Overcurrent Protection and Short-Circuit Current Ratings
Overcurrent Protection and Short-Circuit Current Ratings
This domain covers Article 240 and interrupting ratings: selecting OCPDs, continuous-load sizing, standard ratings, tap rules, series ratings, and SCCR for equipment.
Core rules
- Continuous loads are calculated at 125% for branch-circuit and feeder OCPD sizing, unless the Code says otherwise.
- Standard OCPD ratings are in 240.6; the "next size up" rule is limited to 800 A and below and only when the calculated load does not exceed the next standard rating.
- Equipment must have an SCCR at least equal to the available fault current; series-rated combinations must be listed.
- Feeder taps are limited by 240.21(B); transformer secondary taps by 240.21(C).
- Motor branch short-circuit protection uses Article 430, not the 125% continuous-load rule.
Worked example
A 38 A continuous load: 38 × 1.25 = 47.5 A. The next standard OCPD is 50 A (240.6). Because the calculated 47.5 A does not exceed 50 A, a 50 A breaker is permitted under the next-size-up rule (800 A or less), provided the conductors have an ampacity of at least 47.5 A. A 42 A continuous load gives 52.5 A; the next standard size is 60 A, still acceptable if conductors are sized for 52.5 A.
Traps
- Rounding 47.5 A down to a 40 A breaker.
- Applying the next-size-up rule to motor overload protection.
- Ignoring available fault current and equipment SCCR.
- Using the 80% rule backward when sizing continuous loads.