A CT clamp (current transformer clamp) is the small device that lets a smart EV wallbox see how much power your house is using in real time. The wallbox throttles or boosts the EV charge accordingly — preventing main-fuse trips, enabling dynamic load balancing, and unlocking solar diversion. Almost every modern UK smart wallbox uses one. Most installs require it.
What a CT clamp actually does
A CT clamp is a non-invasive current sensor that snaps over the live tail going from your incoming meter to the consumer unit (fuse box). It measures the magnetic field around the cable and translates it into an instantaneous current reading — usually in amps. That reading is sent (wired or wireless) to the wallbox, which uses it to:
- Prevent overload — if the house already draws 70A and your fuse is rated 100A, the wallbox limits the EV charge to 30A so the fuse doesn't trip.
- Solar PV diversion — see when solar export is positive (you're sending power to the grid) and route it into the EV instead.
- Dynamic load balancing — share a single circuit between two wallboxes for multi-EV households.
When you need a CT clamp
Most modern smart wallboxes require a CT clamp to be fully functional:
- Required: MyEnergi Zappi (for solar diversion), Ohme Pro models, Easee with Equalizer, Wallbox Pulsar Plus with Power Boost.
- Optional but recommended: Hypervolt Home 3 Pro, Project EV Pro Earth — included internal current monitoring but external CT improves accuracy.
- Not required: basic 7kW wallboxes without solar or load-balancing features.
Specifications
UK domestic CT clamps are typically:
- 100A or 200A rated — match to your incoming supply (most UK homes are 100A; some larger or commercial properties are 200A).
- 16mm or 24mm split-core — the inside aperture, must be wide enough to clip over your supply tail. 24mm covers nearly every UK domestic supply.
- Three-phase variants for properties with three-phase supply — three clamps, one per phase, supplied as a set.
Branded CT clamps
Most wallbox brands ship a CT clamp with the unit (or as a paid add-on). We stock them as direct replacements:
- MyEnergi Harvi — wireless CT for Zappi installs where running cable to the consumer unit is impractical. Battery-powered, 5-year battery life.
- Ohme CT clamp — wired clamp for Ohme Pro Earth. Required for solar diversion mode.
- Easee Equalizer — wired sensor for whole-house load monitoring on Easee installs.
- Universal 100A 24mm — works with most non-branded wallboxes that accept a generic CT input.
Wired vs wireless
Wired CT clamps connect to the wallbox via a thin signal cable, typically 5-15m, run through the loft or under floors during install. Wireless CT clamps (like the MyEnergi Harvi) use a 433MHz radio link — useful when running cable from the consumer unit to the wallbox is impractical or expensive. Wireless adds £80-120 to the bill of materials but can save £200+ in install labour.
Installation — must be by a qualified electrician
CT clamps install around the live tail INSIDE the consumer unit or just before it. This means working live or with the supply isolated at the meter — both regulated activities under BS 7671 18th Edition. Self-installation is unsafe and breaches the Electricity at Work Regulations 1989. Every wallbox install we arrange includes the CT clamp where required, fitted by an OZEV-approved electrician.
Three-phase considerations
If you have three-phase supply, you need three CT clamps (one per phase) supplied as a matched set. Single-phase clamps will not work on three-phase boards. See three-phase charging context.
Common faults
The most common CT-related failure is wrong direction of installation — the clamp has an arrow showing the direction of current flow toward the load. If installed reversed, the wallbox sees negative current and behaves erratically. Always check polarity during commissioning. Replacement clamps are inexpensive (£25-50) and 5-minute swap by a qualified electrician.