For a conductor with ampacity 25 A, what is the maximum protective device rating that should be used?

• A. 15 A
• B. 20 A
• C. 25 A
• D. No greater than the conductor ampacity; use the closest standard size not exceeding 25 A (e.g., 20 A or 25 A depending on standards)

Answer: (D)

The protective device must be rated no higher than what the conductor can safely carry. Ampacity is the maximum current a conductor can carry continuously without overheating; the overcurrent protection should trip before the insulation is damaged. So you choose the largest standard device rating that does not exceed the conductor’s ampacity. In this case, the conductor can carry up to 25 A, so you use a protective device rated at 25 A if that standard size exists; if not, you go to the next lower standard, such as 20 A. Using a device larger than 25 A would let too much current flow before tripping, risking overheating.