors count for the bundling adjustment?
Only the ones carrying current. An equipment grounding conductor never counts. A neutral counts in some configurations and not others — on a balanced three-phase four-wire circuit it usually does not, and on a circuit with significant harmonic content it does.
The bands are 4 to 6 conductors at 80%, 7 to 9 at 70%, 10 to 20 at 50%, 21 to 30 at 45%, 31 to 40 at 40%, and 41 or more at 35%.
Do the corrections multiply or do I apply the worst one?
They multiply. Ambient correction and bundling adjustment are separate physical effects — hot air around the raceway, and conductors heating each other inside it — so a conductor in a hot attic bundled with nine others gets both.
The termination limit is different in kind. It is not a correction factor at all, it is a cap: the answer is the lower of the derated figure and the termination column figure, whichever binds first.
Notes
NFPA 70 is copyrighted and no table, note or commentary from it is reproduced here. The code itself is free to read from the NFPA. What this page contains is short statements of rules in this project's own words — the 110.14(C) thresholds and the bundling percentages — and arithmetic.
The ambient correction factor is computed from the conductor heat balance, verified against the published factors at twelve combinations of insulation rating and ambient. Below the 30 °C base ambient the correction is held at 1.00 rather than computed, because the formula overstates the uprate the code permits and overstating ampacity is the unsafe direction.
Base ampacities are supplied by you, from your own table. A valid result here is not an inspection pass: it is arithmetic on the figures you entered, and it cannot know about a mis-read column, a conductor type restriction, or any of the many other rules that apply to a real installation. Tell me if something disagrees with your code book.