Transformer Calc NEC

About this app

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 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.

What's new: • Secondary conductor sizing now follows NEC 240.21(C): the conductor is sized so the secondary breaker never exceeds its ampacity — no next-size-up on the secondary (e.g. a 75 kVA 208V secondary is 350 kcmil, not 300) • Large transformers that exceed a single conductor now show parallel sets per NEC 310.10(H), e.g. "2 × 600 kcmil", instead of a blank result • More precise three-phase current (exact √3) Thanks for using ElecTools! Send feedback to support@tradetools.dev.

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1. First observed Oct 16, 2026 · 4:59 PM $0.99