Overcurrent Protection and Short-Circuit Current Ratings
The Overcurrent Protection and Short-Circuit Current Ratings domain tests Article 240 and the performance requirements in 110.9 and 110.10: standard ratings, conductor protection, tap rules, interrupting rating, and series-rated equipment. Expect a one-line diagram with a fault-current value.
Core rules
- Standard ratings in 240.6(A) include 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 A.
- Conductors must be protected at their ampacity per 240.4. The small-conductor limits in 240.4(D) are 15 A for 14 AWG, 20 A for 12 AWG, and 30 A for 10 AWG for general wiring. The next-size-up allowance in 240.4(B) is limited to 800 A and is not permitted for multioutlet receptacle circuits.
- Tap rules in 240.21: a 10 ft tap requires tap ampacity at least 1/10 of the feeder overcurrent device; a 25 ft tap requires at least 1/3 and a single overcurrent device at the load. Both require protection from damage and termination in an enclosure.
- 110.9 requires equipment interrupting rating not less than the available fault current. 110.10 requires components to withstand the available fault current. Series ratings must be listed and marked per 240.86.
Worked example
A 200 A feeder supplies a 100 A panel through a 25 ft tap. The tap conductor must have ampacity of at least 200/3 = 66.7 A. 6 AWG copper at 75°C is 65 A and fails. 4 AWG copper at 85 A has enough ampacity, but the 100 A panel overcurrent device exceeds 85 A. Use 3 AWG copper at 100 A with a 100 A overcurrent device. The 10 ft rule would reduce the multiplier but would not remove the requirement that the tap carry the 100 A load.
Traps
- Applying the next-size-up rule to a tap conductor.
- Assuming a series rating without a listed and marked combination.
- Ignoring available fault current when selecting equipment.
- Applying 240.4(D) to motor branch circuits that are covered by Article 430.
- Overlooking 110.10 component withstand ratings.