The short version: selecting the 90°C column is usually wrong, because the terminations at each end of the run are almost never rated for 90°C. The column belongs to the equipment, not to the wire. This guide explains why, and then covers the one job the 90°C column genuinely does have.
The columns describe insulation, the choice describes equipment
The three columns exist because insulation systems differ in how much heat they tolerate. A 90°C-rated insulation can sit at a higher operating temperature than a 60°C-rated one, so a conductor wearing it can legitimately carry more current before the insulation is in trouble. That much is a property of the wire.
The conductor does not end in mid-air, though. It ends in a lug, a breaker, a receptacle, a disconnect or an appliance terminal, and that equipment is separately listed for a conductor termination temperature. The heat a loaded conductor produces travels into that connection, and the listing states how much of it the connection was designed and tested to take.
So the operative limit is the lower of the two: the insulation and the termination. In practice the termination is nearly always the lower one. Most modern equipment is listed for 75°C terminations, and a great deal of equipment at branch-circuit ratings is listed for 60°C. Very little ordinary equipment is listed at 90°C. That is why the conductor is selected at the termination's rating, and why the 90°C column being printed on the same page as the other two is a trap rather than an invitation.
- The insulation stamp on the wire does not select the column - the equipment listing does.
- Where a run lands on two pieces of equipment with different ratings, the lower rating governs the whole conductor.
- THHN is 90°C wire. Almost every circuit built with it is still sized in the 75°C column.
What choosing wrong actually costs
The 90°C column reads higher than the 75°C column for every size in the table. Read it as a selection column and you conclude that a smaller conductor is adequate - sometimes a full trade size smaller than the one the terminations will accept. The error direction is undersizing, and undersizing is what heats connections, degrades insulation and starts fires at the panel rather than out in the run.
Nothing downstream catches it either. The overcurrent device protects the conductor at its permitted ampacity; if the conductor was selected against a limit its terminations do not share, the breaker will happily pass current the connection was never listed for. The sizing calculators on this site warn whenever the 90°C column is selected for exactly this reason.
The reverse mistake - sizing at 60°C when the equipment is listed for 75°C - costs money rather than safety. It is never unsafe to install a larger conductor than required.
The one job the 90°C column does have
The hot column is not decorative. It is the permitted starting point for derating. When a conductor runs through a hot space or shares a raceway with more than three current-carrying conductors, its published ampacity is corrected downward by an ambient temperature factor and a conductor-count adjustment factor. Both of those factors may be applied to the 90°C value if the conductor's insulation is genuinely 90°C rated.
That matters because the correction factors themselves are gentler in the hot column, and because you start from a bigger number. The catch is what happens at the end: whatever the derated figure comes out to, the conductor still may not be loaded beyond what the termination rating permits. The 90°C column raises the derating floor; it never raises the termination ceiling.
The worked example below is 6 AWG copper in a 40°C space sharing a raceway with six current-carrying conductors. Starting from the 90°C value produces a materially better derated result than starting from the 75°C value - and both remain capped by the 75°C figure, which in this case neither of them reaches.
NEC 2023Every figure in this table is transcribed from the 2023 NEC (NFPA 70, 2023). The code is republished on a three-year cycle and adoption varies by jurisdiction - confirm against your locally adopted edition and your authority having jurisdiction (AHJ) before you build.
| Starting column | Base | Ambient factor | Conductor-count factor | Derated ampacity |
|---|---|---|---|---|
| 90°C (derating start) | 75 A | 0.91 | 0.80 | 54.6 A |
| 75°C (termination rating) | 65 A | 0.88 | 0.80 | 45.8 A |
The 60°C column is not obsolete
It is tempting to treat 60°C as a historical artefact for old cloth-insulated wiring. It is not. Plenty of currently manufactured equipment carries a 60°C termination listing, particularly at smaller branch-circuit ratings, and some wiring methods are restricted to that column by their own listing regardless of what the conductor inside is rated for.
The consequence is real: at 60°C, conductor sizes step up noticeably compared with the same circuit at 75°C. If your equipment label says 60°C, that is the column, and a table lookup performed in the 75°C column has produced an answer for a different job.
When you cannot establish the termination rating at all, the conservative reading is the lower column. Guessing high is the one direction that has a safety cost.
A separate ceiling applies to the smallest conductors
Independently of all three columns, the smallest conductors carry an overcurrent-protection ceiling of their own. It is why 14 AWG copper is a 15 A conductor in practice even though the 90°C column reads 25 A for it, and why 12 AWG copper stops at 20 A. The rule has specific exceptions that this site's tables do not model, so treat it as a ceiling to be aware of rather than a complete statement of the requirement.
The interaction with derating is worth seeing once. Twelve AWG copper in a raceway with six current-carrying conductors derates from 30 A in the 90°C column to 24 A - but the small-conductor ceiling of 20 A still applies, so the usable figure is 20 A. Starting from the hot column bought nothing at all here, because a different limit was already binding.
That is the general shape of the whole subject: several limits apply at once, and the conductor is governed by whichever is lowest. The temperature column is only one of them, and the one people most often read in isolation.
Frequently asked questions
Can I use the 90°C column if I am running THHN?
Not to select the conductor. THHN's 90°C insulation rating permits you to begin derating calculations from the 90°C value, but the finished conductor still has to satisfy the temperature rating of the terminations it lands on, which for ordinary equipment is 60°C or 75°C.
How do I find out what my terminations are rated for?
It is stated in the equipment listing and is normally marked on the equipment or in its instructions - on the breaker, the panelboard label, or the appliance's terminal block. Where two pieces of equipment on the same circuit differ, the lower rating governs the conductor between them.
Why does the ampacity table publish a 90°C column at all?
Because derating starts there. Ambient temperature correction and conductor-count adjustment may be applied to the 90°C value for conductors with 90°C insulation, which produces a better derated result than starting lower. The derated figure is still capped by the termination rating.
Is sizing at 60°C ever wrong?
It is never unsafe, only more expensive than it needs to be if your equipment is actually listed for 75°C. Sizing at a column above the termination rating is the error that matters, because it produces a conductor smaller than the connection was listed to accept.
Does the 90°C column change the breaker size?
No. The overcurrent device is selected from the standard ratings against the load and the conductor's permitted ampacity, and the smallest conductors carry a separate overcurrent ceiling regardless of which column you read. Choosing a hotter column does not license a larger breaker.