Transformer Calc NEC

NEC transformer overcurrent protection calculator for electricians, apprentices, and electrical engineers. Size primary and secondary OCPD per NEC Table 450.3(B), calculate full-load amps (FLA), and select conductors from Table 310.16 - all per NEC 2023 (NFPA 70). Works 100% offline on the job site.

Whether you're sizing overcurrent protection for a 75 kVA, 480-to-208V three-phase transformer, or checking if primary-only protection requires 125% or 300%, this calculator gives you accurate NEC-compliant results in seconds. Use it as your go-to transformer protection reference - from kVA rating to OCPD and conductor selection.

TRANSFORMER CALCULATOR FEATURES:
- Primary overcurrent protection per NEC 450.3(B) - 125%/167%/250%/300% rules
- Secondary overcurrent protection per NEC 450.3(B) - 125%/167%/250% rules
- Primary-only and primary+secondary protection methods
- Full-load ampere (FLA) calculation: single-phase and three-phase
- Next-standard-size-up applied only where Note 1 permits (125% cells); 167%/250%/300% treated as ceilings
- All standard OCPD ratings per NEC 240.6(A), including fuse-only ratings 1, 3, 6, 10, 601 for small transformers
- Conductor sizing from NEC Table 310.16 (75°C column, printed code values)
- Copper and aluminum conductor support
- 40 standard kVA ratings: 0.05–500 kVA single-phase, 3–2500 kVA three-phase
- 7 voltage presets: 120V, 208V, 240V, 277V, 460V, 480V, 600V
- Single-phase and three-phase support
- NEC article references for every calculation result

NEC REFERENCES INCLUDED:
- NEC Table 450.3(B) - Transformer overcurrent protection (600V or less)
- NEC 450.3(B) Note 1 - Next-size-up rule for primary-only protection
- NEC 240.6(A) - Standard overcurrent device ratings
- NEC Table 310.16 - Conductor ampacity at 75°C

PROTECTION METHOD RULES (printed Table 450.3(B)):
- Primary Only, 9A or more: 125% maximum — next standard size up permitted (Note 1)
- Primary Only, 2–9A: 167% maximum — ceiling, no round up
- Primary Only, under 2A: 300% maximum — ceiling, no round up
- Primary + Secondary: primary 250% maximum (ceiling); secondary 125% (Note 1) for 9A or more, 167% ceiling under 9A

CALCULATION RESULTS INCLUDE:
- Primary full-load amps (FLA) with formula
- Secondary full-load amps (FLA) with formula
- Primary OCPD size (standard rating per NEC 240.6(A))
- Secondary OCPD size when applicable
- Primary and seco