The step people forget after doing the derating correctly

You derate the conductor, get a good number, and then a two-line rule in Article 240 caps it anyway. Skipping that step is how a correct calculation produces a wrong answer.

The rule and the timing

For the small conductor sizes, overcurrent protection is limited to a fixed maximum regardless of what your ampacity calculation produced (NEC 240.4(D)).

The timing is the whole point. The limit applies **after** any correction for ambient temperature and any adjustment for the number of conductors. You do the derating first. Then this caps the result.

That direction matters because it only ever cuts. A derated ampacity that lands above the limit does not raise the limit. It gets cut down to it.

Look up the actual values in your own code book. There are eight rows, they are short, and they are the most cited limits in the trade.

The three smallest rows carry conditions the others do not

The entries for 18 AWG copper, 16 AWG copper, and 14 AWG copper-clad aluminum are conditional. The larger entries are not.

Those conditions cover how much of the load can be continuous, and what type of overcurrent device is protecting the conductor.

Quoting a value from one of those three rows without the conditions attached is a wrong answer, and it is a wrong answer that sounds authoritative. If a question involves 18 or 16 AWG, read the conditions before you answer.

The next-higher rule, and the three conditions on it

Separately, you are sometimes allowed to round the protective device up to the next standard rating above the conductor ampacity (NEC 240.4(B)). Most people remember that. Fewer remember it carries three conditions, and the permission depends on all three (NEC 240.4(B)).

• The conductors are not part of a branch circuit supplying more than one receptacle for cord- and plug-connected portable loads.
• The conductor ampacity does not already correspond to a standard device rating.
• The next standard rating selected does not exceed 800 amperes.

Fail any one of them and you round down, not up.

Above 800 amperes the permission reverses

This is the part that separates people who memorized the rule from people who understand it.

Below 800 amperes you may round the device up to the next standard size. Above 800 amperes, the conductor must equal or exceed the device rating (NEC 240.4(C)). No rounding up.

Two rules, opposite directions, and the 800 ampere line is the entire question. An exam item that hands you a 1000 ampere scenario is checking whether you noticed you crossed it.

What counts as a standard rating

The list of standard ampere ratings for fuses and inverse time breakers lives at 240.6(A). You cannot answer a rounding question without it, and it is worth a tab.

One thing about that section gets misread constantly: nonstandard ratings are permitted. The table defines what counts as standard for the purpose of the next-higher rule. It does not forbid other sizes from existing.

So a question asking what devices are available and a question asking what the next standard size is are different questions with different answers.

Something new in the 2026 edition nobody has written about

Circuit breakers that can be adjusted remotely over a network carry their own restricted-access rules (NEC 240.6(D)). The 2026 edition revised the section rather than creating it, so a 2023 book has a version of it too.

A breaker whose trip setting can be changed from somewhere else is now a code question, not just an IT question. The section requires local restricted access by one of several listed methods, and separately requires that remote access be either through a local nonnetworked interface, or through a networked interface where the breaker and its software have been evaluated for cybersecurity or a documented network risk assessment exists.

It references ANSI/ISA 62443, UL 2900, the NIST cybersecurity framework and NEMA CY70001.

This is brand new material. Most study guides in circulation predate it entirely, which makes it a plausible source of questions and an implausible thing for your competition to have covered.

The sequence, so nothing gets skipped

1. Find the base ampacity for the conductor from the ampacity table at the correct temperature column.
2. Apply the ambient correction factor.
3. Apply the adjustment factor for more than three current-carrying conductors.
4. If the conductor is one of the small sizes, apply the cap in 240.4(D). It only cuts.
5. Select the protective device, checking whether you are permitted to round up and whether you are above 800 amperes.

Step four is the one that disappears under time pressure, because by then the calculation feels finished.

What this page cites

• NEC 240.4(B) The next higher standard rating, and the three conditions on it.
• NEC 240.4(C) Devices over 800 amperes, where the permission reverses.
• NEC 240.4(D) Small conductors. Read the conditions on the three smallest rows.
• NEC 240.6(A) Standard ampere ratings for fuses and inverse time breakers.
• NEC 240.6(D) Remotely accessible adjustable-trip circuit breakers. Carries the revision marker in the 2026 edition, not the new-material marker, so it predates this cycle.