Quick Summary (TL;DR)
Choose a current transformerAliExpress price (CT) for the energy monitor, not just for the amperage of the circuit. First identify the monitor’s supported CT type and electrical output: a 100 A:50 mA current-output clamp, a 100 A:333 mV voltage-output clamp, and a manufacturer-specific 100 A accessory are not interchangeable merely because each says “100 A”. Wrong matching can produce wrong readings, sensor saturation or an unsafe secondary circuit.
For a supported DIY/ESP32AliExpress price energy-monitoring design, a split-core CT with the right rated current, aperture, insulation rating, and a correctly matched analogue front end can be a practical option. A current-output CT requires a correctly specified burden circuit; a voltage-output CT normally contains its burden internally. A bare ESP32AliExpress price ADC pin is not a direct CT input, and a CT alone does not establish real power or kWh without appropriate voltage/phase measurement or an explicitly approximate method.
For Shelly and Emporia systems, start with compatible manufacturer-specified clamps and approved installation arrangements rather than buying generic low-cost CTs. For Home Assistant, choose the complete monitoring system on the basis of safe metering, real-power accuracy and trustworthy kWh entities—not the CT’s claimed precision in isolation. Distribution-board work should be specified and performed by a competent electrician with the supply properly isolated and verified safe.
The six checks before buying any CT clamp
- Meter compatibility: identify the exact model and revision, its CT-input specification, connector and supported calibration settings.
- Output: current-output (for example 50 mA), low-voltage output (for example 333 mV or 1 V), or a proprietary assembly; confirm the specification, not just the label.
- Primary current: choose a suitable rated-current range that covers the normal maximum load without driving the sensor or input into saturation.
- Opening: measure the insulated conductor or busbar clearance; the CT must close completely without pressure on the cable or neighbouring equipment.
- Measurement objective: current only, real power, solar import/export, individual circuits, or billing-grade energy; different objectives need different complete systems.
- Safety and installation: documented insulation/environmental rating, certified installation suitability and an electrician’s route for safe isolation, termination and future service.
A useful starting question is “What exact CT model does the meter manufacturer approve?” If there is a definite answer, it usually eliminates the most common integration mistakes. Generic CT selection makes more sense with an input stage explicitly designed for configurable CTs, such as certain OpenEnergyMonitor systems or a suitably engineered analogue measurement board.
What is a current transformer—and what does it actually measure?
A current transformerAliExpress price measures alternating current using the changing magnetic field around a conductor passing through its core. That conductor forms a one-turn primary winding in the usual clamp-on arrangement. The CT secondary produces a related electrical signal, allowing the attached energy monitor to estimate line current while remaining galvanically isolated from the primary conductor when the device and installation are correctly specified.
The field around a single live conductor tracks its current. If you clamp around a normal multicore mains flex containing both line and neutral, their currents usually run in opposite directions and largely cancel. You will often see almost zero. To monitor a particular circuit, the CT must surround the individual appropriate conductor, not the complete supply cable. Accessing that conductor within a distribution board is not a casual homeowner task.
A standard iron/ferrite CT does not measure steady DC current. Solar battery strings, DC chargers or a battery inverter DC bus require a suitable DC-capable current transducer (often Hall-effect or shunt-based), a correctly rated isolated measurement system, or inverter-provided data. Do not purchase an AC split-core CT expecting it to measure a DC battery cable.
The CT itself reports current, not a billable energy reading. Real electrical power depends on voltage and the phase relationship between voltage and current. A meter that assumes a fixed voltage or power factor may give an estimate appropriate to certain loads, but it cannot reproduce all real-world appliances, motor loads and import/export directions accurately. OpenEnergyMonitor’s installation guide explains the primary-conductor and direction principles.
Current-output vs voltage-output CTs: the compatibility trap
The most important specification is the secondary output. Traditional metering CTs are specified by a current ratio such as 100 A:50 mA, 100 A:5 A or 100 A:1 A. Some modern home-energy clamps are instead specified by a voltage such as 333 mV at rated current. A voltage-output CT incorporates a burden resistor inside its assembly; a current-output CT normally requires a designed load (burden) in the meter or external measurement circuit.
| CT or sensor example | Signal at rated primary current | What the monitor needs | Important buying check |
|---|---|---|---|
| 100 A:50 mA | 50 mA AC secondary | Compatible current input and specified burden resistor | Verify burden, ratio, transient protection and safety |
| 100 A:333 mV | 333 mV AC output, normally from internal burden | Input rated for 333 mV CT output | Do not add an arbitrary extra burden; check voltage scaling |
| 100 A:1 V | 1 V AC output at rated current | Input and calibration intended for that exact output | Not automatically interchangeable with 333 mV |
| 100 A:5 A / 100 A:1 A | Industrial current-secondary level | Appropriately rated metering CT input | Do not connect to ESP32AliExpress price or a low-voltage CT jack directly |
| Shelly-specific CT | Model-specific manufacturer-calibrated combination | Named compatible Shelly EM/3EM/Pro 3EM input | Buy exact compatible accessory; avoid generic substitutions |
| Emporia Vue 3 main / branch CT | 0.333 V at 200 A / 50 A respectively for rigid CTs | Corresponding Vue 3 main or branch port | Correct sensor type, plug and channel are required |
Example: a 100 A:50 mA clamp has a nominal current ratio of 2000:1. At 20 A through the primary it produces approximately 10 mA RMS in its secondary, provided the CT remains inside its specified operating range and is connected to a suitable burden. By contrast, a 100 A:333 mV sensor ideally provides roughly 66.6 mV RMS at 20 A. The numerical result differs because the monitor interfaces are different—not because one of the units is inherently more accurate.
The connector does not establish compatibility. A familiar 3.5 mm plug or small terminal block can carry different secondary signals, have different pin assignments, and be used with different protection schemes. Do not mix branded monitor inputs and unverified CTs on the assumption that their plugs fit.
How to choose the CT current rating: 20 A, 50 A, 100 A or 200 A?
Choose a rating that covers the expected normal maximum current, taking account of the circuit’s protection, its intended load and the monitor manufacturer’s specified range. Oversizing can reduce useful resolution at low current in some combinations; undersizing risks saturation and clipped readings. Do not assume all CTs have the same accuracy across their entire range—look for a manufacturer curve or specification.
| Monitoring job | Common CT range to investigate | Reason | Critical caveat |
|---|---|---|---|
| 16 A domestic circuit | 20 A or manufacturer-defined branch CT | Good match to typical maximum current | Only if accepted by the input and properly installed |
| 32 A EV charger circuit | 50 A branch CT | Headroom while remaining near useful range | Single-phase vs three-phase changes number of CTs |
| 63 A single-phase feeder | 80–100 A CT | Covers feeder rating with useful headroom | Check input range and any higher transient currents |
| 80–100 A home main | 100–120 A class CT or approved vendor bundle | Practical whole-home class for many homes | Must match actual service and CT specification |
| Up to 200 A service | 200 A main CT system | Supports larger services | Main-port input rating and conductor aperture must match |
| Large busbar or crowded panel | Approved flexible/Rogowski solution | Physical access may favour a flexible loop | Requires supported integrator, power and calibration |
These are illustrative screening choices, not universal substitutions. For example, the OpenEnergyMonitor emonPi3/emonTx6 installation guidance recommends choosing a CT that broadly matches the current range and identifies its 333 mV compatible CT options. However, Emporia explicitly says that its 200 A Vue 3 main CTs remain suitable for smaller mains—including a 100 A board—so you should not replace them with a generic “closer-rated” clamp in pursuit of theoretical low-current improvement.
Check the rating of the whole channel, not simply the label of the clamp. The CT, analogue input and firmware may limit readings independently. A larger CT does not necessarily increase the range of an existing 50 A branch port. Likewise, the current rating is not permission to install a sensor in an enclosure or on a busbar where its insulation, temperature or clearances are unsuitable.
CT aperture: the overlooked measurement that determines whether it fits
The CT window is the space through which the insulated conductor passes. There must be enough room not merely to clear the cable diameter, but also to close the core fully, route the CT lead without damaging insulation, and maintain enclosure clearances. A CT clipped over bent, compressed or partially trapped cable insulation is not an acceptable solution.
For example, Emporia documents a 22 mm inner window on its standard Vue 3 rigid 200 A main CT, and a 10 mm inner window on the rigid 50 A branch CT. It also lists a large-aperture 50 A branch option for cramped or larger-conductor cases. These are model-specific product dimensions, not universal standards. They matter as much as a 50 A or 200 A label when selecting a kit.
A split-core sensor normally opens around an accessible insulated conductor. A solid-core CT is a closed ring and usually requires the wire to be disconnected and threaded through. That can yield compact or precise installations in a controlled design, but is rarely an attractive retrofit to an occupied consumer unit. For crowded boards or rigid busbars, a manufacturer-supported flexible sensor may help, yet a Rogowski coil is an active measurement system in practice: it needs an appropriate integrator and cannot simply replace a passive CT by changing a plug.
| Physical format | Strength | Compromise | Best suited to |
|---|---|---|---|
| Split-core hinged CT | Retrofit without separating the conductor | Core closure and mechanical fit matter | Most approved home-panel installations |
| Solid-core / ring CT | Simple closed magnetic path and stable mounting | Conductor generally must be disconnected | New panels or designed meter cabinets |
| Flexible Rogowski loop | Can route around large conductors or busbars | Requires matched electronic integrator and supply | Challenging approved industrial/main installations |
| PCB/Hall-effect DC sensor | Can measure DC in the right design | Different supply, isolation and conductor design | DC systems, not a drop-in AC CT substitute |
One CT per phase—or one per circuit?
The CT count must follow the supply topology and the information you want. In a typical European single-phase supply, one appropriately placed main CT can monitor the main live conductor for the entire property, assuming the meter also measures or estimates the relevant voltage. For a three-phase supply, the usual architecture is one CT per phase with the correct phase-voltage pairing and an energy meter that actually supports the topology.
A three-phase appliance may need three CTs if phase-by-phase current and power are required. A single clamp around all three phases generally produces a vector sum that may be close to zero; it is not a reliable “total power” measurement. Similarly, a split-phase North American home typically needs the monitor’s approved two-leg arrangement, not a European single-phase assumption. Four-wire wye, delta and other networks have different support constraints; verify the exact meter topology with the manufacturer and electrician.
Whole-home monitoring and branch-circuit monitoring are different purchases. Shelly Pro 3EM is a three-channel phase meter; it cannot turn one CT into a sixteen-circuit monitor. Emporia Vue 3 is designed with main CTs plus optional multiple branch CTs. The best arrangement depends on whether you are trying to control solar export, identify household baseload, find large appliances, or monitor an EV circuit.
Solar homes require special attention to meter location and sign convention. A grid-tie CT upstream of a solar connection measures net grid import/export; a CT on an inverter output measures generation; and a CT on a major load measures consumption on that branch. These are related but not identical energy streams. You cannot recover all of them from one reading without additional independent information.
CT direction, phase matching and negative readings
For an AC system that calculates true real power using the voltage waveform, CT orientation affects the sign of power and sometimes the import/export interpretation. A manufacturer arrow, K/L or P1/P2 marking convention is only meaningful in the context of that system’s documentation. Some devices can invert a channel in software; others expect the physical clamp to be oriented correctly.
Where there is a three-phase meter, the current measurement for L1 generally needs the matching L1 voltage reference, similarly for L2 and L3. An incorrect phase pairing can make current look plausible while measured active power is seriously wrong—even negative—particularly at non-unity power factor. Moving a plug at random is not a substitute for confirming conductor identity, voltage references and manufacturer instructions.
A negative grid-power value is not inherently a defect: it may be a correct export reading from rooftop solar. But sustained negative consumption at night, implausibly low total kW with all three phases loaded, or a sensor that never changes under a known load is a useful diagnostic clue. Resolve CT direction, phase mapping and software conventions before trusting accumulated kWh in Home Assistant.
Accuracy: CT percentage, phase error and low-current performance
Product pages often highlight a percentage such as ±1% or ±2%. Ask what it actually describes: CT ratio at a defined current, current reading at a certain power factor, entire meter power measurement, or energy accuracy under a specified test. Those are not identical claims. CT phase shift, voltage sensing error, ADC timing, calibration and low-load behaviour can all influence energy readings.
For an ideal sinusoidal system, real power is P = VRMS × IRMS × power factor. If the CT introduces phase error or the meter pairs current with the wrong voltage channel, its power reading may be materially wrong even though the current number looks convincing. Harmonic-rich loads introduce additional complications; a system built to read only apparent power is not a substitute for a capable real-power meter.
At very low load currents, the relative error can be much larger than the headline claim. For instance, a house drawing 80 W overnight on a 100 A clamp may sit near the system’s noise and measurement threshold. Oversizing the CT by an order of magnitude could make this harder to resolve on certain analogue designs. Conversely, a too-small CT will clip as the EV charger or heat pump ramps up. Check the accuracy band and usable minimum current in the complete monitor plus sensor specification.
Emporia’s published Vue 3 CT accuracy documentation distinguishes the current measurement from whole-system power results under differing conditions. For Shelly, the Pro 3EM specifications publish measurement ranges and accuracy bands for its matched CT system. Neither specification should be transplanted into a DIY ADC circuit as if the entire system inherited it.
Burden resistors: what they do, and why a wiring shortcut can be dangerous
A current-output CT attempts to drive secondary current proportional to the primary. The burden resistor converts that current to a measurable voltage: approximately Vsecondary = Isecondary × Rburden within the CT’s normal range. For example, a nominal 100 A:50 mA current-output CT feeding an 18 Ω burden would develop about 0.90 V RMS at 100 A, ignoring real-world tolerances and saturation. That calculation does not establish that 18 Ω is suitable for your ADC, protection arrangement or CT.
The burden value and its power rating must be chosen with respect to the CT ratio, secondary current, intended peak voltage, analogue input biasing, transient protection and saturation curve. A larger burden is not always better: demanding too much secondary voltage can drive the CT core into saturation and distort the waveform. A CT with a built-in burden is already a voltage-output device and normally should be connected to the input the manufacturer specifies, without an arbitrary second load.
Critical safety rule: do not leave an unburdened current-output CT secondary open-circuit while it encircles an energised conductor. Dangerous voltage can develop at the CT leads. OpenEnergyMonitor explicitly advises connecting the CT secondary before fitting the sensor and removing the CT from the live-current path before disconnecting it, unless you have verified the appropriate voltage-output/burden-protected arrangement. This is an engineering safety issue, not merely a cause of inaccurate readings. Do not attempt to repair an open-secondary condition by handling leads in a live board; use a qualified electrician and an approved isolation/shorting procedure.
For an ESP32 design, even a nominal 333 mV AC signal cannot simply be wired to a microcontroller pin. ESP32 ADC inputs must remain within their device-specific voltage limits and cannot read negative half-cycles directly. Correct biasing, over-voltage protection, filtering, safe ground isolation, an appropriate ADC or metering IC, and software calibration are part of the design. You should not expose hobby electronics or a USB-connected computer to a mains-enclosure fault path.
Shelly, Emporia and OpenEnergyMonitor: practical compatibility
| Platform | CT buying approach | What is documented | Avoid |
|---|---|---|---|
| Shelly EM, 3EM or Pro 3EM | Use the exact CT models Shelly specifies for your meter revision | Shelly cautions that units are designed and calibrated for its own CTs | Generic CT swaps based only on primary ampere rating |
| Emporia Vue 3 | Use Vue 3 main 200 A CTs and branch 50 A CTs for the appropriate ports | Emporia describes 0.333 V rigid CT outputs, connector/window sizes and approved alternatives | Treating main and branch sensors/ports as interchangeable |
| OpenEnergyMonitor emonPi3 / emonTx6 | Choose the documented 333 mV voltage-output CT family and supported current calibration | Manufacturer documentation covers ranges and installation direction | Assuming a current-output SCT-013-000 is a direct plug-in replacement |
| Custom ESP32 / ESPHome meter | Specify CT, burden/voltage-output, analogue front end and isolated voltage reference as one circuit | Design-specific calibration and protection required | Direct CT-to-ADC wiring, unverified voltage references or mains DIY |
Shelly: its support guidance says it does not recommend CTs from another brand for Shelly EM, 3EM or Pro 3EM; the devices are calibrated for specified Shelly clamps. Also remember that “Shelly EM”, “Shelly EM Gen3” and “Shelly Pro 3EM” are different products with differing input arrangements. Double-check model, generation and bundle before ordering accessories. The official Shelly CT compatibility answer is more relevant than a marketplace listing promising universal support.
Emporia: Vue 3’s rigid 200 A main CT and 50 A branch CT sensors use a 0.333 V output at their respective rated current, but that does not remove the need for correct Vue ports and firmware scaling. Emporia supplies dimensional and port details, including approved large-aperture and flexible alternatives, in its CT sensor guide and installation guidance. Its documentation discusses specialised conversions, but those are not general advice to reterminate CT leads yourself; follow the manufacturer’s complete instructions and electrical safety requirements.
OpenEnergyMonitor: systems designed for 333 mV voltage-output CTs are more accommodating when you need a choice of ratings and apertures. Yet the unit version still matters: older DIY emonTx circuits and newer emonPi3/emonTx6 designs do not necessarily have identical CT-input assumptions. Use the actual product installation guide, not a forum example for a previous PCB revision.
Custom ESP32: this route is most suitable for people who want to design and validate the acquisition circuit rather than buy a turnkey revenue meter. The site’s existing ESP32 CT clamp monitor guide covers the firmware and electronics side, while our comparison of pulse, CT, PZEM and Modbus monitoring helps decide whether a CT design is the right starting point at all.
Choosing a CT for solar, an EV charger or a heat pump
Solar import/export and battery systems
For rooftop solar, buy a monitor that can record signed active power and separate the grid connection from generation. A grid CT correctly installed at the point of common coupling is often useful for net import/export. But it does not automatically reveal total household consumption and generation independently. Multiple correctly placed channels or an inverter integration are needed for a full energy-flow model. Battery power on the DC side is a separate electrical measurement problem.
If you also plan to automate a solar diverter, avoid treating CT data as a fail-safe electrical control signal. Network dropouts, stale readings, low-current noise and incorrect sign can produce unwanted behaviour. Use devices with documented safe operating modes, appropriately rated switching hardware, and any required grid-compliance features.
EV charging
A 7 kW-class single-phase charger generally draws roughly 30–32 A at nominal 230 V when operating near full power. A supported 50 A branch CT is a sensible scale for monitoring that circuit; a three-phase charger needs an appropriate three-phase arrangement rather than a single clamp claiming to represent all phases. Confirm the actual charger, circuit protection, maximum current and whether dynamic load balancing requires the charger manufacturer’s own CT/meter.
Measurement alone is not load management. A charger can be certified or configured to use a particular current-sensing accessory for protective load balancing. A Home Assistant graph from a separate CT may be useful for analytics without being a permissible replacement for that safety/control arrangement.
Heat pumps and electric heating
A heat pump may combine a compressor, circulation pumps, immersion or backup heater and defrost loads. Circuit-level monitoring must include whichever conductors carry the load you want to assess. If the outdoor unit and backup heater are on separate circuits, one CT will not show their combined energy unless the upstream topology makes that valid. Choose CTs that tolerate startup/inrush behaviour without compromising normal-running resolution, and evaluate the rated branch circuit rather than only the headline running power.
For a realistic coefficient-of-performance (COP) comparison, electricity consumption alone is not enough; you also need credible delivered-heat measurement. Do not label an electrical kWh reading as a full COP result. Home Assistant can help aggregate separate system sensors, but accuracy depends on the underlying metering.
Calibration and commissioning: what a qualified installer should verify
A successful commissioning check should establish correctness before chasing tiny calibration offsets. If a sensor is on the wrong conductor or phase, adjusting a multiplier may hide the fault rather than fix it. Work within manufacturer settings and accepted test procedures; the steps below describe what the system owner can request from a qualified installer, not instructions to work inside an energised board.
- Identify: label each measured circuit and phase and record sensor make, model, CT ratio and channel assignment.
- Physical check: ensure sensor is fully closed, seated as intended, mechanically secure and clear of cable strain and insulation damage.
- Electrical check: confirm all manufacturer voltage-reference and phase-assignment requirements are satisfied.
- Polarity/sign: check import/export direction against a known operating condition, especially on properties with PV.
- Sanity comparison: compare a stable, appropriately sized known load and the installed meter against a suitable independent measurement; allow for power factor and measurement uncertainty.
- Long-term behaviour: inspect low-load values, high-load clipping, missing data and any anomalous counter resets.
- Documentation: retain a simple CT/channel map, calibration settings, firmware version and commissioning observations.
For instance, an electric heater with a largely resistive load is easier to sanity-check than a variable-speed motor. But never claim a “certified ±1% calibration” after comparing against a single household appliance; meaningful accuracy verification requires suitable traceable instruments, controlled test conditions and a known reference meter.
Home Assistant: CT readings, watts and kWh are different things
Home Assistant does not normally communicate with a bare CT. It receives entities from the monitoring device—Shelly, a supported gateway, ESPHome, Modbus or another integration. Select a monitor that produces useful values locally at your desired update rate and, for long-term energy tracking, presents energy totals in Wh or kWh with supported statistics attributes.
A real-time power sensor in W is useful for instant graphs and automations; a cumulative energy sensor in kWh is the usual basis for Home Assistant’s Energy dashboard. Home Assistant’s documentation specifies appropriate device_class and state_class attributes, such as energy and total or total_increasing where applicable. A net signed total needs suitable semantics: naively treating an import/export value that goes up and down as total_increasing can distort long-term statistics.
Home Assistant provides an Integral integration for producing approximate energy from sampled power. It cannot reconstruct missed transitions or repair CT/voltage errors. Frequent samples and stable sensor availability help, but vendor cumulative kWh counters are often preferable when they are correctly implemented. For troubleshooting sensor eligibility, see our Energy dashboard troubleshooting guide and the official Home Assistant energy FAQ.
For more robust integration—especially where tariff calculations, solar automation and high loads are involved—consider a dedicated DIN-rail meter with RS485/Modbus rather than trying to make a CT alone perform every metering function. Our SDM120/SDM630 Modbus guide explains a complementary route, subject to the meter’s specific configuration and professional installation.
Real-world buying costs: the complete kit matters more than the clamp price
Comparing a single £10–£20 listing for a CT with a complete, installed energy monitor is misleading. The cost of a useful measurement point includes the right CT, compatible main unit, voltage references, appropriate protection and enclosure, installation labour, network availability, configuration and any extra channels for three-phase power or multiple circuits. Retail prices vary by region and promotion, so specific live prices should be verified at purchase rather than assumed here.
| Cost component | Questions to ask before buying | Why it affects value |
|---|---|---|
| Meter/monitor | Does it include power, energy and phase-capable voltage measurement? | CT alone typically measures only current |
| CTs and spares | How many approved CTs are in the kit; are replacements available? | Three-phase and branch monitoring multiply sensors |
| Aperture and cable length | Will the clamp close and reach the input without unsafe extensions? | Mechanical fit can invalidate an otherwise cheap choice |
| Installation | Is a qualified electrician needed and is there panel space? | Affects actual installed cost and compliance |
| Connectivity and data | Local interface, internet dependency, retention and export? | Determines long-term usefulness and outage behaviour |
| Monitoring expansion | Can more branch, solar or EV channels be added? | Avoids buying a second system later |
A good-value result might be a manufacturer CT set that fits an already installed compatible monitor, or a complete new metering system whose channel count closely matches your needs. Replacing a working monitor purely because an untested universal clamp is cheaper is often false economy. For affiliate shopping, prioritise exact device model, revision and CT part number in the product description; do not assume a marketplace seller’s generic compatibility table is authoritative.
Troubleshooting common CT mistakes
| Symptom | Possible explanation | Safe next step |
|---|---|---|
| Nearly zero current on a busy circuit | CT surrounds both live and neutral; wrong conductor | Ask electrician to verify intended single-conductor location |
| Plausible amps, wrong power or negative import | Incorrect orientation, voltage phase pairing or sign convention | Confirm manufacturer channel/phase map and grid flow direction |
| Reading plateaus under high load | CT or monitor input saturation / rating exceeded | Check permitted continuous range and replace with approved rated assembly |
| Very noisy readings at low load | Oversized or poor-quality CT, ADC noise or wiring | Inspect rated low-end accuracy and electronics specification |
| Readings scaled by a consistent factor | Wrong programmed CT ratio or incompatible 333 mV/1 V/current output | Verify exact part number and approved device configuration |
| No sensor in Energy dashboard | Only watts available; invalid statistics metadata or unstable kWh total | Check Home Assistant entity class, unit and Statistics tab |
| Grid import becomes export unexpectedly | CT reversed or supply topology misidentified | Have qualified installer verify location, polarity and live flow |
Never diagnose a suspect CT by unplugging or probing its secondary while it remains around a live conductor. A current-output CT can present an open-secondary hazard. If the physical installation needs correction, isolate according to the appropriate safe system of work and have a competent person perform it.
Safety and legal considerations for UK and European homes
A CT clamp is non-contact with respect to the conductor it measures, but fitting it generally requires entering a mains distribution board. That can expose lethal voltages, stored energy, service conductors and parts that cannot be de-energised by the household main switch. The presence of a hinged plastic clip does not make the task DIY-safe. Seek a registered/competent electrician, comply with local wiring rules, and follow the specific equipment’s installation instructions.
Use CTs with documented voltage insulation, physical/environmental suitability and any installation category/certification applicable to the exact environment. Do not infer a safety certification from the seller’s generic “CE” wording, or from an attractive picture of a yellow clamp. Keep manufacturer instructions with the installation and confirm that cable routing, service clearances, temperature and breaker-cabinet space meet requirements.
In the UK, any applicable BS 7671 and electrical safety requirements still apply; elsewhere in Europe, national implementation and distribution-network rules apply. Sealed utility-company metering compartments are a separate matter and may be restricted. Home Assistant monitoring does not replace a revenue meter, a required protective device, or the safety controls of a certified EV charging/load-management system.
Recommended selection by project type
- One single-phase household total: an approved whole-home monitor and one compatible main CT when the product and electrical arrangement support it. Prioritise correct voltage reference and reliable kWh.
- Three-phase Greek or UK property: a true three-phase monitor with the correct three CTs, phase references and topology support. Verify neutral/delta compatibility rather than assuming “3-phase” is enough.
- Many individual circuits: a multi-branch monitor with manufacturer-approved main and branch sensors, adequate panel space and an installation plan.
- ESP32 experimentation: a documented compatible CT plus engineered burden/ADC and isolated reference, tested away from live distribution-board access. Consider a certified Modbus meter for a production installation.
- Solar grid import/export: a monitor that measures signed real power at the correct grid boundary; separate generation input or inverter data for full flows.
- DC battery or DC charger: a purpose-built DC-rated sensor or device data—not an ordinary AC CT.
- Tight busbar layouts: a manufacturer-approved flexible/Rogowski solution with its dedicated integrator, fitted professionally.
Bottom line: The right CT is defined by the complete meter-channel combination: approved electrical output, primary range, dimensions, insulation and the voltage reference used to calculate power. Buy based on your actual circuit and intended Home Assistant entities, not the biggest ampere number or the lowest clamp price. A correctly selected 50 A branch CT can deliver more useful information than a generic 200 A device; an approved 200 A manufacturer kit can be a better choice than an unauthorised 100 A substitution.
Frequently asked questions
Can I use any 100 A CT with a Shelly meter?
No. Shelly says its devices are calibrated for specified Shelly CT accessories, and does not recommend using third-party clamps. Exact Shelly model and generation matter. A generic “100 A CT” can have the wrong ratio, output type or connector even if its size is similar.
Can I put a CT over the whole 3-core cable?
Usually not for a conventional single-circuit measurement. Live and neutral currents normally cancel inside the sensor when both conductors are in the window. For reliable energy metering an electrician must identify an appropriate single conductor and follow the meter maker’s instructions.
Can a 100 A CT measure 10 A?
Often yes within the matched system’s specified operating range, but low-load relative accuracy depends on the sensor and electronics. A 100 A number normally describes a rating, not an assertion that measurements below 100 A are unusable or equally accurate. Read the product’s low-current specification.
Does a CT work on solar PV or a home battery?
A standard AC CT can measure suitable AC grid and inverter lines when paired with a compatible meter. It cannot directly measure steady DC current on solar panels or a DC battery. For net grid export, placement, polarity and voltage measurement matter.
Do I need three clamps in a three-phase home?
A normal phase-resolved three-phase monitoring arrangement uses a CT on each phase and matching voltage references. The exact implementation depends on system topology and meter specifications. One CT cannot usually substitute for three phase currents.
What is the difference between a 50 mA and a 333 mV CT?
The first specifies a nominal secondary current and needs a designed burden/load. The second specifies a secondary voltage and normally already has a burden built in. They need different compatible inputs and must not be substituted on the basis of primary current alone.
Can a CT replace my electricity supplier’s meter?
No. A consumer CT monitor is useful for analysis and automation, but it does not automatically meet the legal, accuracy or sealing requirements of revenue metering. Treat the official meter and its billing readings as the reference for charges.
Official documentation and further reading
- OpenEnergyMonitor — Current Transformer Installation: single-conductor rule, orientation and open-secondary warnings.
- OpenEnergyMonitor — Interfacing a CT with Arduino: burden and analogue-measurement principles.
- OpenEnergyMonitor — emonPi3/emonTx6 Installation: supported voltage-output CT ranges and installation notes.
- Shelly Support — Third-party CT compatibility: manufacturer recommendation on approved sensors.
- Shelly — Pro 3EM Specifications: channel ratings, supported CT and metering performance.
- Emporia — How CT Sensors Work: 200 A main CTs, 50 A branch CTs and flexible alternatives.
- Emporia — CT Sensor Installation: phase and port rules, dimensions and supported configurations.
- Emporia — CT Accuracy: accuracy expectations and limitations.
- Home Assistant — Energy FAQ: power versus energy and Energy dashboard entity requirements.
- Home Assistant — Home Energy Management: integrated energy monitoring, solar and storage.
Editorial note: This is a specification-based selection guide, not a hands-on comparison or independently calibrated performance test. Source documents were reviewed in October 2026. Product revisions, approvals, specifications and firmware can change; confirm the exact part number and local installation requirements before purchase. No retailer prices or real-world test results have been invented.
