What is dynamic load management for EV charging?

Dynamic load management for EV charging measures the site's actual load at its grid connection, usually per phase and every few seconds, and gives the chargers the capacity that is spare at that moment. It keeps the chargers, together with everything else on the site, within the current the connection can carry. A static scheme hands the charger group an allowance that never changes. Either way, the chargers are throttled, paused or shared rather than tripping the main protection or exceeding the agreed import capacity.

Load management is what lets a car park run eight 22 kW chargers on a supply that could not feed two of them at full power alongside a busy building. It does not create capacity. It moves charging into the capacity that is not being used.

Static and dynamic load management

Static load management is a cluster limit: the chargers share, for example, 32 A per phase between them, whatever the building is doing. A master wallbox or the charging platform divides that allowance between the cars that are connected. It is simple and needs no measurement outside the charger group, but the allowance must fit the building's maximum demand, so it is small exactly when the building is quiet and the cars could take more.

Dynamic load management adds a meter at the grid connection. The load manager reads it, subtracts the building's load from the usable capacity, and hands the rest to the chargers. A wallbox that is described as having "dynamic load management" usually means it can read such a meter directly or through its master unit.

How dynamic load management works

Every control cycle does the same four things:

1. Measure current on each phase at the site's grid connection, import positive and export negative. The meter must see everything behind the connection, including the chargers themselves.
2. Subtract the chargers' own current, which the load manager knows from the chargers' meters or from the limits it sent, to get the building's load per phase.
3. Calculate the headroom per phase: usable capacity minus building load. Usable capacity is the connection's rating less a safety margin.
4. Allocate that headroom to the connected cars and send each charger its new limit.

The allocation rule decides who charges:

The constraints that shape the allocation

The 6 A floor

An AC charger tells the car how much current it may draw through the IEC 61851-1 control pilot signal. The lowest current the signal can advertise is 6 A per phase. At 230 V that is 1.38 kW on one phase and 4.14 kW on three. Below 6 A, the only way to reduce a car further is to pause it. A load manager therefore never divides the headroom into shares smaller than 6 A: it decides how many cars can charge, then shares the headroom between those.

Some vehicles end a session, or need the cable reconnected, after a long pause. Test the paused-and-resumed case with the vehicles that will actually use the site.

Phases and single-phase cars

A three-phase car on a three-phase charger draws the same current on every phase, so the most loaded phase sets the limit for all three. A single-phase car draws on one phase only. If every charger brings its single-phase supply out on L1, a car park full of single-phase cars loads L1 alone while L2 and L3 sit idle. Installers rotate the phase order from one charger to the next (L1-L2-L3, then L2-L3-L1, then L3-L1-L2) so single-phase cars spread across the three. A phase-aware load manager then needs to know which phase each charger's first conductor is on.

Work every headroom figure per phase, with the three-phase power calculator or the kW to amps calculator to move between kW and amps. A kW headroom summed over three phases hides the phase that is full.

Solar export

When the site has PV, its connection current can be negative: it is exporting. Dynamic load management can count that export as headroom and charge from surplus solar. A car needs at least 1.38 kW of surplus to charge on one phase, or 4.14 kW on three, before the 6 A floor allows it to start. Chargers that switch between one-phase and three-phase charging use the lower threshold on cloudy days. The solar self-consumption guide explains how to measure the export that this uses.

Loop time and margin

The load manager reacts only as fast as its slowest step: the meter's update interval, the calculation, the command to the charger, the charger applying it and the car following the new pilot signal. While the loop catches up, the building can add load. Size the safety margin to cover the largest load step the site can make within one loop: if the loop takes 30 s and a 40 A compressor can start in that time, the margin is at least 40 A. Measure the loop at commissioning by stepping the limit down and timing the fall on the charger meter.

Worked example: eight cars on a 100 A supply

A workplace has a 400 V three-phase supply with an import capacity of 100 A per phase (69 kW at 230 V phase voltage). The load manager keeps a 10% margin, so 90 A per phase is usable. It runs eight 22 kW chargers, each 32 A per phase.

At 11:00 the building draws 55 A on L1, its most loaded phase. The headroom is 90 − 55 = 35 A per phase, about 24.15 kW.

Equal share over all eight cars is not possible: 4.375 A is below the 6 A floor. The load manager must choose how many cars charge. Five cars at 7 A use the whole 35 A; the other three wait, or rotate in when a session finishes.

At 22:00 the building draws 20 A. The headroom is 70 A per phase, and 70 ÷ 8 = 8.75 A per car, so all eight cars charge at 6.04 kW each, 48.3 kW in total.

With static load management the allowance has to fit the building's maximum demand. If that is 75 A per phase, the chargers get 90 − 75 = 15 A per phase at all hours: two cars at 7.5 A, or 10.35 kW in total. Over a ten-hour night that is 103.5 kWh, against 483 kWh with dynamic load management at the 22:00 headroom.

The EV charging capacity calculator repeats the full-power count hour by hour. These are the same site and chargers, with the building's load in kW for each hour from 07:00 to 14:00 (each amp per phase is 0.69 kW, so the 11:00 value of 37.95 kW is the 55 A case above):

The calculator counts chargers at full power only. Shared current below 32 A, the 6 A floor and phase balance are the load manager's job, as in the table above.

When the measurement or the link fails

Dynamic load management depends on three links: meter to load manager, load manager to chargers, and charger to car. Each needs a defined behaviour when it stops.

• Meter reading lost or stale. The load manager must stop using the last value after a set age and fall back to a limit that is safe with the building at its maximum demand. In the example, that is the static 15 A per phase.
• Charger loses the load manager. A charger that keeps its last high limit can overload the site when the building's load rises. Over OCPP, the fallback is built from stored charging profiles, such as a low TxDefaultProfile beneath a short-lived dynamic one. The OCPP guide sets out the profile rules and how to test them. Chargers controlled over Modbus TCP often have a vendor-defined safe current applied when the set point is not refreshed in time: read the charger's documentation and test it.
• Car does not follow. The charger meter shows whether the car actually reduced its current. A load manager that trusts its own commands without reading the result cannot see a car that ignores the pilot signal.

Keep the measurement and the headroom calculation on site. A cloud platform can schedule and bill, but a decision that has to protect the main fuse within seconds should not depend on an internet round trip. When the charging platform is in the cloud, it should receive the headroom with its age and apply a fallback when the value is too old.

Rules that require controllable charging

In Great Britain, the Electric Vehicles (Smart Charge Points) Regulations 2021 apply to charge points sold for homes and workplaces since 30 June 2022; public charge points and rapid charge points of 50 kW or more are excluded. A charge point must be able to change its charging rate or timing in response to signals, must remain able to charge if it loses its network connection, and must apply a randomised delay of up to 600 s in normal operation (it must be capable of up to 1,800 s). Its pre-set default charging hours must fall outside the peak hours of 8:00 to 11:00 and 16:00 to 22:00 on weekdays, and the owner can override them.

In Germany, under Section 14a EnWG and the Bundesnetzagentur's decision BK6-22-300, a wallbox rated above 4.2 kW and commissioned since 1 January 2024 must accept a reduction by the network operator, which may cap it at no less than 4.2 kW. On an 11 kW wallbox that is 6.1 A per phase, so in practice 6 A. A site with several controllable devices can take the operator's limit through an energy management system, which then shares it between them. The Section 14a guide has the minimum-power formula.

In the EU, Article 14(1) of the recast Energy Performance of Buildings Directive, Directive (EU) 2024/1275, covers new non-residential buildings with more than five parking spaces and those undergoing major renovation. They need at least one recharging point for every five spaces (one for every two in office buildings), pre-cabling for at least 50% of the spaces and ducting for the rest. The pre-cabling and ducting must be dimensioned so that the required recharging points can be used at the same time and, where appropriate, to support a load or recharging management system, to the extent that this is technically and economically feasible and justifiable. Article 14(6) requires the recharging points to be capable of smart recharging and, where appropriate, bi-directional recharging. The national transposition sets the details.

Load management also changes the bill. A site that charges under a demand or capacity charge can keep its monthly peak down by holding the chargers below a target rather than the connection rating: see peak shaving vs load shifting. Fleets that move charging to cheap hours, or sell that flexibility, use the same control, covered in EVs as grid assets.

Where EpiSensor fits

EpiSensor measures the site that the chargers share. A ZEM electricity monitor at the incomer reports current and power per phase, and a second ZEM on the charger board separates the charging load from the building. Edge on the Gateway calculates the headroom from the most loaded phase, with its age, on site and without a cloud round trip.

Edge is not a charge point management system and does not speak OCPP. It publishes the headroom over MQTT, HTTP or Modbus TCP. Where a charger or its load-management unit accepts a current limit over Modbus, an Edge automation can write the limit itself; the charger's own timeout then provides the fallback. Otherwise the charging platform or the chargers' own load manager reads the headroom and applies it. Before choosing one, confirm that it accepts an external site limit (in kW or in amps per phase), how often it reads it, what it does when the value is old, and how it handles headroom below 6 A per waiting car. The EV charging load management application lays out the meters, the calculation and the hand-off in full.