CT clamps and load balancing for EV chargers

What a CT clamp does

CT stands for current transformer. The clamp is a split-core ring that closes around a live cable without touching the copper. It measures how much current passes through that cable and sends the reading to the charger. Ohme describes it as monitoring “the total load going through a home”. The usual place is the cable leaving your smart meter, which is why Ohme says to attach it “closest to the cable exiting the smart meter”. The arrow on the clamp must point the way electricity flows into the house, and the clamp must be closed and locked.

A charger uses that one reading for two jobs: load balancing and solar diverting.

Load balancing and your main fuse

A 7.4kW charger can draw about 32A for hours. Your home is protected by a main fuse (the cut-out), commonly 60A, 80A or 100A, and the oven, shower and tumble dryer share it. Load balancing watches the total and lowers the charger’s output when the house needs the power, then raises it again. Ohme says it “will temporarily reduce the power of the EV charger to prevent a loss of power”. Pod’s Solo has the same idea under the name Auto Power Balancing: its status light flashes yellow when the limit is reached and returns to green when demand drops.

This matters for the network operator too. The Energy Networks Association says an installer can connect first and notify the operator within 28 days where the maximum demand, including the new charger, is 60A or less per phase. Above that the installer can still use that route if the charger has a fail-safe load-limiting scheme that keeps total demand at 60A or less. Where new demand would reach 60A to 100A, or the supply is looped, the installer must apply first. The operator answers within 10 working days.

Balancing can sometimes avoid a fuse upgrade. Upgrades are usually free: UK Power Networks upgrades 30A or 60A supplies to 80A or 100A at no charge, though Citizens Advice found one £200 assessment fee, and 22% of people with a low-carbon technology needed one. A limiter is not a substitute where the supply is unsafe. Your installer measures the demand and decides.

Solar diverting

For solar the clamp must see both import and export at the meter. Ohme requires one to charge from solar, and without a working reading Solar Boost and Solar Only are unavailable. Place it on the live cable as close to the meter box as possible. The myenergi zappi uses a grid clamp, and its modes (Eco, Eco+, Fast) rely on it. Easee needs the Equalizer, and Andersen’s standard install includes a solar integration package that uses a solar clamp. Hypervolt’s supplied clamp must sit at the supply origin to detect export, and a second one protects a home battery.

One trade-off: during Octopus-managed windows on Intelligent Octopus Go, Hypervolt runs in Boost and its solar modes are unavailable. See our solar charging guide before choosing both.

Which chargers include a clamp

Octopus Charge is not in this table. Octopus does not document load balancing for it, and its Eco mode says nothing about how export is measured.

Fitting and common faults

The clamp wires join the charger’s data cable, and Ohme says the installer checks the reading in Installer Mode; if usage is too low, switching on a kettle helps. The faults makers warn about are installation errors: a clamp facing the wrong way (myenergi and Ohme both stress direction), left open, or moved. Pod notes that a smart meter fitted afterwards can disturb its Energy Clamp, which can trigger the flashing red safety mode. Ohme adds that a missing, misfitted or faulty clamp can cap its charger at 16A, about 3.7 to 3.8kW, and suggests checking that the clamp reading rises when the home uses power. If you have your meter swapped, tell your installer.