NEC 450 transformer OCPD calculator. kVA to conductor sizing. Offline.
50+ downloads
About Transformer NEC 450 Calculator
Transformer NEC 450 Calculator is a tools app developed by ZkDev.
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Transformer NEC 450 Calculator has been downloaded 84 times. In the last 30 days, the app was downloaded 22 times.
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App Information
Transformer NEC 450 Calculator is FREE to download. The APK download size is 20.44 MB. The latest version available is 1.0.8. The last update was on June 10, 2026.
Technical Requirements
The app has a content rating of Everyone. The app has been available on Google Play April 2026.
Description
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.
Recent changes: What's new: • 167%/300% rules now follow the printed Table 450.3(B): ceilings without round-up (Note 1 applies to 125% cells only) • Fuse-only ratings 1, 3, 6, 10, 601 per NEC 240.6(A) — correct OCPD for small transformers • Conductor table corrected to the printed NEC 310.16 75°C values • 600V preset added; 40 standard kVA ratings listed
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