Phase Academy Interactive Power Systems Lab

Commercial Load Build-Up Calculator

NEC Art. 220
CALCULATION SUPPORT ONLY. This tool applies paraphrased NEC methodology to help build up a load calculation. It does not certify compliance. The calculator is internally tested for the implemented assumptions and includes warnings for unsupported cases; final compliance requires PE/AHJ/AHJ-adopted-edition review.
Local AHJ adoption varies. Confirm which NEC cycle your jurisdiction has adopted, and confirm all table values against that edition, before applying any result. 2026 NEC is not implemented — it relocated former Article 220 load-calculation material into Article 120.

Dashboard

Calculation-support snapshot. Adjust inputs in the sidebar. Connected load, calculated demand, conductor/OCPD basis, and service rating are shown as distinct quantities. All results require PE/AHJ review.

Load Stack (Demand-Adjusted)

Each bar is a category after applicable demand factors.

Connected vs Calculated Demand

Demand factors reduce the calculated load only where the NEC permits for the selected occupancy.

Sizing Basis

Calculation support only. This tool applies stated NEC rules to help build up a load calculation. Items labeled “calculator-applied” mean only that the calculator applied that paraphrased rule — not that the design is code-compliant. Items labeled “requires PE/AHJ review” are conditions the calculator cannot fully evaluate. Table values must be confirmed against the NEC edition your AHJ has adopted.

Step-by-Step Calculation Trace

Every number has a source. NEC references are edition-specific for the selected edition.

Assumptions & Warnings

Calculated Load Summary

Motor Contribution

OCPD & Sizing Basis

Calculation Notes

Proposed Design Notes

Compares your proposed ratings (sidebar → “Proposed Design Notes”) against the calculated sizing basis, plus an informational Table 310.16 ampacity note. This is not a conductor-sizing verification; final selection requires PE/AHJ review.

    Load Balance Analysis

    Distribution of calculated demand across categories.

    What-If Scenarios

    See how the calculation changes under different conditions.

    NEC Reference Panel

    Article and table numbers for the selected NEC edition. Paraphrased logic — exact NEC language is copyrighted by NFPA.

    Why This Matters

    A commercial load calculation determines the minimum service size, feeder conductors, and overcurrent protection. Under-sizing creates hazards; over-sizing wastes money and can mask design problems. The NEC provides a structured method so the answer is defensible.

    Mixed-use buildings add complexity: occupancies carry different lighting densities and demand-factor eligibility. The calculation must respect each occupancy separately.

    Key Concepts — Click to Explore

    What is a demand factor? ▶

    A demand factor acknowledges that not all connected loads run at full capacity at once. The NEC permits reducing certain categories based on diversity. Not all qualify: most non-dwelling general lighting is 100% (no reduction). Receptacles get a reduction per 220.44; kitchen equipment per Table 220.56 with conditions.

    Why does the largest motor get special treatment? ▶

    NEC 430.24 requires feeder ampacity of at least 125% of the largest motor FLC plus 100% of the others. For the service load, 25% of the largest motor is added (430.24/220.50). It is a conductor ampacity requirement, not an inrush margin.

    Connected vs calculated demand load? ▶

    Connected load is the sum of nameplate ratings. Calculated demand load is the connected load modified by NEC-permitted demand factors where applicable, plus the motor treatment. They are different values; the NEC does not permit demand factors on all categories.

    How does the edition change lighting? ▶

    2017 Table 220.12 carries higher VA/ft2 values and general lighting is a continuous load (125% applied at sizing). The 2020 table was revised to lower, energy-aligned values that already include the continuous factor. 2023 moves the unit loads to Table 220.42(A), demand factors to Table 220.45, show-window/track to 220.46, and non-dwelling receptacle demand to 220.47; school/university dropped from 3.0 to 1.5 VA/ft2. Hospitals were removed from the lighting demand table in 2020 (100%); hotel/motel demand is 60/50/35 in 2020/2023.

    What is the non-coincident rule? ▶

    NEC 220.60 permits using only the larger of loads that do not operate simultaneously - heating vs cooling being the classic case. Common loads (blowers running in both modes) are always included in addition.

    Common Mistakes

    • Applying warehouse/hotel lighting demand factors to offices. Most non-dwelling occupancies are 100%.
    • Copying 2017 table values into 2020/2023. The 2020 table was revised down; office dropped from 3.5 to 1.3 VA/ft2, and school/university is 3.0 in 2020 but 1.5 in 2023.
    • Adding a second 125% to 2020/2023 table lighting. Those values already include the continuous factor.
    • Using nameplate FLA instead of table FLC for motors. NEC 430.6 requires table values for conductor/OCPD sizing.
    • Adding motor feeder amps at motor voltage directly to service amps. Convert to VA at the service basis; report feeder ampacity separately.
    • Assuming aux heat is non-coincident with a heat-pump compressor. Confirm the control interlock.

    Test Your Knowledge

    Edition-aware questions. Use the calculator to help — that is the point.

    Self-Test

    Runs the in-browser regression suite. The same DOM-free core (PSP-CALC-CORE) is exercised in Node via commercial-load-calc.test.js (see the repository). The calculator is internally tested for the implemented assumptions and includes warnings for unsupported cases; final compliance requires PE/AHJ/AHJ-adopted-edition review.

    
      
    

      
    
        Calculation support only. This tool assists with learning NEC load-calculation methodology. It is not a substitute for engineering judgment, plan review, or AHJ interpretation.
        The calculator is internally tested for the implemented assumptions and includes warnings for unsupported cases; final compliance requires PE/AHJ/AHJ-adopted-edition review.