Overview
The OCPD sizing calculator determines the correct fuse or circuit breaker rating for a circuit based on the conductor ampacity, load type, and NEC Article 240 requirements. It handles the “next standard size up” rule (240.4(B)) and special sizing rules for motors, transformers, and HVAC equipment.
Standard Sizing Rules
For most circuits, the OCPD is sized per NEC 240.4:
- The OCPD rating must not exceed the conductor ampacity
- Exception: NEC 240.4(B) allows rounding up to the next standard size when the ampacity doesn’t match a standard rating
- Standard OCPD ratings: 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, 600A
- The 240.4(B) exception does NOT apply to circuits rated over 800A
- For continuous loads: OCPD must be rated at 125% of the continuous load (or 100% for breakers listed for 100% continuous operation)
Motor Circuit OCPD
Motor circuits have specific maximum OCPD percentages per NEC 430.52:
| Motor Type | Inverse-Time Breaker | Dual-Element Fuse | Instantaneous Trip |
|---|---|---|---|
| Design B (most common) | 250% | 175% | 800% |
| Design A | 300% | 175% | 800% |
| Wound Rotor | 150% | 150% | 800% |
| Synchronous | 175% | 175% | 800% |
| DC | 150% | 150% | 250% |
If the calculated percentage doesn’t correspond to a standard size, round up to the next standard size. If the motor won’t start at the calculated size, NEC 430.52(C)(1) allows increasing to the next higher standard size.
NEC References
- NEC 240.4 — protection of conductors
- NEC 240.4(B) — next standard size up rule
- NEC 240.6(A) — standard ampere ratings
- NEC 430.52 — motor branch circuit OCPD sizing
- NEC 450.3 — transformer OCPD sizing
- NEC 440.22 — air conditioning equipment OCPD sizing
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