EV Charger Load Management: CT Clamps and Maximum Demand

Quick Answer: Dynamic load management lets a 7kW (32A) EV charger share a property's existing supply without exceeding the main fuse rating. A current transformer (CT) clamp on the incoming tails measures the whole-house demand in real time, and the charger reduces its charging current so total demand stays below the main fuse limit (typically 60A, 80A, or 100A). It is the standard, low-cost alternative to a DNO supply upgrade where maximum demand would otherwise be exceeded, and is referenced in BS 7671 and the IET Code of Practice for EV charging.

Summary

Adding a 7kW EV charger to a home means adding up to 32A of continuous load. In a property already running an electric shower, an induction hob, and perhaps a heat pump, the combined potential demand can exceed the main supply fuse rating — historically 60A or 80A in older homes, 100A in modern ones. The traditional answer was a DNO supply upgrade: expensive, slow (weeks of lead time), and sometimes impossible without network reinforcement. Dynamic load management solves the same problem far more cheaply by making the charger a "good citizen" that backs off when the rest of the house is busy.

The mechanism is a current transformer (CT) clamp — a split-core sensor placed around the meter tails (or the supply cable) that measures total household current draw. The charger's controller reads this continuously and modulates its own charging current so that household + charger never exceeds a configured limit (set below the main fuse rating with a safety margin). When the house is quiet, the car charges at full 7kW; when the oven, shower, and kettle come on, the charger throttles down — even pausing — then ramps back up as demand falls.

This article explains maximum demand assessment, how CT-clamp load management works, the difference between static and dynamic limiting, multi-charger load balancing, and the design and installation points. It supports ev charger installation types and ev charger installation pricing guide; for supply fundamentals see three phase supply and cable sizing current capacity.

Key Facts

Quick Reference Table

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Approach CT Clamp? Behaviour Best For
No management No Full 32A always Ample spare capacity
Static limiting No Fixed reduced current Modest fixed headroom
Dynamic load mgmt Yes Real-time modulation Tight supplies (most retrofits)
Multi-charger balancing Yes Shares current across points Multiple chargers, one supply
Solar-aware (eco) Yes Diverts surplus PV PV-equipped homes

A 7kW charger draws 32A; on an 80A supply with a 9.5kW shower (41A) and other loads, dynamic management is usually needed to stay within the fuse rating.

Detailed Guidance

Assessing maximum demand

Before deciding whether load management is needed, assess the property's maximum demand. This is done by applying BS 7671 diversity factors to the connected load (or, better, by measuring actual demand over time with a logger). Compare the result against the main fuse rating. If the existing demand plus the charger's 32A could exceed the fuse — accounting for the realistic worst case of multiple high loads coinciding — then either the supply needs upgrading or the charger needs managing. The assessment is part of the EV circuit design, not an optional extra; over-loading the main fuse causes nuisance fuse failures (and a DNO call-out) at best.

How the CT clamp works

A current transformer is a split-core ring that clips around a conductor and produces a small secondary current proportional to the current flowing through it — without any electrical connection to the conductor. Placed around the incoming meter tails (or the main supply conductor), it measures the total current the whole property is drawing. The CT's output is wired back to the charger (or a separate load-management controller), which converts it to a real-time demand figure.

Correct installation matters: the CT must be on the right conductor (the live tail carrying total household current, positioned so the charger's own load is or isn't included as the manufacturer specifies), oriented the correct way (CTs are directional — an arrow indicates current flow), and wired per the manufacturer's instructions. A reversed or mis-placed CT gives wrong readings and defeats the protection.

Dynamic vs static limiting

Throttling behaviour and the 6A floor

The charger communicates its available current to the car via the control pilot signal (IEC 61851). It can modulate down to a minimum of around 6A (≈1.4kW) before it must pause charging entirely — the standard does not allow signalling below this floor. So in dynamic management, as household demand rises the charger steps down toward 6A and, if demand is still too high, pauses, resuming when capacity frees up. This is normal, safe behaviour, though it does extend charge times during high household-demand periods. Set customer expectations accordingly.

Multi-charger load balancing

Where more than one charge point shares a supply (dual-charger homes, small car parks), a load-management system divides the available current dynamically between the connected vehicles, keeping the aggregate within the supply limit. If one car finishes, its share is reallocated to the others. This is essential for multi-point installations on a constrained supply and is a key reason commercial/workplace installs use managed systems.

Setting the limit and margin

The load-management target is set below the main fuse rating to retain a safety margin and allow for measurement tolerances and brief surges — for example, limiting total demand to around 90A on a 100A supply. The exact margin follows the manufacturer's guidance and the designer's judgement. Setting the limit at or above the fuse rating defeats the purpose; setting it too conservatively needlessly slows charging.

Frequently Asked Questions

What is a CT clamp and why does an EV charger use one?

A CT (current transformer) clamp is a split-core sensor that clips around the incoming supply cable and measures how much current the whole house is drawing, without any electrical connection. The EV charger reads this to know how much spare capacity is available, then adjusts its own charging current so the house plus charger never exceeds the main fuse rating. It's what allows a 7kW charger to be added to a home that couldn't otherwise take the extra load.

Will load management slow down my charging?

Only when the rest of the house is drawing heavily. With dynamic management the car charges at full 7kW whenever there's spare capacity (overnight, for instance), and throttles back only when high loads like the oven, shower, or hob are running. Since most charging happens overnight at off-peak rates when household demand is low, the practical impact is usually minimal. It charges fast when it can and backs off when it must.

Do I need load management or a supply upgrade?

It depends on your maximum demand versus your main fuse rating. If adding a 32A charger could push your peak demand over the fuse rating, you need either dynamic load management (cheap, immediate) or a DNO supply upgrade (expensive, weeks of lead time, sometimes requiring network reinforcement). Load management is the first choice for most retrofits because it solves the problem without involving the DNO.

Can two EV chargers share one supply?

Yes — a multi-point load-balancing system uses a CT clamp to measure total demand and divides the available current dynamically between the connected chargers, keeping the aggregate within the supply limit. When one car finishes, its capacity is reallocated. This is standard for dual-charger homes and small workplace car parks on a single supply.

Regulations & Standards