Open Energy Monitor Current Transformer Value Calculator

Calculator inputs

CT primary rated current Example: 100 A.

CT secondary rated current Enter milliamps. Example: 50 mA.

Burden resistor Ohms across CT secondary.

ADC reference voltage Use measured controller reference voltage.

ADC resolution Usually 10, 12, or 16 bits.

Bias center voltage Usually half of the ADC reference.

Target peak headroom Percent of safe peak swing.

Calibration trim factor Use 1 when no correction is known.

Load voltage Used for power estimates.

Power factor Enter between 0 and 1.

Measured actual current Clamp meter or trusted reference.

Monitor reported current Current shown by the monitor.

Line frequency Usually 50 Hz or 60 Hz.

Samples per cycle Sampling density per mains cycle.

Cycles to measure Used for sample window timing.

Example data table

CT primary	CT secondary	Burden	ADC reference	Base calibration	Notes
100 A	50 mA	18 ohm	3.3 V	111.111111	Common current-output CT setup.
50 A	50 mA	33 ohm	3.3 V	30.303030	Higher burden gives stronger ADC signal.
200 A	100 mA	12 ohm	5 V	166.666667	Check peak headroom before commissioning.

Formula used

The calculator uses RMS current transformer relationships and ADC scaling.

CT ratio: ratio = primary rated current / secondary rated current.
Burden RMS voltage: Vburden = secondary current × burden resistor.
Peak burden voltage: Vpeak = Vburden × √2.
ADC step: Vstep = ADC reference / 2^ADC bits.
RMS ADC counts: counts = Vburden / Vstep.
Base current calibration: calibration = CT ratio / burden resistor.
Trimmed calibration: trimmed = calibration × trim × actual current / reported current.
Clipping current: Iclip = headroom RMS × CT ratio / burden resistor.
Suggested burden: Rb = target peak headroom / √2 / secondary current.

How to use this calculator

Enter the CT primary rating printed on the sensor label.
Enter the CT secondary rating in milliamps.
Add the installed burden resistor value in ohms.
Enter the ADC reference voltage and resolution.
Set the bias voltage, usually half of the reference.
Check the headroom result before trusting high-load readings.
Compare measured current with reported current for trimming.
Download the CSV or PDF report when needed.

Accurate CT Setup Matters

An Open Energy Monitor system depends on a correct current transformer value. The value links sensor current, burden resistance, ADC range, and calibration. A small mistake can shift every daily, weekly, and monthly energy report. This calculator keeps the important checks together. It estimates the CT ratio, the burden voltage, RMS ADC counts, clipping margin, calibration value, and a trimmed value from real measurements.

Burden and ADC Headroom

The burden resistor turns CT secondary current into a voltage. That voltage must stay inside the analog input range. The calculator compares the expected peak burden voltage with the available bias headroom. A healthy design leaves margin for waveform peaks, supply changes, and CT tolerance. If the burden is too small, the signal becomes noisy. If it is too large, the signal clips and readings flatten near heavy load.

Calibration Logic

Most Open Energy Monitor sketches use a current calibration constant. For a current-output CT, the base value is the turns ratio divided by burden resistance. A correction factor can adjust for resistor tolerance, CT ratio tolerance, and ADC reference error. When you enter measured current and monitor reported current, the page gives a practical trim factor. This helps match the monitor to a clamp meter or another trusted reference.

Advanced Electrical Checks

The calculator also estimates CT burden VA, apparent power, useful sample count, and measurement window time. These values help when testing different sensors or moving from a 5 V board to a 3.3 V board. They also show whether the selected CT remains comfortable near rated load. For best results, use RMS values, confirm the burden resistor value, and measure the real reference voltage at the controller.

Practical Use

Start with the CT nameplate primary and secondary ratings. Enter the installed burden resistor and ADC details. Review the peak headroom percentage first. Then copy the calibration value into your sketch. Apply the trimmed value after a field comparison. Export the table when you need a record for commissioning, troubleshooting, or later sensor replacement.

Safety Note

Current transformers should never be opened while carrying current. Short the secondary before wiring changes. Keep mains conductors enclosed. Use suitable isolation, fusing, and local electrical rules during installation work.

FAQs

What is the current transformer calibration value?

It is the multiplier that converts ADC voltage samples into primary current. For a current-output CT, it usually equals CT turns ratio divided by burden resistance, then corrected by measured trim.

Should I enter RMS or peak current?

Enter RMS ratings from the CT nameplate. The calculator converts RMS burden voltage to peak voltage internally for headroom and clipping checks.

Why does burden resistance matter?

The burden resistor converts secondary current into voltage. A larger burden raises signal level, but it can also cause clipping or overload the CT near rated current.

What is a safe headroom percentage?

Many designs work well below 80 percent peak headroom. Lower values leave more margin. Very low values may reduce measurement resolution and increase noise sensitivity.

Can I use this for voltage-output CT clamps?

This version is designed for current-output CTs with an external burden. Voltage-output clamps already include burden parts, so use their rated output voltage carefully.

How do I trim the calibration?

Measure a steady load with a trusted meter. Enter that actual current and the monitor reading. The calculator scales the working calibration by their ratio.

Why is the ADC reference important?

The reference defines each ADC count. If the real reference differs from the assumed value, the calculated current can drift from the true load current.

Can the PDF export work without libraries?

Yes. This page creates a simple text PDF directly in the script. It is intended for compact calculation records and quick commissioning notes.