Electrical measurement
Three-phase power, kW and kWh, power factor, current transformers, Rogowski coils and submetering.
Every reading comes from a chain of current sensor, voltage reference and meter, and it is only as good as the weakest part.
Start with the three-phase power formula if you need to calculate a load. Start with selecting current transformers if you are specifying hardware.
Power fundamentals
7 guides · about 60 minutes
- Three-phase power formula and measurement Three-phase power formula P = √3 × V × I × PF, with the kVA, kvar, current and kWh forms, a worked example, star and delta values and unbalanced loads. 7 min
- kW vs kWh: the difference between power and energy kW is power, the rate energy is used; kWh is energy, the amount used. 1 kW for 1 hour is 1 kWh. Worked examples, everyday conversions and both units on a bill. 6 min
- Power factor, kW and kVA explained kW is active power, kVA apparent power and kvar reactive power. Power factor is kW ÷ kVA, or kWh ÷ kVAh over a period, with tariffs, harmonics and correction. 11 min
- True RMS: meaning, formula and energy metering True RMS means the meter computes the root mean square, √(mean of x²). The RMS formula, form factors, and why average-responding meters misread distortion. 8 min
- Total harmonic distortion (THD): meaning, formula and limits Total harmonic distortion (THD) compares the harmonics of a voltage or current with its fundamental. Formula, THD vs TDD, IEEE 519 and EN 50160 limits. 7 min
- Negative power readings and meter reconciliation Why a meter shows negative kW: real export, a reversed CT or a CT on the wrong phase. Check each phase, then reconcile a main meter with its submeters. 9 min
- Solar self-consumption: how to calculate it from meter data How to calculate solar self-consumption and self-sufficiency from generation, import and export meters, with worked intervals and a three-step battery ledger. 12 min
Metering and CTs
9 guides · about 107 minutes
- What is a current transformer? How a CT works A current transformer (CT) gives a small secondary current in fixed proportion to a large primary current. How CTs work, ratio, burden, classes and safety. 8 min
- Selecting current transformers (CTs) for energy monitoring Choose a metering CT: 1 A, 5 A, 333 mV or Rogowski output, a ratio for the real load, Class 0.5S vs 0.5, burden in VA and polarity checks. 13 min
- Rogowski coils for energy monitoring: how they work and how to choose one A Rogowski coil is an air-core current sensor whose output follows di/dt. How it works, the integrator, sizing, low-load accuracy and Rogowski coil vs CT. 15 min
- Conductor size, voltage drop and CT aperture Convert AWG and mm², estimate conductor resistance and voltage drop, and check measured cable dimensions against current-sensor apertures. 9 min
- High-accuracy electricity metering An energy meter's accuracy class is its error limit: Class 2 is ±2 %, Class 1 ±1 %, Class 0.5S ±0.5 %. IEC 62053 and MID tables, CT error and accuracy checks. 16 min
- Operational monitoring, revenue metering and settlement Choose metering for energy analysis, billing, market settlement, power quality and protection. One meter is not proof for every purpose. 7 min
- Electricity submetering in commercial buildings Choose submetering points, reconcile the metering tree with the main meter, reuse existing meters and find installation faults that corrupt data. 17 min
- Wireless vs wired energy monitoring for retrofits How to choose between wireless and wired energy monitoring in an existing building: power, data path, reporting rate, installation work and failure behaviour. 9 min
- Energy monitoring commissioning checklist Commission energy monitoring: point schedules, source-to-report tolerances, timestamps, daylight saving, failure tests, receiver checks and acceptance. 13 min
More in Electrical measurement
- CT meters: how CT-operated electricity meters workWhat a CT meter is, when a supply needs one instead of a whole-current meter, how the CT ratio and multiplier work, and how to check the readings.
- Power quality analyser or energy meter: which does a site need?What a power quality analyser records that an energy meter does not, Class A and Class S, how long to measure, and when Modbus meters are the better answer.
- Split-core current transformers: how they work, accuracy and installationHow a split-core CT works, its output types, accuracy at low current, how to fit and test one, and when a solid-core CT or a Rogowski coil is the better choice.
Calculators
All calculators- Three-phase power Work out active, reactive and apparent power for a balanced three-phase load from voltage, current and power factor.
- kW to amps Convert kW or kVA to amps, or amps to kW or kVA, for DC, single-phase and three-phase circuits.
- kW, kVA, kvar and power factor Find the rest of the power triangle (kW, kVA, kvar and power factor) from two known values.
- Transformer current Work out the full-load primary and secondary current of a transformer from its kVA rating and voltages.
- Power factor correction Estimate the kvar needed to raise a lagging power factor to a target, and the current it saves.
- Voltage unbalance Calculate voltage unbalance from three line-to-line readings using the NEMA method.
- THD and TDD Calculate voltage or current THD, or current TDD, from measured harmonic magnitudes.
- AWG to mm² Convert conductor sizes between AWG and mm².
- CT ratio and burden Work out a CT's secondary current and check its rated burden covers the meter and the lead loop.
- Conductor resistance Estimate the DC resistance of a conductor run at its operating temperature.
- Voltage drop Voltage drop on a cable run, and the smallest cable size that keeps it within 3% or 5%.
- Current transformer selection Size the aperture, check a candidate CT and match its output to the meter.
- Ohm’s law Find volts, amps, ohms or watts from any two known values.
- Motor full-load current Find a motor’s full-load current from its kW or hp rating, efficiency and power factor.
- Transformer losses Separate the loss that remains while energised from the loss that rises with current.
EpiSensor products
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