Dynamic load balancing CT clamps

How current sensing lets an EV charger respect a site import limit, and the installation details that decide whether it works.

Dynamic load balancing reduces EV charging when other loads leave too little capacity at the property supply. When household demand falls, the charger can increase again within the configured limit.

A current transformer, or CT, is a common way to obtain the live measurement. A split-core CT closes around one insulated conductor and produces a smaller signal related to the current in that conductor. The charger or energy controller interprets that signal and adjusts charging.

The clamp is only the sensor. Safe load control depends on its position, direction, phase mapping, communications, settings and failure behaviour.

Dynamic rather than fixed limiting

A fixed charger limit assumes a set amount of capacity is always available. It can be safe but unnecessarily slow when the rest of the home is quiet.

Dynamic control uses a site measurement. If cooking, heating or another large load starts, the controller reduces EV current. When that load stops, it releases more capacity to the car. A vehicle may pause if the available current falls below what the charging system supports, then resume when enough capacity returns.

The commissioning limit must come from the actual supply arrangement and design assessment. It should not be guessed from a label, an app default or the charger’s maximum rating. The cut-out fuse, service capacity, looped supply, phase arrangement and any DNO condition can all be relevant.

Where the CT is placed

For whole-site import control, the sensor must measure at a point that includes the loads the charger is supposed to coordinate with. On a single-phase supply it normally encloses one current-carrying conductor, not a complete cable containing both line and neutral. If both opposing conductors pass through the aperture, their magnetic effects largely cancel.

On a three-phase supply, the controller generally needs the correct measurement and phase reference for each phase it manages. A single total figure can hide an overloaded phase even where the combined three-phase load looks acceptable.

Solar PV, stationary batteries, secondary consumer units and more than one meter can make placement less obvious. A CT on the wrong side of a branch may omit a load. Reversed orientation or incorrect phase association may turn import into export or make the control respond to the wrong conductor.

Metering and intake areas may contain sealed equipment and exposed hazards. The clamp and its wiring should be installed only at a permitted, safely accessible point. Do not disturb the electricity meter, cut-out or DNO seals to make it fit.

Current is not the whole measurement

A basic CT senses an AC waveform. It does not by itself measure voltage, power factor or direction. The connected controller may combine the CT signal with a voltage reference and calibration to estimate active power and distinguish import from export.

This matters at low loads and where inverters are operating. A value shown by a charger can be perfectly adequate for control without being a billing-grade energy measurement. It should not be treated as a replacement for the electricity meter.

Wired, wireless and meter-based links

The measurement may reach the charger through a dedicated cable, a proprietary wireless transmitter, a local network or a separate energy meter. Each approach can work, provided its range, update rate and loss-of-communication behaviour suit the job.

The design should answer:

how quickly the charger responds to a sudden load
what happens if the sensor value freezes or disappears
whether charging stops, falls to a safe fallback or continues at its last limit
how a low or zero-current period affects a self-powered transmitter
how the system reports a reversed or implausible reading
whether a cloud service is needed for a local safety-related limit

For protection of a supply constraint, a safe local response is preferable to dependence on a remote service that may be unavailable.

Import limiting, solar charging and export limiting

Similar sensors can support three different controls:

import load management keeps site demand within an agreed or designed limit
solar-surplus charging adjusts EV demand around net export
G100 export limitation keeps export from generation within a DNO-agreed limit

They are not interchangeable. A charger’s import control is not a G100 customer limitation scheme. Solar-divert logic also needs to distinguish genuine surplus from discharge by a home battery.

Where several devices respond to the same grid measurement, their priorities and response rates need coordination. Otherwise a battery and charger can chase one another, oscillate around a target or import energy to replace energy that another controller has just exported.

Commissioning tests

The installer should prove the control with real changes in site load rather than accepting a plausible idle reading. Tests should cover:

CT direction and the sign of import and export
correct phase association
a load upstream and downstream of relevant branches
charger reduction as the site approaches its limit
recovery when capacity becomes available
response to sensor or communications failure
simultaneous PV or battery operation where present
the configured limit recorded in handover information

If the charger is later replaced, the supply fuse changes or a heat pump, battery or second chargepoint is added, repeat the maximum-demand and control assessment. The original limit may no longer represent the site.