Transformer NEC 450 Calculator

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). This electrical transformer sizing calculator 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 transformer 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 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 transformer calculation history with labels • Export calculations as PDF reports for permits and records • Dark theme support HOW THE TRANSFORMER CALCULATOR WORKS: 1. Select phase type (single-phase or three-phase) 2. Choose kVA rating from standard sizes 3. Select primary and secondary voltage 4. Choose protection method (primary only or primary + secondary) 5. Select conductor material (copper or aluminum) 6. Get OCPD sizes, conductor recommendations, and NEC references instantly BUILT FOR ELECTRICIANS ON THE JOB SITE: This transformer calculator is designed by electricians, for electricians. Enter your transformer parameters, get the correct OCPD size and conductor recommendation instantly - no complicated menus or unnecessary features. One-hand operation, instant results. PERFECT FOR: • Licensed electricians sizing transformer protection daily • Electrical apprentices studying for journeyman exams • Electrical engineers and designers • Electrical contractors bidding jobs • NEC code exam preparation • Journeyman and master electrician exam study OFFLINE & PRIVACY: • Works 100% offline - no internet needed on the job site • No account required • No data collection - your calculations stay on your device Simplify your transformer protection sizing on the job site.