CT Clamp Load Management for EV Chargepoints: Preventing Main Fuse Trips

Quick Answer: A CT clamp (current transformer) fitted to the main incomer monitors real-time household load and signals the EV chargepoint to reduce its charge current when the total demand approaches the main fuse limit. This prevents the main fuse (typically 80A or 100A) from tripping when the EV chargepoint and high-demand appliances run simultaneously. CT clamp load management is a feature of smart chargepoints and is set up during commissioning; it requires no additional hardware beyond the CT clamp sensor and a signal cable to the chargepoint.

Summary

A domestic supply rated at 80A or 100A must serve all loads simultaneously: cooking, heating, lighting, and now an EV chargepoint drawing 32A. Without load management, a customer running the oven, electric shower, and EV charger at the same time risks tripping the main fuse.

CT clamp load management solves this by making the chargepoint the flexible load. The CT clamp sensor clamps onto the main incomer live conductor and sends a real-time current reading to the chargepoint. The chargepoint uses this to calculate the remaining headroom and adjusts its charge current accordingly — dropping from 32A down to 6A (the IEC 62196 minimum) if required. When the other loads reduce, charging ramps back up automatically.

For electricians, installing a CT clamp load management system involves fitting the sensor, running a signal cable, and configuring the chargepoint via its app or commissioning interface. This article covers the installation, commissioning, and common pitfalls.

Key Facts

• CT clamp (current transformer) — a sensor that clips around a single conductor; outputs a small current proportional to the measured current without breaking the circuit
• Main incomer — the live conductor feeding the consumer unit from the supply cut-out; the correct measurement point for whole-house load
• Headroom — available supply capacity: main fuse rating minus current total demand; the chargepoint charges at up to this headroom (capped at its rated 32A)
• Minimum charge current — IEC 62196 and BS 7671 Chapter 722 require EV chargepoints to operate between 6A and 32A; if headroom falls below 6A, the chargepoint pauses charging rather than go below minimum
• Pause and resume — when headroom drops below 6A, the chargepoint pauses; when headroom recovers above 6A, charging resumes automatically
• Single-phase CT — for single-phase supplies, one CT clamp on the live conductor is sufficient
• Three-phase CT — for three-phase supplies, one CT per phase (three CTs total) is required
• Signal cable — the CT output connects to the chargepoint via a signal cable; typically 2-core twisted pair; run separately from power cables
• Chargepoint compatibility — not all chargepoints support CT load management; confirm with the manufacturer before specifying; most OZEV-approved smart chargepoints include it
• Multiple chargepoints — where two chargepoints share a supply, load management can balance the two chargepoints against each other and the main fuse simultaneously
• Myenergi Zappi / Ohme / Wallbox — popular chargepoints with built-in CT clamp capability
• Dynamic load management — the term for CT-clamp-based real-time current adjustment (vs static scheduling which only considers time-of-use tariffs)

Quick Reference Table: CT Clamp Load Management Parameters

Detailed Guidance

Why Load Management Is Often Necessary

The IET Guidance Note 7 (EV charging) and OZEV guidance both note that many domestic supplies, particularly those in older houses on suburban ring mains, have cut-outs rated at 60A or 80A — not the 100A that is sometimes assumed. A 32A chargepoint represents 40–53% of a 60–80A supply all by itself.

Even at 100A, the simultaneous combination of:

• 32A EV chargepoint
• 30A electric shower
• 13A cooker (at 50% diversity)
• 10A general lighting and sockets

...approaches 85A — dangerously close to the 100A fuse. In winter evenings, when these loads coincide, trips become likely without load management.

The BS 7671 Chapter 722.312.2 assessment for EV chargepoints requires the installer to calculate whether the supply can accommodate the chargepoint without overloading. Where it cannot without risk, load management (or a supply upgrade) is the solution.

CT Clamp Installation

Step 1: Identify the main incomer The CT clamp goes on the live conductor feeding the consumer unit — on the load side of the main supply cut-out fuse, before the main switch of the consumer unit. Do not clamp onto the neutral conductor.

For split-load consumer units, the measurement must be on the incoming live before the split.

Step 2: Fit the CT clamp CT clamps are hinged or split-core sensors that clip around the conductor without cutting it. Ensure:

• The conductor fits within the CT aperture (most domestic CT clamps fit 25mm² or smaller conductors)
• The arrow or marking on the CT is oriented in the direction of current flow (supply to load)
• The CT is securely closed and locked; a partial clip gives false readings
• The CT lead exits the enclosure cleanly; use a suitable gland if the consumer unit is metalclad

Step 3: Run the signal cable Run 2-core signal cable from the CT clamp to the chargepoint. Segregate from power cables where possible (run in a separate conduit or maintain separation). For wall or surface-run installations, signal cables can be surface-clipped. Maximum run is typically 10–15m; check the chargepoint manufacturer's maximum CT cable length.

Step 4: Connect to the chargepoint The signal cable connects to a dedicated CT input on the chargepoint (terminal block; polarity must be correct — check the manual). Some chargepoints accept the CT signal directly; others require an intermediate module (e.g., Zappi's CT clamp connects directly to the unit's CT terminals).

Step 5: Commission load management via the app Log into the chargepoint app and configure:

• Main fuse rating (enter the correct DNO cut-out rating, not an assumed value)
• Minimum guaranteed charge current (some chargepoints allow setting a floor above 6A if the customer always wants at least a minimum rate)
• Load management mode: "eco" (prioritise home energy balance) vs "fast" (maximise charge rate when headroom allows)

Step 6: Test With the chargepoint running, turn on the electric shower or other high-load appliance and confirm the chargepoint reduces its output. Monitor via the app. Most chargepoints show real-time current draw from CT and chargepoint separately.

Three-Phase Installations

For a three-phase supply (most commercial premises; some larger domestic properties), the load management requires three CT clamps — one per phase. The chargepoint must support three-phase CT input. Configure the main fuse rating per phase (typically 80A or 100A per phase for commercial 3-phase).

The chargepoint monitors the most loaded phase and reduces charge current to protect that phase. On three-phase chargepoints, the current is distributed across all three phases (approximately 10.7A per phase at 32A total output). Load management ensures no phase exceeds its capacity.

Multiple Chargepoints and Load Balancing

Where two or more chargepoints are installed on the same site, dynamic load balancing shares the available supply capacity between them:

• Peer-to-peer load balancing: chargepoints from the same manufacturer can communicate directly via Ethernet or Wi-Fi; they negotiate charge rates with each other
• CT clamp shared reference: both chargepoints read the same CT clamp (or a gateway device aggregates the reading); the gateway allocates current to each chargepoint
• Priority charging: some systems allow one chargepoint to take priority (e.g., company vehicle charges first; employee vehicles share remainder)

Where two chargepoints are installed on a 100A supply, the theoretical maximum shared rate is 100A minus standing load. If standing load is 20A, the two chargepoints share 80A — up to 40A each, but together not exceeding 80A.

Solar Integration: CT Clamp for Solar-Aware Charging

Chargepoints such as the Myenergi Zappi use a CT clamp not just to prevent overload but to measure solar generation excess and direct it to the EV. This is called "eco mode" or "solar boost" charging:

• A CT clamp on the solar inverter output (or on the supply cable) measures export
• When the property is exporting to the grid, the chargepoint diverts the excess to the EV instead
• The chargepoint modulates its current continuously to absorb all available solar export
• If solar drops below 6A (the minimum), the chargepoint pauses or (in a hybrid mode) tops up from the grid to maintain minimum charge rate

This requires two CT clamps: one on the main incomer (for overload protection) and one on the solar generation circuit. Setup is done through the Zappi app/myenergi hub.

Frequently Asked Questions

Does load management slow down charging noticeably?

For most customers, no. During the 90% of the overnight charge when the household is asleep and base load is low (2–5A), the chargepoint runs at full 32A. Load management only activates during high-demand periods. The customer rarely notices reduced charging speed because the reduction happens when they're not paying attention, and the car tops up to full before morning regardless.

Can I install a CT clamp on a split-load consumer unit?

Yes. The CT must be on the main incomer cable before the split — it must measure all load, not just one of the split sections. If there is a main switch before the split, the CT goes on the incomer to that main switch. Do not put the CT on one of the split-load tails.

What if the CT clamp reading is wrong?

Symptoms: chargepoint not reducing output during high load (CT reading too low), or chargepoint barely charging at all (CT reading too high). Check: (1) clamp is fully closed; (2) arrow direction is correct; (3) signal polarity at chargepoint terminals; (4) the main fuse rating entered in the app matches the actual DNO cut-out rating. If the cut-out is 80A but the app is configured for 100A, the chargepoint will not throttle at the right level.

Is load management required by regulation?

Not mandatory on every install. However, BS 7671 Chapter 722.312.2 requires the installer to assess the supply adequacy. Where the chargepoint risks overloading the supply, load management (or a supply upgrade) is required. For most installs with electric showers or hobs, load management is a sensible default.

Regulations & Standards

• BS 7671:2018+A2:2022 — Chapter 722: supply assessment; protection requirements for EV circuits

• IEC 62196 — minimum and maximum charge current requirements for Mode 3 chargepoints

• IET Guidance Note 7 — EV charging installation guidance including load assessment

• Electric Vehicles (Smart Charge Points) Regulations 2021 (SI 2021/1467) — smart charging and demand flexibility requirements

• IET Guidance Note 7 — EV Charging — load assessment and CT clamp guidance

• Myenergi Zappi installation manual — CT clamp connection and solar eco mode

• Ohme Home Pro installation guide — CT clamp load management setup

• Wallbox Pulsar Plus — dynamic power sharing — multi-chargepoint load balancing

• ev charger installation types — Mode 3 chargepoint specification and circuit design

• bs 7671 ev wiring requirements — Chapter 722 supply assessment requirements

• three phase ev supply — three-phase load management for 22kW chargepoints

• ev charging commercial premises — multiple chargepoints and load balancing for fleets