TRANSFORMER CALCULATOR

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This Transformer Calculator gives you a quick way to check full-load currents on both the primary and secondary sides of a transformer, for either single-phase or three-phase systems. You enter the transformer kVA and line-to-line voltages, and the tool automatically calculates primary and secondary full-load amps using standard formulas. On top of that, you can optionally enter a primary or secondary overcurrent protection (OCP) value, and the calculator will show you what current that OCP translates to on the opposite side of the transformer based on the voltage ratio.

That makes it handy for everyday design and troubleshooting work: sizing conductors, checking breaker and fuse selections, or getting a feel for how “hard” a transformer will be working at a given OCP level. Instead of doing multiple steps on a calculator (kVA → amps, then multiplying by voltage ratios), you can get everything in one place, with both nominal full-load currents and OCP-based currents clearly labeled.

1. Select the system type
   Choose Single-phase or Three-phase. For three-phase, enter line-to-line voltages (e.g., 600 V / 208 V).
    
2. Enter transformer nameplate data
    
   • Transformer rating (kVA) – e.g., 45, 75, 112.5
      
   • Primary voltage (V) – line-to-line
      
   • Secondary voltage (V) – line-to-line
      
   • Once these are filled in and you hit Calculate, the tool will show:
      
   • Primary full-load current
      
   • Secondary full-load current
      

1. (Optional) Add overcurrent values
    
   • If you enter a Primary OCP (A), the calculator will show
     “Secondary current if primary OCP = X A” – i.e., how many amps the transformer would be capable of delivering on the secondary at that primary OCP, using the ratio I₂ = I₁ × (V₁ / V₂).
      
   • If you enter a Secondary OCP (A), it will show
     “Primary current if secondary OCP = X A” – the equivalent current on the primary side.
      

1. Review results & reset as needed
   Use Reset to clear all fields and start a new scenario. You can run as many what-ifs as you like just by changing kVA, voltages, or OCP values and hitting Calculate again.
    

This tool is meant as a quick estimating aid, not a code book. It doesn’t apply CEC/NEC transformer protection rules or temperature corrections — it simply does the core math cleanly and consistently so you can make faster decisions and then confirm final sizes and protection devices against the applicable electrical code and manufacturer data.