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
- Automatic next-standard-size-up per NEC 240.6(A) with Note 1 logic
- All standard OCPD ratings from 15A to 6000A per NEC 240.6(A)
- Conductor sizing from NEC Table 310.16 (75°C column)
- Copper and aluminum conductor support
- 12 standard kVA ratings from 3 kVA to 1000 kVA
- 5 voltage presets: 120V, 208V, 240V, 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:
- Primary Only (≤9A): Maximum 167% primary OCPD
- Primary Only (≥9A): Maximum 125% primary OCPD (next size up per Note 1)
- Primary Only (any): Maximum 300% if 125%/167% doesn't correspond to standard size
- Primary + Secondary: Maximum 250% primary, plus 125%/167% secondary OCPD

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 secondary conductor sizes with ampacity
- Applicable NEC percentage rules and references
- Protection method notes and explanations

PRO FEATURES (one-time purchase):
- No ads - clean interface for professionals
- Save transfor