NEC 2026 — Article 450 · OCPD, Secondary Taps, Autotransformers, Ground-Fault Protection & Parallel Load Sharing
Calculator Type
NEC 450.3(B) / Table 450.3(B)
Applies to transformers rated 1000 V or less (NEC Table 450.3(B)).
kVA Rating
Phase
Primary Voltage — Line-to-Line (V)
Secondary Voltage — Line-to-Line (V)
NEC Table 450.3(B)
OCPD Location
Primary-only protection: OCPD rated up to 125% (≥ 9 A), 167% (2–9 A), or 300% (< 2 A) of primary FLA. Secondary conductor length is limited per 450.3(B) Note 2.
Recommended Breaker Sizes
NEC 240.6(A) Standard Sizes
Enter the transformer rating on the left to see the recommended breaker sizes.
Disclaimer — Important
This calculator is an independent tool intended to assist electrical professionals. It provides general guidance based on NEC 2023 (NFPA 70) for educational and estimation purposes only. Calculated values represent minimum or maximum values derived from the referenced NEC articles and tables, and do not account for all conditions of use, local amendments, authority having jurisdiction (AHJ) requirements, or project-specific engineering judgments.
Results must be verified by a licensed electrical engineer or electrician and reviewed against the full NEC text, equipment listings, manufacturer specifications, and applicable local codes before use in design, permitting, or construction. The Secondary Conductor Tap and Ground-Fault Protection tabs in particular are threshold-screening design aids, not exhaustive implementations of NEC 240.21(C) or 450.5 — verify all applicability conditions and exceptions directly against the adopted code.
The Parallel Transformers tab applies a general power-engineering load-sharing method (inverse percent impedance), not a literal NEC rule — it is not a substitute for a manufacturer paralleling/compatibility study and must be verified by a licensed engineer before units are physically paralleled.
NEC® and NFPA 70® are registered trademarks and copyrighted publications of the National Fire Protection Association. Users should verify all results against the adopted code edition and local amendments.