Continuous Load Sizing Basis Calculator

The continuous-load rule is one of the most frequently cited and most frequently misapplied provisions in NEC design practice. The rule is straightforward — conductors and OCPDs must be sized for 125% of continuous load plus 100% of noncontinuous load — but two mistakes are pervasive: applying 125% to the total load instead of only the continuous portion, and treating the 100%-rated assembly exception as a preference rather than a listing condition. This calculator eliminates both errors. Enter noncontinuous load amperes (0–200 A) and continuous load amperes (0–200 A), select sizing context (branch circuit, feeder, service conductor, or educational simplified), NEC edition (2017, 2020, 2026), and equipment rating (standard-rated with 125% applied, or listed 100%-rated assembly). The Results tab shows the calculated load (raw sum), the sizing basis (with 125% applied to continuous portion), and the 25% adder value — all clearly separated. The Trace tab walks through the applicable NEC sections: 210.19(A) and 210.20(A) for branch circuits, 215.2(A) for feeders, 230.42(A) and 230.90(A) for services, and 240.4(B/D/E) and 240.6(A) for OCPD selection. The Sandbox tab provides an interactive two-load model (Load A noncontinuous, Load B continuous) so learners can isolate the effect of each classification in real time. The References tab surfaces only the NEC sections active for the selected scope. A Design Challenge mode generates sizing problems with evaluated results. The Learn tab covers the continuous load definition (3-hour rule), the 100%-rated assembly exception as a listing condition — not a preference — edition-specific changes, and a Load Treatment Comparison table. An internal Self-Test harness verifies the calculation engine.

Enlarge

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

Enlarge

Enlarge

Enlarge

Enlarge

Enlarge

Enlarge

Enlarge

• 01State the NEC definition of a continuous load (maximum current for 3 hours or more) and explain why the classification triggers the 125% sizing multiplier
• 02Calculate the correct sizing basis for a circuit with mixed continuous and noncontinuous loads — applying 125% only to the continuous portion, not the total
• 03Identify the applicable NEC section for each circuit type: 210.19(A) and 210.20(A) for branch circuits, 215.2(A) for feeders, and 230.42(A) / 230.90(A) for service conductors and service OCPD
• 04Explain why the 100%-rated assembly exception is a listing condition — requiring the OCPD and its enclosure to be listed and marked together for 100% continuous operation — and identify two common misapplications of the exception
• 05Use the Sandbox tab to vary continuous vs. noncontinuous load split and observe how the sizing basis and 25% adder change as the ratio shifts
• 06Identify at least one edition-specific change between NEC 2017, 2020, and 2026 that affects continuous-load sizing requirements

• —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.