Calculation sequence
Step-by-step method
- Calculate CT and PT ratios.
- Multiply the ratios for a power or energy multiplier.
- Apply the multiplier only to a genuinely unscaled secondary power or energy reading.
ElectricalAdvancedMetering and Instrumentation
CT/PT Meter Scaling Calculator
Calculate CT ratio, PT ratio, combined multiplier, and primary power or energy from an unscaled secondary reading.
Why it matters: An incorrect ratio or a multiplier applied twice can create large commissioning and billing errors.
Calculated result
Engineering evaluation
Scaled primary quantity (Mᵖ)4,800 matching primary power or energy unit
- CT ratio (Kᶜᵗ)
- 120 ratio
- PT ratio (Kᵖᵗ)
- 4 ratio
- Combined multiplier (K)
- 480 ratio
Engineering interpretation
The entered CT/PT ratios produce multiplier 480 and scaled primary quantity 4800.
Important limitations
- Do not apply this combined multiplier to current-only or voltage-only readings. Confirm meter internal scaling, polarity, phase mapping, burden, accuracy, and configuration.
Engineering knowledge center
Understand the calculation—not just the answer
Governing relationships
Kᶜᵗ = CTᵖ/CTˢ
Kᵖᵗ = PTᵖ/PTˢ
K = Kᶜᵗ·Kᵖᵗ
Mᵖ = K·Mˢ
Current substitution
Kᶜᵗ = 600/5 = 120; Kᵖᵗ = 480/120 = 4; K = 480; Mᵖ = 480×10 = 4800.
Equation legend
Inputs and calculated quantities
- CTᵖ
- CT primary rating — A
- CTˢ
- CT secondary rating — A
- PTᵖ
- PT primary rating — V
- PTˢ
- PT secondary rating — V
- Mˢ
- Unscaled secondary reading — kW, kWh, or matching unit
- Mᵖ
- Scaled primary quantity — matching primary power or energy unit
- Kᶜᵗ
- CT ratio — ratio
- Kᵖᵗ
- PT ratio — ratio
- K
- Combined multiplier — ratio
Applications
How this calculation is used
Building design
Document intended instrument-transformer ratios.
Construction
Check nameplates, polarity, wiring, and meter configuration.
Commissioning
Reconcile an unscaled secondary kW or kWh reading with the primary quantity.
Quality controls
Assumptions and limitations
Assumptions
- The entered meter value is genuinely unscaled and requires both CT and PT ratios.
- Polarity, phase mapping, wiring, and ratio nameplates are correct.
Limitations
- Do not apply the combined multiplier to current-only or voltage-only readings.
- Meter internal scaling, pulse constants, billing configuration, accuracy class, and instrument-transformer burden require qualified review.
Field use
Verification procedure
- Follow the approved electrical-safety program and use correctly rated instruments.
- Confirm wiring, ratios, units, operating mode, and measurement boundaries.
- Document simultaneous readings and compare with approved project and manufacturer data.
Common engineering mistakes
- Combining values from different time intervals, phases, or measurement boundaries.
- Treating a screening result as equipment selection, protective-device coordination, or code compliance.
Technical basis
References and source standards
- IEEE Standards AssociationPower quantity, harmonic, transformer, motor, battery, and metering standards; verify the current applicable edition during professional review.
- NIST Guide for the Use of the International System of UnitsElectrical quantity, unit, symbol, and conversion context.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement