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Transformer Protection Calculator: NEC 450.3(B) OCPD Sizing

Size Maximum Primary and Secondary Overcurrent Protection per NEC Table 450.3(B) (1000V and Less)

Free transformer overcurrent protection screen for unsupervised NEC Table 450.3(B) cases. Enter transformer kVA, primary voltage, secondary voltage, phase configuration, and whether a secondary OCPD is provided to check the transformer-protection maximum primary and secondary OCPD sizes with the applicable percentage cells and rounding rules.

The 450.3(B) cells differ by current band and by whether secondary protection is provided, and the Note 1 next-size-up allowance applies only to the 125% cells. This calculator screens the unsupervised 125%, 167%, 250%, and 300% cells; it does not cover Table 450.3(A), supervised installations, conductor protection, panelboard protection, coordination, or impedance-based cells.

Pro Tip: Table 450.3(B) Note 1 round-up applies only to the 125% cells: primary-only protection at 9A or more, and secondary protection at 9A or more. The 250% primary-with-secondary cell and the 167%/300% small-current cells are hard ceilings - when the calculated value is not a standard rating you must drop to the next standard size DOWN, not up. Mixing up which cells allow round-up is the most common mistake on plan reviews. And remember these are transformer-protection maximums; conductor protection per 240.4 and 240.21(C) often forces a smaller device.

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How It Works

  1. Enter Transformer Specifications

    Input kVA rating, primary voltage, secondary voltage, and phase configuration (single-phase or three-phase). Common kVA quick-pick chips are provided.

  2. Select Protection Arrangement

    Choose whether a secondary OCPD is provided. With secondary protection the primary cell is 250% (not exceeding); without it the primary cell depends on the primary current band: 125% (9A or more), 167% (2-9A), or 300% (under 2A).

  3. Review Calculated Ratings

    See primary and secondary full-load amps, the applicable 450.3(B) percentage, the calculated maximum, and the maximum standard OCPD size with the correct round-up or round-down rule for that cell.

  4. Check the Quick-Reference Table

    A built-in 450.3(B) quick-reference table highlights which cell applies to your inputs and shows the rounding rule for each cell.

  5. Export the Sizing Screen

    Generate a PDF with the configuration, FLA values, OCPD maximums, rule basis, source warnings, and residual gaps for plan-review records. Confirm conductor protection and AHJ requirements separately.

Built For

  • Electricians screening transformer-protection maximums before qualified review
  • Electrical designers checking which unsupervised 450.3(B) cell applies before conductor and panelboard checks
  • Plan reviewers using the output as a question list for adopted NEC and AHJ review
  • Inspectors comparing field data against source warnings before acceptance decisions
  • Electrical contractors collecting OCPD maximums before material and coordination review
  • Maintenance electricians checking whether a replacement device needs engineering review
  • Apprentice electricians studying NEC Article 450 rounding boundaries

Features & Capabilities

NEC Table 450.3(B) Logic

Screens the unsupervised Table 450.3(B) cells: primary with secondary protection (250%, not exceeding) and primary-only current bands (125% at 9A or more, 167% at 2-9A, 300% under 2A), plus the two secondary bands (125% at 9A or more, 167% under 9A).

Rounding Per Cell

Applies NEC 240.6(A) standard ratings with Note 1 round-up only on the 125% cells; the 250%/167%/300% ceilings select the next standard size down when the calculated value is not standard.

Full-Load Amp Arithmetic

Calculates primary and secondary FLA from kVA and voltage: I = kVA × 1000 / (V × √3) for three-phase, I = kVA × 1000 / V for single-phase. Nameplate and tap data still need review.

Standard Rating Screen

Uses NEC 240.6(A)-style standard ampere ratings as a screen for the transformer-protection maximum; device listing, interrupting rating, conductor protection, and coordination are separate checks.

PDF Export

Export the screening inputs, transformer-protection maximums, rule basis, source warnings, and residual gaps for review records.

Assumptions

  • Overcurrent protection percentages from NEC Table 450.3(B) for transformers 1000V and below
  • Primary full-load amps calculated as kVA x 1000 / (V x sqrt(3)) for three-phase, kVA x 1000 / V for single-phase
  • Next-standard-size rule per NEC 240.6(A) applied only where permitted by NEC 450.3(B)
  • Standard ampere ratings per NEC 240.6(A) used to screen maximum transformer-protection OCPD sizes
  • Unsupervised-installation cells of Table 450.3(B) only; supervised and impedance-based cells are not modeled
  • Protection arrangement (primary only, or primary with secondary OCPD) selected by user

Limitations

  • Does not cover transformers over 1000V primary - NEC Table 450.3(A) rules differ significantly
  • Does not size conductor overcurrent protection per NEC 240.4 - that is a separate calculation
  • Autotransformers, instrument transformers, and zig-zag transformers have additional NEC requirements not addressed
  • Does not evaluate coordination between primary and secondary protective devices (requires time-current curve analysis)
  • Electronically actuated fuses and adjustable-trip breakers require separate engineering evaluation
  • NEC 450.3 exceptions for supervised locations (qualified persons only) must be verified by the user

References

  • NEC (NFPA 70) Article 450 - Transformers and Transformer Vaults
  • NEC Table 450.3(A) - Maximum Rating or Setting of Overcurrent Protection for Transformers Over 1000 Volts
  • NEC Table 450.3(B) - Maximum Rating or Setting of Overcurrent Protection for Transformers 1000 Volts and Below
  • NEC 240.6(A) - Standard Ampere Ratings for Fuses and Circuit Breakers
  • IEEE C57.12.00 - Standard for Liquid-Immersed Distribution, Power, and Regulating Transformers
  • Mike Holt's Illustrated Guide to Transformer Overcurrent Protection

Frequently Asked Questions

Yes, NEC 450.3(B) permits primary-only protection. For primary currents of 9A or more the maximum is 125% of primary FLA, with Note 1 allowing the next standard size up when 125% is not a standard rating. For 2-9A the ceiling is 167%, and below 2A it is 300% - those small-current cells have no round-up allowance. Secondary conductors must still be protected by other means (e.g., a panelboard main per 408.36 and 240.21(C)).
For secondary currents of 9A or more, yes - the 125% secondary cell carries the same Note 1 round-up allowance as the 125% primary cell. For secondary currents under 9A the 167% cell is a hard ceiling with no round-up. Either way the secondary OCPD is a transformer-protection maximum; conductor and panelboard protection rules often require a smaller device.
In unsupervised Table 450.3(B), 167% is the maximum primary OCPD for primary-only protection when the primary current is 2-9A, and the maximum secondary OCPD when the secondary current is under 9A. It is a not-to-exceed ceiling - no round-up. (With secondary protection provided, the primary cell is 250% of primary FLA, also not exceeding.) Impedance-based cells exist only in the supervised portions of the NEC tables, which this tool does not model.
Control transformers (Class 2, signaling) under 1000VA are governed by NEC 725 or other articles, not 450.3. Transformers rated over 1000VA in industrial control circuits follow NEC 450.3 unless specifically exempted by another article.
The voltage and current calculations account for the winding configuration automatically. The kVA is the same on both sides (minus losses). Primary amps use the primary voltage; secondary amps use the secondary voltage. For 480V delta to 208Y/120V, secondary FLA is based on 208V line-to-line for three-phase loads.
Disclaimer: Calculations are based on NEC Table 450.3(B) for unsupervised installations of transformers 1000V and less; Table 450.3(A) (over 1000V), supervised installations, and impedance-based cells are not modeled. Results are transformer-protection maximums only - conductor protection (240.4, 240.21(C)) and panelboard protection (408.36) can require smaller devices. Local code amendments may modify NEC requirements; always verify with the authority having jurisdiction (AHJ). This calculator does not account for special transformer types (autotransformers, instrument transformers, etc.) that have additional NEC requirements.

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