💡 What This Tool Does
Enter continuous and noncontinuous load amperes for a branch circuit, feeder, or service and get the NEC sizing basis — with the 125% multiplier applied correctly (to the continuous portion only), the 100%-rated assembly exception explained as a listing condition, and edition-specific notes for 2017, 2020, and 2026. It computes a sizing basis and is not a complete conductor/OCPD design tool.
- Set the noncontinuous and continuous load sliders in the sidebar (0–200 A each), and pick the sizing context, NEC edition, and equipment rating.
- Read the Results tab for the calculated load, sizing basis, and 25% adder — then follow the Trace tab to see each NEC step with its section citation.
- Use the Sandbox to isolate the effect of each classification, try the Design challenge to pick a breaker + conductor, and check References for the active code sections.
⚠ Scope Statement
This calculator covers the continuous-load treatment step only: sizing basis = noncontinuous × 1.00 + continuous × 1.25. It does not perform ambient/fill adjustment (310.15), terminal-temperature checks (110.14(C)), voltage drop, motor (Art. 430) or HVAC (Art. 440) rules, taps (240.21), or paralleling (310.10). Local adoption of NEC editions varies — verify the code cycle your Authority Having Jurisdiction has adopted.
🚫 Not Included in This Tool
Required for a real conductor/OCPD design but out of scope here: ambient temperature correction and conductor-count adjustment (310.15), terminal temperature limits (110.14(C)), voltage drop, motor rules (Art. 430), HVAC MCA/MOCP (Art. 440), transformer secondary rules, tap conductors (240.21), parallel conductors (310.10), local AHJ amendments, and manufacturer listing conditions (110.3(B)).
- Load sliders are the maximum expected operating current in amperes for each classification.
- “Continuous” means the maximum current is expected to continue for 3 hours or more (NEC Article 100 definition).
- The 125% factor applies to conductor and OCPD sizing per the article for the selected scope: 210.19/210.20 for branch circuits, 215.2/215.3 for feeders, and 230.42 for service conductors. (For services the 125% requirement is on the conductor only — 230.90(A) sets a maximum on the device, not a 125% minimum.)
- If “Listed 100%-rated assembly” is selected and all three listing confirmations are checked, the 125% multiplier is removed.
- This tool computes a sizing basis only — it is not a complete conductor size. Adjustment/correction, terminal limits, voltage drop, motor/HVAC rules, taps, and paralleling are out of scope.
The other two conditions (ampacity not corresponding to a standard rating, next size ≤ 800 A) are evaluated automatically in the verdict below.
⚠ Educational Check Only
A “pass” means the design satisfies only the limited checks implemented by this educational tool (continuous-load basis + 240.4 conductor protection). It is not a complete conductor-sizing verdict — adjustment/correction factors, terminal temperature limits, equipment listing, and the other exclusions still apply.
X axis: percentage of the total load classified as continuous. Y axis: resulting sizing basis in amperes.
💡 What to Notice
Slide Load B up while Load A stays fixed — the 25% adder grows with only the continuous portion. Two circuits with identical calculated loads can have very different sizing bases depending on how much of each is continuous. That is the entire point of the classification.
Why This Matters
The distinction between continuous and noncontinuous loads is one of the first decision points in any electrical sizing calculation — and one of the easiest to get wrong. Most standard breakers are evaluated for continuous loading at 80% of their rating; if you plan to load one continuously at 100%, the device may not perform within its calibrated parameters. The NEC addresses this with the 125% rule for conductor and OCPD sizing.
Misclassify a continuous load as noncontinuous and every downstream calculation — conductor derating, conduit fill, voltage drop — is built on an inadequate foundation.
The 3-Hour Rule
NEC Article 100 defines a continuous load as one where the maximum current is expected to continue for 3 hours or more. The 3-hour threshold drives all downstream treatment. It is about the duration of the maximum current — not merely how long the equipment is switched on, and not its duty-cycle percentage. A cycling machine can still be continuous if its maximum current persists for 3+ hours.
Load Treatment Comparison
Noncontinuous Load
Definition: Maximum current expected for less than 3 hours
Treatment: 100% of load current
Example: Intermittent office receptacles
Formula: load × 1.00
Continuous Load
Definition: Maximum current expected for 3 hours or more
Treatment: 125% of load current (for sizing)
Example: Commercial lighting during business hours
Formula: load × 1.25
The 100%-Rated Assembly Exception — a Listing Condition
The 125% multiplier may be omitted only where the OCPD assembly — the device together with its enclosure / switchboard / panelboard — is listed and marked for operation at 100% of its rating, and installed per that listing. The allowance appears on both the conductor and device side: 210.19(A)(1) Exception and 210.20(A) Exception for branch circuits, 215.2(A)(1) Exception No. 1 and 215.3 Exception for feeders, and 230.42(A)(2) for services. It is a property of listed equipment, not a designer preference.
- Misapplication #1: treating it as a checkbox preference — selecting “100%-rated” on paper without verified listed equipment.
- Misapplication #2: using a 100%-rated device in an enclosure that is not listed and marked for 100% operation with that device — the listing applies to the combination, not the breaker alone.
Edition-Specific Notes
- NEC 2017: The 125% framework lives in 210.19(A)(1)/210.20(A), 215.2(A)(1)/215.3, and 230.42(A)(1)/230.90(A). The ampacity table used here is designated Table 310.15(B)(16). 10 A is not a standard inverse-time breaker rating (fuses only, per 240.6(A)).
- NEC 2020: Same 125% framework; Article 310 was reorganized, renaming the ampacity table to Table 310.16. 10 A is still not a standard inverse-time breaker rating.
- NEC 2026: Clarifies the calculation-vs-sizing distinction — the 125% factor is a conductor/OCPD sizing requirement, not a load-calculation requirement (load calculations are reorganized under Article 120 and do not automatically apply 125%). 10 A remains a standard inverse-time breaker rating (first added in the 2023 cycle).
Common Mistakes
- Applying 125% to the entire load. The multiplier applies only to the continuous portion.
- Assuming all lighting is noncontinuous. Commercial lighting that runs through business hours almost always qualifies as continuous.
- Confusing duty cycle with duration. A cycling motor is continuous if its maximum current is expected to continue for 3 hours or more — not merely because its run time accumulates to 3 hours across the day.
- Treating the sizing basis as a finished conductor size. Adjustment, correction, and terminal limits still apply after this step.
- Assuming the breaker automatically protects the conductor. 240.4(D) caps small conductors (14/12/10 AWG Cu) regardless of table ampacity.
- Treating 100%-rated as a checkbox preference. It requires listed equipment and matching installation conditions.
What to Notice in This Lab
- Move the continuous slider and watch every tab update — the navy “25% adder” segment only exists because of the continuous portion.
- Switch Equipment Rating to “Listed 100%-rated assembly.” The basis drops only after you check all three listing confirmations — that is the exception working as a listing condition.
- In the Design challenge, try 14 AWG on a 20 A breaker: it fails on 240.4(D) even though the 75°C table shows 20 A.
- Try 6 AWG (65 A) on a 70 A breaker: it passes only while all 240.4(B) conditions are satisfied.
- Switch the scope to Service conductor and read the OCPD text — 230.90(A) is a maximum on the device, unlike the minimums in 210.20(A)/215.3.