Adding an EV charger to a home with a modest supply fuse often raises a simple, urgent question: will the lights stay on? The answer is yes, provided your wallbox uses a CT clamp for dynamic load balancing. A CT clamp (current transformer clamp) clips onto the meter tail inside your consumer unit area, reads your home’s total current draw in real time, and signals the wallbox to reduce EV charging current whenever the rest of the house needs more power. Here is how it works.
What is a CT clamp and what does it measure?
A CT clamp (current transformer clamp) clips onto the live conductor of your meter tail and measures the total current your home is drawing at any moment. It sends that reading to the wallbox, which uses it to decide how much current is safe to divert to EV charging, all without tripping the supply fuse.
The “current transformer” part describes the physics: a toroidal core encircles the cable and induces a small, proportional voltage signal without any direct electrical contact, so there is no cutting and no splicing. CT clamps used with EV chargers are typically rated 100 A or 200 A, with an output signal of 0-333 mV (from a split-core clamp) or via a Rogowski coil for higher accuracy on larger conductors. The signal type must match what the wallbox expects. Compatibility is not universal.
Where the CT clamp sits in the circuit
The CT clamp must be fitted to the live conductor of the meter tail, the cable between the DNO cut-out fuse and your consumer unit. This is the only point in your domestic wiring that carries the full supply current before it is split across circuits. It is the only position where the device can see total house load, not just a single circuit.
Fitting it anywhere else (on a sub-circuit breaker output, for example) would give the wallbox only partial information and the load balancing would fail to protect the supply fuse.
How dynamic load balancing works step by step
Dynamic load balancing works as a continuous feedback loop. The CT clamp samples total house current several times per second. The wallbox firmware compares that reading against the supply fuse rating and adjusts the pilot signal to the car accordingly. If you boil a kettle, the charger backs off within seconds; when the kettle finishes, it ramps back up. Charging is slower during peak household demand but the fuse is never exceeded.
Here is the sequence in plain steps:
- CT clamp measures total current on the meter tail live conductor, continuously and in near real time.
- Signal is sent to the wallbox via a low-voltage signal wire (or wirelessly, via a hub such as the Myenergi harvi).
- Wallbox calculates headroom: supply fuse rating minus current house draw equals the maximum available EV charge current.
- Pilot signal adjusts: the wallbox modulates the control pilot to the vehicle to draw more or less current.
- Loop repeats every few seconds, tracking household demand automatically.
- DNO cut-out fuse rated 60 A (label on diagram)
- Meter tail: live conductor, CT clamp clips here (red highlight, label CT clamp clips here; electrician only)
- Consumer unit splits into two branches:
- Branch 1: House loads, drawing 0-28 A dynamically (label: House loads: up to 28 A)
- Branch 2: EV charger, drawing 0-32 A dynamically (label: EV charger: 0-32 A (adjusted by CT data))
- Dashed signal line: CT clamp → wallbox (label: Real-time current signal)
- Annotation box: Total draw must not exceed 60 A, and the CT clamp enforces this in real time
- Arrow direction: power flows down from fuse; signal flows sideways from CT to wallbox
Why your supply fuse rating matters
Every UK domestic property is protected by a DNO (Distribution Network Operator) cut-out fuse, the sealed fuse at the top of your meter cabinet that you cannot open or replace yourself. Most UK properties have a 60 A, 80 A, or 100 A DNO fuse, and the rating is fixed by the DNO based on the service cable feeding your property.
That rating sets a hard ceiling on everything your home can draw simultaneously.
The 60 A fuse problem and how CT balancing solves it
A 32 A (7.4 kW) EV charger on a 60 A supply leaves only 28 A headroom for the rest of the house. Most evenings that is fine: background domestic demand sits at 5-10 A. But add an electric shower or heat pump and you are at risk of the DNO fuse tripping.
Without CT load balancing, the charger draws a fixed 32 A regardless of what else is running. With CT load balancing, the wallbox automatically backs off. If the house draws 40 A, the charger is throttled to 20 A. The fuse is never exceeded.
The alternative is a DNO fuse upgrade, which typically costs £500-£1,500 or more and can involve multi-week wait times. For most homeowners, CT load balancing is the faster and more cost-effective route.
CT clamp specifications: what to look for
Not all CT clamps are interchangeable. Wallbox compatibility, both the hardware input and the communications protocol, must be verified before purchase. Confirm the following attributes against your wallbox’s datasheet:
| Attribute | What to check | Typical value |
|---|---|---|
| Current rating | Must exceed your DNO supply fuse rating | 100 A (for 60/80 A supply) or 200 A (for 100 A supply) |
| Output signal | Must match the wallbox input spec | 0-333 mV (split-core) or Rogowski coil |
| Comms protocol | Proprietary (brand-specific) or open (Modbus) | Check wallbox datasheet |
| Number of cores | Single-phase: 1 clamp; three-phase: 3 clamps | Most UK homes: single-phase, 1 clamp |
| Cable diameter | Clamp jaw must fit the meter tail conductor | Typically 16-35 mm² for UK domestic tails |
| Installation requirement | Meter-tail location = qualified electrician | DNO notification may apply |
Our EV charging accessories guide covers the broader range of hardware you may need alongside a CT clamp installation.
Which wallboxes support CT clamp load balancing?
CT load balancing is not a universal feature. It must be designed into the wallbox firmware and hardware. Checking compatibility before purchase is essential because retrofitting is not possible on wallboxes that lack the necessary input circuitry.
The following wallboxes have confirmed CT clamp load balancing support as of mid-2026:
- Columns: Wallbox | Max charge current | CT clamp supported | Clamp rating | Output signal | Protocol
- Row 1: Zappi v2 (Myenergi) | 32 A / 7.4 kW | Yes | 100 A | 0-333 mV split-core | Myenergi proprietary (harvi hub)
- Row 2: Ohme Home Pro | 32 A / 7.4 kW | Yes | 100 A | Ohme CT dongle | Ohme cloud + local
- Row 3: Wallbox Pulsar Plus | 32 A / 7.4 kW | Yes | 200 A | 0-333 mV split-core | Power Boost (Modbus-based)
- Row 4: Hypervolt Home 3 | 32 A / 7.4 kW | Yes | 100 A | 0-333 mV split-core | Hypervolt app / local
- Row 5: Generic dumb 7.4 kW charger | 32 A / 7.4 kW | No | N/A | N/A | No load balancing
Key notes: the Zappi transmits CT data wirelessly via a harvi hub rather than a direct cable. The Wallbox Pulsar Plus uses open Modbus for its Power Boost feature. Generic dumb chargers cannot receive CT data at all. Always confirm current compatibility with the manufacturer before purchasing, as firmware updates can change supported configurations.
Installation: why this is electrician-only work
The CT clamp hardware itself is non-invasive. It clips around the conductor without cutting or interrupting the cable. That simplicity is appealing, but it obscures where the clamp must go: the meter tail.
The meter tail is the cable between the DNO’s sealed cut-out fuse and your consumer unit’s main switch. It is DNO infrastructure. In the UK, any work at or near the meter tail, including simply opening the meter cabinet and routing a signal wire to a CT clamp, requires a qualified electrician. Work of this nature must also comply with BS 7671:2018+A2:2022 (IET Wiring Regulations), Section 722, which governs the installation of EV charging circuits, including earthing arrangements and protective devices.
The IET Code of Practice for EV Charging Equipment Installation, 5th edition (2023) is equally clear: EV charging equipment installation must be carried out by a competent person. Depending on your DNO and the nature of the work, a notification or application to the network operator may also be required before or after installation.
The cost of a qualified installation is a known quantity; the cost of an unnotified or unqualified intervention near DNO infrastructure is not. If you are planning an outdoor setup, our guide to outdoor EV charging sockets covers the weatherproofing and wiring standards that apply.
Do you need CT load balancing? A quick decision guide
CT load balancing is not always necessary.
You are likely to need it if: your DNO supply fuse is 60 A or 80 A and you want a full 7.4 kW (32 A) charger, especially if the household runs high-draw appliances (electric shower, heat pump, cooker) in the same hours.
You may not need it if: your supply fuse is already 100 A, or you are installing a lower-rated charger with comfortable headroom.
When in doubt, discuss your supply rating and household load profile with your installing electrician before specifying a wallbox.
Frequently asked questions
What does CT stand for in CT clamp?
CT stands for current transformer. The clamp contains a toroidal transformer core that encircles the conductor and induces a proportional low-level voltage signal without any direct electrical contact with the wire. The output signal, typically 0-333 mV, is then read by the wallbox controller.
Can I fit a CT clamp myself?
No. Although the clamp clips onto the cable without cutting it, it must be placed on the meter tail, which is DNO infrastructure. Any access to or work near the meter tail requires a qualified electrician. The IET Code of Practice for EV Charging Equipment Installation, 5th edition (2023) requires that EV charging installation is carried out by a competent person, and notification obligations to the DNO may apply.
Does every EV charger support CT load balancing?
No. CT load balancing must be built into the wallbox firmware and hardware. Confirmed compatible models as of mid-2026 include the Zappi v2 (Myenergi), Ohme Home Pro, Wallbox Pulsar Plus, and Hypervolt Home 3. Always verify compatibility with the manufacturer before purchasing a wallbox or CT clamp.
What current rating CT clamp do I need?
This depends on your DNO supply fuse. For a 60 A or 80 A supply, a 100 A-rated clamp is typical. For a 100 A supply, a 200 A-rated clamp is often preferred by installers to provide measurement headroom. The clamp’s output signal format (0-333 mV or Rogowski coil) must match the wallbox input. Check the wallbox datasheet before ordering.
Will load balancing make my EV charging much slower?
Only when household demand is high. Overnight, the wallbox runs at full 32 A. Throttling is automatic and proportional, and most drivers never notice it because overnight demand is minimal.
Do I still need a CT clamp if I upgrade my DNO fuse?
Not necessarily. A 100 A fuse may provide sufficient headroom on its own. But DNO upgrades carry cost (£500-£1,500+) and wait times. CT load balancing is often the faster and more cost-effective alternative.
Is load balancing the same as power sharing between two chargers?
No. CT-clamp load balancing manages total house consumption against the supply fuse: it protects a single supply. Power sharing (twin-charger balancing) splits available EV charging capacity between two or more wallboxes. Some wallboxes, including the Zappi within the Myenergi ecosystem, support both functions independently.
Where exactly does the CT clamp go on the meter tail?
The clamp clips over the live conductor of the meter tail, the cable running from the DNO cut-out fuse to the main switch of your consumer unit. It must be positioned before the consumer unit so it measures total supply current. Placement on any downstream cable or sub-circuit would give the wallbox incomplete data and the load balancing would not protect the DNO fuse correctly.
