Overcurrent Protection on the C-10 Exam
Overcurrent protection looks simple until the exam turns a two-line question into a trap. You know a breaker trips on overload. Fine. But the C-10 wants to know whether you can size that breaker to a specific conductor, apply the rounding rule correctly, and recognize when the rule doesn't apply at all. That's Article 240, and it earns its own cluster of questions on the trade portion.
Here's the mental model that keeps you out of trouble: an overcurrent protective device (OCPD) protects the conductor, not the equipment. The breaker's job is to open the circuit before the wire overheats. Once that clicks, most of Article 240 stops being memorization and starts being logic.
Breakers vs. fuses — what the exam actually tests
Both do the same core job: interrupt current on overload or short circuit. The exam rarely asks you to pick one over the other on philosophy. What it does test:
- Standard ampere ratings. From 240.6(A), the standard ratings are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 amps, and up from there. Learn the small end cold — nearly every sizing question lives below 100A.
- Fuses carry the same standard ratings, plus a few additional ones (1, 3, 6, 10, and 601 among them). If a question offers a 45A protection value, it's legal for both.
- Continuous loads at 125%. A breaker feeding a continuous load — three hours or more — must be sized at 100% of the noncontinuous load plus 125% of the continuous load, per 210.20(A) and 215.3. This shows up constantly, and the 125% multiplier is easy to drop under time pressure.
If you're shaky on where load current comes from in the first place, tighten up your dwelling load calculation fundamentals before you drill overcurrent — the two topics feed each other on the exam.
The "next standard size up" rule
This is the single most-tested concept in Article 240, and the one people blow. The rule lives in 240.4(B).
When a conductor's ampacity doesn't line up with a standard OCPD rating, you're allowed to go up to the next standard size — but only if all three conditions hold:
- The conductor isn't part of a multi-outlet branch circuit supplying receptacles for cord-and-plug-connected portable loads.
- The conductor's ampacity doesn't correspond to a standard rating.
- The next higher standard rating is 800A or less.
Above 800A, you round down to the next standard size instead (240.4(C)). A conductor rated 350A on a feeder can't be protected at 400A — you drop to a device that doesn't exceed 350A.
Quick example. You pull the ampacity of a conductor from Table 310.16 and land on 115A after your corrections. There's no standard 115A breaker. The next standard size up is 125A, and since you're well under 800A, 240.4(B) lets you protect that conductor at 125A. Clean.
The small-conductor rule that overrides everything
Now the trap. 240.4(D) sets hard ceilings on small conductors regardless of what their ampacity table says:
| Copper conductor | Max OCPD |
|---|---|
| 14 AWG | 15A |
| 12 AWG | 20A |
| 10 AWG | 30A |
Aluminum runs one step behind: 12 AWG aluminum caps at 15A, 10 AWG aluminum at 25A.
Why it's a trap: 12 AWG copper at 90°C shows an ampacity of 30A in Table 310.16. A student who only remembers the next-size-up rule will "protect" it at 30A. Wrong — 240.4(D) hard-caps 12 AWG copper at 20A no matter what the ampacity column says. When 240.4(D) applies, it wins. Expect the exam to bait you with that exact setup.
OCPD placement — where the breaker goes
The general rule is 240.21: overcurrent protection goes at the point where the conductor receives its supply. A branch-circuit conductor gets its breaker at the panelboard it originates from. A feeder gets its protection at its source. That's the default answer unless a tap rule applies.
Article 240 also nails down a few placement specifics worth knowing:
- 240.24 — OCPDs must be readily accessible, generally installed so the center of the operating handle sits no more than 6 ft 7 in above the floor. They can't go in bathrooms of dwelling units, over the steps of a stairway, or where exposed to physical damage.
- OCPDs must protect the ungrounded conductors (240.15). You don't put an overcurrent device in a grounded neutral except under narrow conditions where it opens all conductors at once.
That neutral distinction ties straight into how grounding and bonding get tested — another spot where the exam checks whether you know which conductor does what.
Tap rules — know the conditions, not just the length
A "tap" is a conductor that connects to a feeder without its own overcurrent protection at the point of connection. Normally that's forbidden, but 240.21(B) carves out exceptions. The two you'll see most:
The 10-foot tap rule (240.21(B)(1)): a tap up to 10 ft long is allowed if the tap conductor's ampacity is at least the load it serves and at least the rating of the device it terminates in, the tap doesn't extend past the enclosure it enters, and — the qualifier people forget — the tap ampacity is at least 10% of the feeder OCPD rating.
The 25-foot tap rule (240.21(B)(2)): a tap up to 25 ft is allowed if its ampacity is at least one-third of the feeder OCPD rating, it's protected from physical damage, and it terminates in a single device rated no more than the tap's ampacity.
The exam tests the qualifiers — the 10% figure, the one-third figure, the "single device" limit — far more than the raw footage. Memorize "10 and 25" and nothing else, and you'll miss the question. These are edge cases; get the core sizing rules solid first and treat taps as the finishing layer.
How this shows up on test day
Overcurrent questions usually arrive as short calculations: here's a conductor, here's a load, pick the breaker. The winning approach is a fixed order of operations — find the load, apply 125% if it's continuous, get the conductor ampacity, choose the OCPD using 240.4(B), then check 240.4(D) to make sure a small-conductor cap doesn't override your answer. Run that sequence every time and the traps fall apart.
The only way it becomes automatic is reps. Working timed C-10 practice questions on overcurrent sizing until the order of operations is muscle memory is what separates people who pass from people who retake.
These concepts are laid out here to help you study — always work from the current adopted code and confirm any specific figure against the 2025 California Electrical Code. Get Article 240 down and you've locked in a reliable block of points on the trade exam.