Transformer Calculator
Convert kVA to primary and secondary current, then apply Table 450.3(B) overcurrent protection for transformers 1000 V and less. Article 450 protects the transformer windings — not the conductors. Size primary and secondary conductors and their protection under 240.21(C) (transformer secondary conductor / “tap” rules), 215, and 240.4. The 450.3(B) percentages below are the same in the 2020 and 2023 editions.
How the percentages work. Primary-only protection: 125% of primary current when that current is 9 A or more (next higher standard size permitted — Note 1); 167% when primary current is 2 A or more but less than 9 A; 300% when primary current is less than 2 A. Primary-and-secondary protection: primary device not more than 250% (next size up is not permitted on this column); secondary 125% when secondary current is 9 A or more (Note 1 next size up permitted), or 167% when less than 9 A. Supervised-location rows of Table 450.3(B) are not included here.
Secondary conductors. A secondary OCPD at the transformer does not automatically protect the secondary conductors for their full length. 240.21(C) sets when secondary conductors may run to a remote OCPD (length, ampacity, physical protection, and termination rules). Check the adopted edition and the applicable 240.21(C) subsection before treating this result as a conductor size.
Reference only — not engineering, legal, or code-compliance advice. Results are estimates for qualified persons and must be verified against the NEC edition adopted where the work is done (your selected edition), manufacturer instructions, and the local AHJ. Use is subject to the Terms of Use & Disclaimer. — Morningstar Electric LLC