Every shop runs on the same shorthand. Twelve is twenty, ten is thirty, eight is forty. It gets an apprentice through the first year and it carries most journeymen through a career of residential branch circuits, because on those circuits it happens to land on the right answer. Then somebody sizes a feeder out of the 90-degree column, the inspector writes it up, and it turns out the shorthand was covering two separate rules that were never the same rule.
The useful thing about this particular confusion is that clearing it up takes about ten minutes and then stays cleared up. It is not a hard idea. It is just that nobody ever laid the two rules side by side.
Where the twenty amps actually comes from
It does not come from the ampacity table. A 14, 12 or 10 AWG copper conductor has its overcurrent protection capped by the small-conductor rule in Article 240 — 240.4(D) — which is a flat limit written into the overcurrent article, not a number pulled out of Table 310.16. You can read the article numbering yourself through NFPA's free access portal, which requires a free account.
That matters because the two rules answer different questions. The ampacity table answers "how much current can this conductor carry under these conditions before the insulation cooks." The small-conductor rule answers "regardless of that, what is the largest breaker I am allowed to put in front of it." On a 12 AWG branch circuit in ordinary conditions the second rule bites first, which is why the shorthand works and why it teaches the wrong lesson.
Three columns, and the lowest common denominator wins
Table 310.16 gives you three ampacity columns, sorted by the temperature rating of the conductor's insulation: 60, 75 and 90 degrees Celsius. THHN is a 90-degree dry insulation. That rating describes what the insulation survives. It says nothing whatsoever about what the rest of the circuit survives.
And the rest of the circuit is the problem. A conductor does not exist on its own. It lands on a lug at both ends, and that lug is attached to a breaker, a disconnect, a panelboard or a piece of equipment that has its own temperature limit. Heat generated in the conductor migrates into the termination. Article 110.14(C) is the rule that forces you to account for this: you take the conductor ampacity from the column that matches the lowest temperature rating of any termination, conductor or device in the circuit.
In practice that comes down to two cases. Equipment rated 100 amperes or less, or provided for 14 through 1 AWG conductors, is treated as a 60-degree termination unless it is listed and identified for higher. Equipment over 100 amperes, or for conductors larger than 1 AWG, is treated as 75 degrees unless listed for more. Almost every breaker and lug you handle in the field is stamped 60/75C AL-CU, which means the 75-degree column is your practical ceiling and the 90-degree column is not available to you as a final answer.
Aluminum changes the arithmetic but not the method. Aluminum and copper-clad aluminum have their own columns in the same table, and the same 110.14(C) limits apply to the terminations, which is why a service conductor sized in aluminum off the 75-degree column is routine and one sized off 90 is a correction notice. Parallel conductor sets follow the same logic run by run.
The rule is not bureaucratic caution either. Terminations are tested as assemblies at a rated temperature, and the heat that arrives at a lug comes from the conductor behind it as much as from the connection itself. A lug qualified at 75 degrees that is fed by a conductor operating at 90 does not fail immediately; it relaxes, loses clamping force over thermal cycles, and the resistance at the joint climbs. That is the mechanism the rule is written against.
So what is the 90-degree column for
It is a starting point, not a destination. When you have to apply ambient temperature correction or an adjustment factor for more than three current-carrying conductors in a raceway, you are permitted to start from the ampacity of the conductor's own insulation rating — the 90-degree column for THHN or XHHW-2 — run the multipliers, and then compare the result against the terminal limit. Whichever number is lower is the number you live with.
That is the whole method, and it fails in both directions. One crew never opens the 90-degree column at all and buys more copper than the job needs. The other crew derates off 90, gets a comfortable number, and forgets the second half of the comparison entirely. The second mistake is the expensive one, because it shows up as heat at the terminations rather than as a line item on the bid.
Why this shows up on the exam and on the callback
Licensing boards know this is the soft spot. Texas publishes its numbers, and in FY2025 only 20.56 percent of candidates passed the journeyman calculations paper — roughly one in five. The calculations paper is where terminal ratings, correction factors and adjustment factors all arrive at once, and a candidate who is running on shop shorthand will get partway through and then choose a plausible wrong answer with confidence.
The field version is slower and less forgiving. Undersized conductors landed on a hot lug do not fail on the day of the rough-in. They fail after a few summers of load, as a discolored termination, a softened insulation jacket and eventually an arcing connection inside a panel. The U.S. Fire Administration counted 23,700 residential fires from electrical malfunction in 2023, with 305 deaths and $1.50 billion in loss, and over the ten-year trend the dollar loss is up 28 percent even after adjusting for inflation.
None of that is an argument for being conservative for its own sake. It is an argument for doing the comparison every time: pull the ampacity from the column your insulation allows, apply the corrections the installation demands, then check the answer against the weakest termination in the circuit and take the smaller of the two. If somebody asks you why the conductor is bigger than the table says it needs to be, the answer is the lug, not the wire.
Terminal ratings, correction factors and the order you apply them in are laid out visually in Volume 1 of the Illustrated Guide to Understanding the National Electrical Code, which may be worth a look if the columns have never quite sat still in your head.