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.

Recent changes:
Added a link to TradeTools — the one-app version of this calculator and 61 more, for HVAC, electrical, plumbing, fire protection, solar and carpentry. Nothing else changed.