OCPD Sizing Calculator

Overcurrent protective device sizing sits at the intersection of three competing NEC requirements — the conductor must be protected (240.4), the continuous load must be multiplied by 125% (210.20(A)), and the next standard size up is often permitted (240.4(B)) but not always (240.4(D) caps small conductors). Navigating all three correctly, for different device types, is a source of persistent errors in practice. This calculator applies the full NEC 240.4 logic tree for general branch circuits and the 430.52 table for motor branch circuits. Enter load current as a single value (all continuous or all noncontinuous) or split into continuous and noncontinuous portions; select OCPD type (inverse-time circuit breaker, dual-element time-delay fuse, non-time-delay fuse, or instantaneous-trip breaker); and specify the conductor — material, size, and temperature rating. The Results tab shows the calculated sizing basis, the next standard size up (NEC 240.6(A)), whether 240.4(D) applies a hard cap for small conductors, and a final pass/fail against conductor protection. The Charts tab visualizes the sizing logic path. The Trace tab walks every decision step with code citations. A Code Logic tab diagrams the 240.4(B)/(C)/(D) rule tree. Seven preset scenarios target the most common traps: Noncontinuous Branch, Continuous Commercial, Borderline Next-Size-Up, Conductor Too Small (FAIL), Small Conductor Trap (12 AWG cap), 100% Listed Assembly, and Motor Branch Circuit (430.52). A Design Challenge tab generates sizing problems with evaluated results and a knowledge-check quiz is built into the Learn tab.

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A live look at the app you'll launch — every control on the left drives the visualizations on the right in real time.

Example data — not a real field report

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• 01Apply the NEC 240.4(B) next-standard-size-up rule and identify when it is permitted versus when 240.4(D) imposes a hard cap based on conductor size
• 02Calculate the sizing basis for a branch circuit serving continuous and noncontinuous loads using 210.20(A) and determine the correct OCPD rating for an inverse-time breaker and a dual-element fuse
• 03Explain why a 12 AWG conductor is capped at 20 A under 240.4(D) regardless of the continuous-load calculation result
• 04Apply NEC Table 430.52 to size an OCPD for a motor branch circuit given FLC and device type, and explain why motor OCPDs are intentionally larger than conductor ampacity
• 05Determine when the 100%-rated assembly exception removes the 125% continuous-load multiplier from the OCPD sizing and identify what listing conditions must be met
• 06Trace the complete 240.4(B)/(C)/(D) decision tree for a borderline case where the calculated sizing basis falls between two standard OCPD ratings

• —Not a design document, stamped calculation, or approval record.
• —Not a substitute for engineering analysis by a licensed professional.
• —Not a reproduction of NEC tables — values are computed and cited, and you must verify against the code edition enforced in your jurisdiction.
• —Not connected to live equipment and not a safety device.