Quick Summary (TL;DR)
For EV charger dynamic load balancing, choose the current transformerAliExpress price (CT) or energy meterAliExpress price approved for the exact charger, firmware and energy-management function — not merely a clamp labelled 100 A. The correct selection must match sensor output type, turns ratio, phase count, measurement location, conductor diameter, cable run and commissioning settings. Some chargers accept a proprietary CT directly; others require a compatible external meter that communicates digitally.
Size a CT to measure the full expected current in the conductor it monitors, with suitable headroom and a sufficiently large window. The supply limit and charging-circuit limit are separate. A 32 A charger connected to a home with an 80 A service normally needs to monitor the whole-site current, not just the 32 A charger branch; a sensor selected solely for charger current could therefore be unsuitable. A larger rated CT is not automatically more accurate at the low currents that matter for solar charging.
Three-phase systems generally need per-phase measurements, not one net kW number. The grid sensor must be in the approved measurement position where all relevant loads and generation are visible, and the installer must confirm polarity, phase assignment, response and communication-loss behaviour. Never treat a Home Assistant automation as a substitute for certified EVSE protection and the manufacturer-supported load-management path.
What a CT clamp actually does for an EV charger
A CT is a sensor that measures alternating current flowing through a conductor without needing to carry the full load current through the monitoring electronics. Its output is a small current or voltage proportional to the measured current. In an approved EV charging system, a CT or a meter incorporating CTs reports what the site is drawing so that the charger can adjust the current it offers to the vehicle. The precise arrangement varies across equipment.
Imagine a house with an EV chargepoint, cooker, heat pump and immersion heater sharing one supply. A chargepoint set to a constant 32 A has no knowledge of the other loads. Dynamic load management instead considers the remaining permitted headroom at the relevant supply or sub-board. When household demand rises, the chargepoint can reduce its requested charging current or temporarily suspend charging in a supported way.
The CT does not independently interrupt dangerous currents, replace an MCB/RCBO or prove that a circuit is correctly installed. It is an input to a controller with defined limits, commissioning checks and fault behaviour. Protection design, supply capacity, earthing arrangements and legal notification remain matters for the appropriately qualified installer.
| Component | What it knows | What it does not establish |
|---|---|---|
| Grid/service CT | Current flowing through the monitored service conductor; direction if the meter supports it | Actual power or energy on its own without suitable voltage reference, power-factor measurement or calculation |
| External energy meterAliExpress price | Depending on model, per-phase current, voltage, real power, import/export and energy | Compatibility with the charger unless its protocol and version are supported |
| Charger controller | Charging current limit and available signals from its approved sensors | Safe use of an unsuitable supply cable or unapproved sensor |
| Home Assistant | Useful visibility, trends and slower tariff/solar logic when supported | An independently certified real-time overload-protection path |
First identify which current you need to measure
This is the most important distinction when choosing the CT rating. The measured conductor is not necessarily the charger feeder. For dynamic EV charger load balancing, the controller generally needs a view of the entire supply section that must not be overloaded. For a charger-only energy display, it may instead measure the charger branch. For solar surplus operation, the meter often needs net import/export at the grid connection. One sensor position may serve several functions, but only if the equipment specifically supports it.
| Use case | Typical measurement boundary | Key installation question |
|---|---|---|
| Whole-property import limiting | Main supply or approved meter position covering all relevant incoming and outgoing currents | Does this position capture the EV, other house loads, and any parallel supply paths? |
| Sub-board or shared garage limit | Feed to the constrained sub-board | Is the other garage load captured as well as the EV? |
| Charger consumption metering | Dedicated EVSE branch or charger’s own certified meter | Is the reading AC input to EVSE, charger output or a session estimate? |
| Solar import/export tracking | Grid point of connection, sometimes with additional PV/battery monitoring | Can the system distinguish export from battery discharge and phase imbalance? |
| Multiple charger load sharing | Approved shared-meter architecture plus compatible controllers | Will all chargers use one coordinated site limit or fight each other? |
A grid CT placed downstream of a junction or in only one of several parallel feeders may miss some of the site load. Conversely, a CT installed exclusively on the chargepoint cable cannot tell the charger that the house has just turned on two electric showers. Have the installer establish a simple single-line diagram showing the grid connection, all boards, solar/battery tie-in, meter, CTs and chargers before ordering parts.
CT compatibility: why the amperage printed on the clamp is insufficient
A 100 A label describes a measurement rating; it does not define the sensor interface. Current-output CTs, voltage-output CTs with an internal burden, and complete digital meters are different technologies. A charger expecting a 100 A : 40 mA current-output CT will not necessarily read a 100 A : 50 mA or 100 A : 1 V sensor correctly, even when every item carries a 100 A label. Connectors and conductor diameter also vary.
Check the complete supported accessory list for your exact charger and hardware revision. This is especially important if you plan to use more than basic load balancing: an accessory supported for one feature may be excluded from another, such as solar charging.
| Charger ecosystem | Documented approach | Practical buying implication |
|---|---|---|
| myenergi zappi | myenergi specifies its CT characteristics, grid-CT rules and approved wired/harvi configurations; the documented 100 A CT has a 2500:1 ratio (100 A primary produces 40 mA secondary). | Use the matching approved myenergi CT and connection architecture. Verify three-phase and harvi limits rather than buying an arbitrary 100 A clamp. |
| Ohme Home Pro / ePod | Ohme states these chargers come with a CT for household load balancing; the commissioning threshold is configured by the installer. | Check whether the included CT and permitted cable routing suit the actual service before buying an extra sensor. |
| Tesla Gen 3 Wall Connector | Tesla UK describes Dynamic Power Management using a Tesla-approved power meterAliExpress price, supplied separately, rather than an interchangeable standalone CT. | Budget for the complete approved meter, CT and communications path; confirm the regional configuration. |
| Wallbox | Wallbox lists distinct compatible power meters and CT-based accessories: N1 CT 80 A/100 A, EM330 three-phase versions and others. Its 100 A Temco clamp is listed as not compatible with Solar Charging. | Confirm charger model, desired function and the exact meter variant; matching the amperage alone is not enough. |
These examples are based on official manufacturer documentation available at the time of writing: myenergi CT specification, Ohme CT guidance, Tesla UK power management and Wallbox meter catalogue. Product revisions, regions, software support and accessories can change; verify before purchase.
How to size the CT: rated current, turns ratio and headroom
The CT primary rating should cover the largest plausible current in the conductor it monitors, including the commissioning-defined supply limit and any relevant export scenario. For example, a house may have a nominal 80 A single-phase supply, while the fitted wallbox is rated to draw only 32 A. A CT surrounding the main incoming conductor still needs an appropriate rating for total service current. However, an 80 A-rated or 100 A-rated CT is not automatically compatible with that charger — the maker decides the sensing range and calibration.
Do not confuse the CT primary rating with the main fuse. A CT rated for a higher primary current does not authorise the installer to set the charger above the service or circuit limit. Similarly, do not choose an unnecessarily high-ratio sensor to “future-proof” a small installation unless the metering system explicitly supports that ratio and retains adequate resolution at ordinary household currents.
| Nominal sensor ratio | Secondary output at 100 A primary | What it demonstrates |
|---|---|---|
| 100 A : 50 mA | 50 mA | Current-output CT: 2,000:1 turns ratio |
| 100 A : 40 mA | 40 mA | Current-output CT: 2,500:1 turns ratio; used by the referenced myenergi CT |
| 100 A : 1 V | 1 V | Voltage-output CT; not interchangeable with current-input electronics without explicit support |
| 100 A : 333 mV | 333 mV | Another voltage-output system; a connector match does not establish electrical compatibility |
| 5 A secondary industrial CT | 5 A at its specified primary rating | Metering-class input for a suitable external meter, not for a low-level charger CT port unless expressly designed for it |
Worked example — avoiding a ratio mismatch. Suppose a controller expects a 100 A : 40 mA transformer, but an otherwise physically identical 100 A : 50 mA unit is fitted. At a real 40 A primary current, the expected secondary is 16 mA, while the substitute produces 20 mA. A controller that interprets this using its fixed conversion could overstate current by 25%. The reverse substitution could understate measured current. Either case invalidates reliable load limiting. This is a simplified linear-ratio demonstration, not permission to use mismatched parts.
Also confirm the CT’s operating frequency, accuracy and permitted current range. Residential EV chargers commonly run on 50 Hz mains in the UK and Europe, but that does not remove the need for model-specific approval. An accuracy figure may apply only over a defined range and specified test conditions. For details of measuring CT behaviour outside EV charging, see our general CT selection guide.
A practical headroom calculation — and where it can go wrong
Consider a single-phase supply with an installer-verified site import limit of 60 A, a 32 A chargepoint circuit limit and 38 A of non-EV household demand. Ignoring extra design margins for this illustrative calculation, theoretical EV headroom is 60 − 38 = 22 A. The allowed EV current is the lower of 22 A and the separately commissioned 32 A charger maximum: 22 A, equivalent to roughly 5.1 kW at 230 V. If household demand rises to 55 A, only 5 A would remain, typically below the approximately 6 A AC charging minimum; a supported controller should then reduce, pause or manage charging according to its design.
The input to this calculation must be the non-EV site current. A live CT measurement at the grid point while the vehicle is already charging contains the EV current too. A controller using total-site current must account for its own present EV contribution correctly; naïvely subtracting the entire site reading from the limit as if it were only household load will give the wrong headroom. Mature EVSE load-management firmware handles this as part of its control logic. The arithmetic here explains the constraint, not a replacement control algorithm.
Real installations also require approved protective-device coordination, phase-specific limits, measurement latency and a margin appropriate to the installer’s design. A site may need a lower limit for the service headroom even if the supply fuse has a higher nominal rating. Do not change these settings to chase faster charging without professional review.
CT window size: will the clamp actually fit?
A common reason for ordering the wrong CT is measuring only the cable’s conductor cross-sectional area and forgetting the outside diameter of its insulation and available space. The sensor’s aperture or window needs to close fully around the intended insulated conductor without forcing its jaws against adjacent hardware or cables. The installer may need a different approved CT housing, a compatible remote meter or a different authorised metering position.
- Confirm the external conductor diameter. A label such as 25 mm² or 35 mm² describes nominal copper cross-sectional area, not the cable’s outside diameter.
- Check physical access. Main meter tails and board entries may be tight, sealed, inaccessible or subject to utility restrictions. A dimensionally fitting CT may still be impossible to install legally or safely.
- Check closure and retention. A split-core CT must latch properly and remain mechanically stable under normal maintenance conditions.
- Use approved sensor locations. The best electrical measurement point is not always accessible; discuss alternate approved meter architectures with the installer rather than moving the clamp arbitrarily.
- Allow for the environment. Temperature, moisture, outdoor routing, electromagnetic interference and enclosure clearance matter, especially in garages and exterior meter cupboards.
CT clamps are not simply clipped around the whole multi-core cable. If live and neutral currents pass through the same aperture in opposite directions, their magnetic fields tend to cancel, leading to an insignificant or misleading net reading. An installer should follow the manufacturer’s exact conductor-selection and orientation diagrams. Never attempt to pull apart or rearrange live meter tails to gain access.
Single-phase versus three-phase CT requirements
On a single-phase supply, a typical approved grid-sensing architecture may use one sensor on the correct supply conductor. On a three-phase supply, a phase-aware energy-management system commonly needs one suitably assigned CT per phase or a compatible three-phase meter with individual per-phase measurements. This matters because the service limit is often expressed in amperes per phase, not just aggregate kilowatts.
A three-phase charger cannot assume that spare capacity on L2 compensates for an overloaded L1. Consider a nominal per-phase site limit of 25 A and unrelated household currents of 17 A on L1, 9 A on L2 and 20 A on L3. The mathematical headroom is 8 A, 16 A and 5 A respectively. For balanced three-phase charging, the smallest phase headroom (5 A) limits what the controller can offer on all active phases. Because 5 A is below the usual 6 A minimum AC charging current, a controller may need to pause, defer or move to a supported one-phase mode. It cannot simply sum the three headrooms (29 A) and treat that as safe charging current.
Some systems support netting of solar export across phases for energy accounting, but that does not mean they may exceed current limits on a heavily loaded phase. Distinguish billing netting, total real power, CT phase assignment and protective constraints. If phase switching is offered, confirm the charger, vehicle, contactor logic and electrical installation explicitly support it.
| Three-phase issue | Typical misleading symptom | Correct commissioning question |
|---|---|---|
| Only one CT on a three-phase site | Aggregate household consumption appears far too low | Does the product require all phases to be monitored? |
| CTs assigned to wrong phases | App shows odd negative/positive readings or inconsistent power factor | Are CT phase numbers aligned with voltage references and firmware mapping? |
| Site power netted but phase current omitted | Overall import looks modest while one phase approaches its limit | Does load balancing enforce a limit independently on every required phase? |
| Single-phase EV on three-phase home | The chosen charge phase reaches its limit first | Which phase feeds the charger, and how is that monitored? |
| Generation on a different phase | Solar dashboard suggests spare power that a constrained charger phase cannot safely use | Are billing/netting assumptions being confused with phase current limits? |
For additional background on grid-phase measurements, read Single-Phase vs Three-Phase Energy Monitoring for Home Assistant.
Direction, polarity and correct phase association
A CT’s orientation tells a compatible system which direction to regard as positive; this is important when the same measurement point sees both grid import and solar export. It is tempting to memorise a universal arrow rule, but there is no safe universal orientation instruction across all charger brands and CT types. A manufacturer may define the arrow relative to grid import, the consumer unit or the monitored source. If a neutral conductor is approved, conventions may differ from the live conductor arrangement. Follow the drawing for the installed model, not a photograph of someone else’s board.
One verified example: myenergi documents its grid CT arrow as pointing in the direction of import for a live conductor, while Tesla documents an energy-meter CT orientation relative to its power source. Phase-pairing can be as important as direction: a CT on L2 assigned to an L1 voltage reference can produce erroneous real-power or direction calculations even when a current-only reading looks plausible. This is why the commissioning process must test the installation against known conditions.
Owners can help validate the results without opening equipment: ask the installer to compare the charger or meter display with an independent approved reading while a known household load is switched on normally, and again while known solar generation is exporting. A visibly inconsistent sign, improbable current, or unexplained phase imbalance is reason to stop trusting automatic limiting until the installer resolves it.
CT cable length, extensions and communications
A supplier may ship a CT with several metres of attached lead, while the charger is 10–30 metres away in a garage or carport. Whether extension is allowed depends on the CT output type, conductor pair, electrical noise, maximum distance and termination rules. Current-output CTs, voltage-output CTs and digital meters impose different requirements. Do not assume ordinary two-core cable or an arbitrary RJ connector will work.
For example, myenergi lists a 5 m lead for its standard CT and specifies twisted-pair extension methods in its guidance. Tesla’s dynamic management relies on a supported meter-to-Wall-Connector communications link; the architecture is not the same as simply extending a CT wire. Where the site topology makes a direct CT run awkward, a compatible meter with an approved wired protocol may be the better design.
| Interface option | Potential advantage | Questions to settle before purchase |
|---|---|---|
| CT directly into charger | Small component count and manufacturer-tested integration | Maximum length, CT ratio, approved connector, routing and noise immunity |
| CT into dedicated energy meterAliExpress price | Per-phase voltage/power plus a digital connection | Supported meter make/model, firmware, protocol, termination and power supply |
| Brand-specific wireless bridge | Avoids a difficult signal run in supported layouts | Radio range, power, update rate, missing-signal behaviour and current-limit constraints |
| Cloud or home-automation telemetry | Good for dashboards and tariff decisions | Whether safety load limiting remains local and functional when network or cloud fails |
Treat the secondary connection of a current-output CT as an electrical safety matter: depending on CT design and protection, an open secondary while the primary conductor carries current can produce hazardous voltage. Never unplug, extend, probe or reterminate a CT on an energised installation except under a manufacturer-approved professional procedure. This warning is particularly important for unfamiliar third-party or industrial 5 A secondary CTs.
The difference between load balancing and solar-surplus charging
Load balancing protects a defined supply or branch capacity by reducing discretionary EV demand. Solar-surplus charging attempts to absorb generation that would otherwise be exported. They can share measurement hardware but solve different problems. A system may support one without the other; Wallbox’s published accessory list explicitly shows that its Temco 100 A clamp is not compatible with the Solar Charging feature, despite being a real current sensor.
For solar-aware charging, the meter needs reliable grid import/export information, and the control system needs a policy for when a home battery is charging or discharging. An EV may be drawing energy from stationary storage while the grid meter sits near zero. A grid CT alone usually cannot distinguish that from truly surplus solar generation; additional battery telemetry or manufacturer-supported settings may be necessary. Read our detailed solar surplus EV charging guide for control priorities, and home battery backup and EPS guide for important inverter operating boundaries.
Avoid using an EV charger’s minimum charging power as a proxy for CT sizing. At nominal 230 V, the usual 6 A minimum means about 1.38 kW single-phase, while three active phases near 230 V phase-to-neutral mean about 4.14 kW total. These are useful planning figures, but the current sensor still needs to measure the relevant supply currents — which may be much higher than the charging current or even be negative during export.
Failure behaviour: what happens if the CT cable or meter stops reporting?
A system may be called “smart” yet have very different missing-sensor responses. You need the documented behaviour of your model and current firmware: some chargers reduce to a defined fallback current, others stop solar mode or suspend charging, and still others raise an alarm. Do not assume every charger stops safely at zero current. Ohme, for instance, states that a Home Pro with enabled load balancing can revert to a maximum of 16 A if its CT is disconnected, incorrectly installed or faulty; its documentation specifically reminds installers that sufficient spare supply capacity must remain for that state.
A 16 A fallback is not automatically suitable for every marginal supply. Ask the installer to record the actual fault-state response, whether the site can safely accept it and whether the commissioning configuration implements the manufacturer’s approved protection requirements. If the site relies on a CT-based limit to avoid exceeding a constrained main supply, this is not a minor detail.
- Loss of a CT signal: which alarm, fallback current or charging suspension is specified?
- Loss of meter power: can the charger detect it, and how quickly?
- Loss of data link: is load control independent of internet access, Wi-Fi and Home Assistant?
- Reboot or firmware update: do the site, phase and EVSE limits persist?
- Solar or battery transition: does the system continue to respect the import limit when generation falls?
- Multiple chargepoints: are their combined currents still controlled after one charger disconnects from the communications network?
Tesla UK states that its supported Wall Connector power-management features do not require an internet connection to operate. This should not be generalised to all brands, and it does not imply every optional integration can function without a local network.
Installer handover and acceptance checks
Because the CT is used for current limiting, selecting it is only half the job. The installation should be commissioned by a qualified professional using the approved equipment procedure and with the distribution board made safe as required. The homeowner’s useful role is to insist on observable evidence that the system behaves correctly; no homeowner should have to open a live board or tamper with sealed supply tails to verify the work.
- Record the exact parts: CT model/part number, primary rating, output specification, window size, charger and meter firmware, and any supported bridge.
- Document the boundary: where grid import, sub-board load and EV branch load are measured, including PV and battery tie-ins.
- Confirm the actual limits: service, relevant per-phase values, chargepoint circuit rating, shared-board restrictions and any agreed margin.
- Observe direction and phase values: under normal load, and under inverter export where applicable, readings should make physical sense.
- Observe controlled load changes: with the charger running, a qualified installer verifies that increasing ordinary demand reduces available charging current in the approved manner.
- Confirm insufficient headroom behaviour: a designed test should establish how the charger responds at its minimum current and near the import threshold.
- Check sensor loss response: obtain evidence from the manufacturer-approved commissioning workflow of its documented fallback, without unsafe disconnections.
- Recheck after the system changes: new heat pumps, solar, a second EVSE or a service upgrade can invalidate earlier assumptions.
A simple handover sheet showing phase limits, sensor positions and commissioning results is far more valuable than a photograph of a CT clipped to a cable. If the homeowner changes charger settings, it should be limited to the user-adjustable controls the installer has expressly identified as safe.
Common CT clamp and load-management problems
| Symptom | Possible explanation | What to check without live-board work |
|---|---|---|
| Charger always runs at a low current | Site limit set conservatively; other load is high; CT is missing; fallback mode is active | Charger alarms, commissioning record, site power history; refer any wiring or limit change to installer |
| Charging remains at full output when cooker switches on | Grid CT may miss part of supply; feature may be disabled; incorrect meter binding | Is the proper load-management mode active? Does the site reading change as expected? |
| Import appears as export | CT orientation or phase reference mismatch | Compare displayed direction with utility meter and verified generation state; installer to correct |
| One phase shows implausible power | Wrong CT assignment, reversed CT, or phase reference mismatch | Compare per-phase reading patterns under normal known appliance use |
| Solar-only charging drains home battery | Grid CT cannot distinguish battery output from solar; competing inverter controls | Inspect battery SoC and discharge trace; use supported battery priority configuration |
| Meter intermittently disappears | Out-of-spec cable, poor termination, bridge coverage, device power or firmware | Review fault history, link status and the vendor’s installation limits |
| Home Assistant power chart looks fine but supply trips | Energy dashboard measures net kW or has delayed samples; site limit is per phase | Ask installer to inspect native metering/protection logs and phase limits |
| CT fits physically but values are wrong | Turns ratio/output mismatch, wrong setting or unsupported accessory | Check model number against official compatibility and approved sensor data |
What to buy: three common installation scenarios
Scenario 1 — one 7.4 kW charger on a single-phase house. If the charger provides supported dynamic load balancing, begin with its own manufacturer-approved CT or compatible meter kit. Confirm that the main service-current rating and CT aperture suit the intended service conductor, not merely the 32 A chargepoint circuit. An included CT can remove the need for another purchase, but not the installation and commissioning effort.
Scenario 2 — an 11 kW three-phase charger. Find out whether the manufacturer specifies three individual CTs, a complete three-phase meter or a proprietary energy-management unit. Confirm per-phase limits, phase alignment and whether data are available locally. A single large 100 A clamp is not a substitute for measuring all three phases.
Scenario 3 — solar, a stationary battery and two chargepoints. Treat this as a system architecture rather than a CT shopping exercise. You may need an approved site meter, battery/inverter data, a multi-charger load-sharing function and a clear owner for the solar-diversion logic. Check that all chargepoints and the EMS are supported together and agree on behaviour during cloud, local-network or meter faults. Installing two independent CT-equipped chargers is not equivalent to coordinated site-wide load management.
| Purchase line | Why it matters | Avoid this assumption |
|---|---|---|
| CT or complete meter kit | The charger may require exact accessory part number and special communications | “Any 100 A clamp will work.” |
| Installation labour and commissioning | Correct measurement location and fault tests may dominate effort | “The sensor clips on, so it is a DIY installation.” |
| Bridge/RS485/communication accessories | Some garages need extra supported cable or interface modules | “If it is connected to Wi-Fi, it can do protection over the cloud.” |
| Phase-specific monitoring | Per-phase readings are important for capacity and safety | “Three-phase is just single-phase multiplied by three.” |
| Later expansion | Second EVSE, PV and battery may need additional approved hardware | “A charger’s CT automatically controls every other device.” |
For a decision between CT sensors and alternative meter types, see EV Charger Dynamic Load Balancing: Which Meter and Controller?. For measured charging cost, which requires an appropriate energy boundary rather than just current limiting, read EV Charger kWh vs Utility Meter kWh.
Can Home Assistant use the same CT data?
Often yes, but there are two separate questions. First, does the vendor provide supported current, power or energy telemetry that Home Assistant can read? Second, does the local charger continue to enforce its capacity constraints without Home Assistant? The second requirement is non-negotiable for a safe design. Home Assistant is well suited to showing current, electricity costs and solar status, or requesting discretionary charging when tariff conditions are favourable. It must not be the only element preventing a supply overload.
A CT by itself is a sensor, not a standard network device. Data are normally exposed through the charger, its approved meter, a documented Modbus interface or another supported integration. A generic ESP32 current-monitoring project is excellent for non-critical analysis, but it should not be substituted for a certified, manufacturer-supported EV load-management loop. If the site’s network disconnects, the approved charger subsystem should retain the necessary capacity protection according to its specified design.
Frequently asked questions
Is a 100 A CT suitable for an 80 A house fuse?
Possibly, if the CT is expressly supported by the charger or meter, its measurement range covers the design current, its aperture fits the approved conductor and the installer confirms all other requirements. The rating alone proves none of those points. Also ask whether a communication bridge imposes a lower effective operating limit than the sensor itself.
Can I use a 200 A CT instead of a 100 A CT?
Not by default. Larger primary ratings can use different ratios and may reduce low-current resolution or exceed the controller’s specified input configuration. Select from the approved sensor list, and only change metering ratios where the vendor explicitly supports configuration and calibration.
Do I need three CT clamps for a three-phase charger?
Frequently a three-phase site needs three phase measurements, but the correct hardware may be three discrete CTs, a multi-channel meter or a different manufacturer-approved arrangement. In some setups the chargepoint itself is single-phase even though the house supply is three-phase; this still does not mean the installer may ignore site limits on other phases.
Can one CT monitor live and neutral together?
That is generally inappropriate for measuring the current of a normal single-phase load: currents in the two conductors cancel in the CT aperture. Follow the official metering diagram. Do not attempt any work on supply tails or sealed equipment yourself.
Does the CT need its own power supply?
A passive CT does not normally have an independent mains supply, but an external metering module, gateway or communications bridge often does. Some CT arrangements have particular low-current or energy-harvesting constraints. Assess the complete package rather than just the clamp.
Can a Shelly, ESP32 or Home Assistant sensor replace the charger CT?
Only if the charger manufacturer explicitly supports that exact metering arrangement for its load-management function. Reading equivalent current in a dashboard is not the same as providing an approved, fault-handled safety-control input. Use independent third-party monitors for additional insight if desired, not as an assumed replacement.
Why does my solar charger show negative power in the daytime?
It may correctly be showing exported power, or it may have a reversed CT or wrong phase mapping. Compare against the utility meter and verified inverter state. The meaning of negative values is vendor-specific; have the installer correct erroneous metering before trusting automatic solar or import-limit operation.
How long can the CT cable be?
There is no single universal distance. Cable type, CT technology, extension practices, electrical noise and the manufacturer’s approved limits determine it. For example, myenergi publishes a maximum supported cable length with twisted-pair extension guidance; a charger connected to a separate digital meter follows a different set of rules.
If the CT fails, will charging stop?
Not necessarily. Manufacturer-defined responses differ, and a fallback may be a non-zero current. Ohme specifically documents a 16 A fallback in certain load-balancing failure conditions. The installer should verify that the permitted fallback is safe for the property and that the owner can recognise a metering fault.
Bottom line: select the whole metering system, not the cheapest clamp
The sensible buying order is charger compatibility → measurement boundary → phase count → rated current and ratio → physical aperture → communications → fault behaviour → installer validation. A CT clamp costs little compared with the consequences of an incorrectly limited EV charging installation. The most robust solution is generally the one officially supported by the charger manufacturer and demonstrably tested against the particular property’s supply and connected equipment.
For households adding PV, storage or a second EV, document the measurement and control architecture now. A correctly placed approved grid meter, sensible per-phase limits and one clear load-management authority make future upgrades far easier than purchasing disconnected devices that merely display compatible-looking numbers.
Manufacturer documentation and related reading
- myenergi: CT Clamp Information — turns ratio, primary rating, physical aperture and cable requirements.
- myenergi: Critical Role of Grid CT Sensors — site positioning, load limiting and harvi-specific restriction.
- myenergi: CTs in a Three-Phase System — phase assignment considerations.
- Ohme: CT clamp guidance — manufacturer setup requirements for Home Pro and ePod.
- Ohme: Home Pro installation overview — documented sensor-failure fallback considerations.
- Tesla UK: Wall Connector Power Management — approved meter-based dynamic limiting and internet-independent operation.
- Wallbox: Power meter accessories — N1 CT models, EM330 versions and feature-specific compatibility.
- ESP32.co.uk: How to Choose Current Transformers for a Home Energy Monitor
- ESP32.co.uk: EV Charger Dynamic Load Balancing
- ESP32.co.uk: EV Charger kWh vs Utility Meter kWh
Safety and editorial note: This article is a purchasing and system-design guide, not instructions to work inside distribution boards, remove sealed covers or connect CTs to energised conductors. EV charging installations involve potentially lethal mains voltages and country-specific electrical requirements. All CT placement, wiring, ratio configuration, protective-device selection, load-limit commissioning and any fault-state tests must be carried out by a qualified electrician or approved installer following current manufacturer instructions and applicable regulations. Product support depends on precise model, region, hardware revision and firmware.