Practice: overcurrent device sizing
Each problem uses new numbers, so you practice the method rather than memorizing an answer.
Pick the answer: sizes and ratings come from your code book.
Worked solution
Formula
| Symbol | Meaning | Unit |
|---|---|---|
| Ampacity | Conductor ampacity after any derating and the termination limit | A |
| P | Overcurrent device rating, a standard size from 240.6(A) | A |
Worked example
A feeder uses 4/0 AWG copper conductors, and every termination is listed and marked for 75 °C. No correction or adjustment factors apply, and the feeder does not supply receptacles for cord-and-plug equipment. What is the largest standard overcurrent device that may protect these conductors?
- Look up Table 310.16, 75 °C copper column, for 4/0 AWG: 230 A.
- That is not a standard rating, and the next size up is 800 A or less, so 240.4(B) lets you round up.
- Largest overcurrent device (240.6(A)): 250 A.
Answer: 250 A
Overcurrent device sizing, explained
The breaker or fuse exists to protect the wire. Its rating normally may not be more than the conductor's ampacity. Because breakers come only in standard sizes, the NEC gives some room: if the ampacity falls between two standard ratings, you may use the next one up, as long as that next size is 800 A or less.
That round-up is not allowed when the circuit is a multioutlet branch circuit feeding receptacles for cord-and-plug equipment, and it is not allowed above 800 A. Over 800 A, 240.4(C) says the device rating must be at or below the conductor ampacity, so you round down.
Small conductors have their own cap. 240.4(D) limits the device on 14, 12 and 10 AWG conductors, regardless of what their 75 °C or 90 °C ampacity is. The ampacity you start from is the final ampacity: after correction, adjustment and the termination limit.
Motor circuits, air conditioning equipment and a few others follow their own rules in Articles 430 and 440. These problems are general circuits and feeders.
How to solve it on the exam
- Find the conductor ampacity: table value, then any correction and adjustment, then the termination limit.
- If the ampacity is a standard rating in 240.6(A), that is the largest device.
- If it is not, and the next standard size up is 800 A or less, you may use the next size up (240.4(B)), unless the circuit supplies receptacles for cord-and-plug loads.
- If the device would be over 800 A, use the standard size at or below the ampacity (240.4(C)). For parallel sets, add the ampacity of all sets first.
- For 14, 12 or 10 AWG, check the 240.4(D) cap and use the smaller value.
Common mistakes
- Rounding up above 800 A.
- Rounding down when 240.4(B) allows the next size up.
- Putting a 25 A breaker on 12 AWG copper because its 75 °C ampacity is 25 A.
- Using the 90 °C table ampacity as the conductor ampacity without derating or the termination check.
- Using only one parallel set's ampacity instead of the total.
Exam tips
- Tab 240.4 and 240.6(A) together. Most questions need both.
- Memorize the small conductor limits: 14 AWG copper 15 A, 12 AWG copper 20 A, 10 AWG copper 30 A.
Overcurrent device sizing calculator
Questions people ask
Can I round up to the next breaker size?
Yes, if the conductor ampacity is not a standard rating, the next standard size is 800 A or less, and the circuit is not a multioutlet branch circuit for cord-and-plug receptacles. This is 240.4(B).
What happens above 800 A?
You may not round up. Under 240.4(C), the overcurrent device must be rated at or below the ampacity of the conductors it protects.
What is the largest breaker for 12 AWG copper?
20 A, under 240.4(D), unless a specific section such as those for motors or air conditioning equipment allows more.
What are the standard breaker sizes?
They are listed in 240.6(A). In the common range they go 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, then 110, 125, 150, 175, 200 and up.
Sources
About the tables: The exam is open book. These problems cite the 2026 NEC section and table so you practice finding them in your own book. We never print NEC text or tables.