Code Applied by the app elec.conductor.small-protection
Small conductor protection
The largest breaker the code lets protect the small copper sizes, 15 A on #14, 20 A on #12 and 30 A on #10, which is why a 20 A circuit is #12.
| When | The app uses |
|---|---|
| #14 copper | 15 A |
| #12 copper | 20 A |
| #10 copper | 30 A |
- Source
- NEC · 240.4(D) · read the public text
- Editions checked
- 2017 · 2020 · 2023
- Used by
- Courses
- Amendments
No state or local amendment to this rule is on file. Not on file means the model code as written — check your jurisdiction.
A model code the authority having jurisdiction adopts.
The breaker protects the wire. For the three small copper sizes the code caps the breaker whatever the wire's table ampacity says: #14 at 15 A, #12 at 20 A, #10 at 30 A. So a 20 A receptacle circuit is #12. A load of 23 A (the course's dishwasher, 4800 VA at 208 V) needs a breaker over 20 A; its engineer chose 30 A, the size the maker's label names, and a 30 A breaker wants #10 wire.
On a bid the gauge sets the wire price and, through conduit fill, the conduit size. Going up a size for a long run is the voltage-drop question; going down a size under this cap is never an option.
What the app does with it
The electrical course teaches it on two cards (Chapter 1, Read a row, and Chapter 4, Why #12), and check-lesson-rules holds the cards' numbers to this rule. The app reads the gauge you write in a line type's conductors, but it does not compare a circuit's breaker with its wire.
Verify against your edition
The three rows are 240.4(D)(3), (5) and (7). 240.4(G) sends motor and air-conditioning circuits to their own articles (a rooftop unit is sized from its nameplate under Article 440, not from this cap), and aluminum has its own figures. Local amendments still govern.
Settled 2026-09-27 by Claude on the owner's delegation, from the dossier's research and the model code text; not checked against a printed book or a local amendment.
Cited by section, not reprinted: the code text belongs to its publisher. Company practice is not a rule here — it lives with your pricing.