As take-up of electric vehicles (EVs) increases, electricians and designers are being asked more and more to include EV charging points in residential and business installations. An important part of making that happen is ensuring accurate calculation of maximum demand—the maximum level of electrical loading an installation will be likely to have under normal conditions.

Unless properly calculated, EV chargers may overload the electrical supply of a site and trigger nuisance tripping, network service limitations, or dangerous installation. Let us simplify maximum demand for EV chargers, the standard calculation methods, and how to use them in the field.

What Is Maximum Demand?

Maximum demand is the largest electrical load a system will carry at any particular time. It is not connected load but is a more practical figure with usage diversity and load profiles taken into account.

In the case of EV chargers, maximum demand calculations can aid in computing:

• – Whether the existing supply can support additional loads
• – If a service upgrade is needed
• – How to configure load management systems

Four Recognised Methods for Calculating Maximum Demand

As per AS/NZS 3000:2018 Clause 2.2.2, the four primary ways of determining maximum demand are:

1. Assessment Method

It’s estimation-based on load from general patterns of use. It’s suitable for EV chargers because:

• – Chargers are not used continuously
• – Charging often occurs at predictable times (e.g. overnight)
• – Diversity can be applied in multi-unit sites

Clause Reference: AS/NZS 3000:2018 Cl. 2.2.2(c)

Load Management Considerations

Evaluation may cover impact of load control systems, including:

• Primary Load Management: Active systems that control when and how EVs charge in order to remain within a site’s level of demand.
• Backup Load Management: Fail-safes which activate if the primary one is failing, to prevent EV charging from degrading critical building loads.
  

Example:
In a carpark with 10 x 7kW EV chargers (each drawing ~32A), a load controller might restrict simultaneous charging to three vehicles. This limits the calculated demand to 96A instead of 320A.

2. Measurement Method

Most suitable for installations already in place, this method employs actual data to calculate the maximum demand.

How It Works:

• – Capture 12 months of data from a smart meter
• – Calculate the peak maximum demand over that time
• – Include the new EV load to determine if it exceeds the supply capacity

Example:
Site has 500A supply
Smart meter shows historical peak of 320A
Plan to install three 22kW chargers (~32A each) = 96A
New projected demand: 320A + 96A = 416A → No upgrade required

Clause Reference: AS/NZS 3000:2018 Cl. 2.2.2(d)

3. Limitation Method

This approach applies physical hardware—e.g., circuit breakers or load-limiting relays—to limit how much load an EV charger can take.

It is simple and appropriate for:

• – Single-dwelling installations
• – Sites with minimal infrastructure

However, it’s not suited for complicated buildings because:

• – It lacks flexibility
• – It does not consider load diversity
• – It offers no dynamic control

Clause Reference: AS/NZS 3000:2018 Cl. 2.2.2(b)

4. Application (Summation) Method

This is the standard procedure applied in design stage for new installations. It entails addition of all pertinent loads, multiplication by diversity factors where necessary, and use of Appendix C in AS/NZS 3000.

Under domestic conditions, Appendix C identifies how EV charger contributions are accommodated in demand:

• – For installations with up to 5 dwellings, each charger is considered to contribute 100% of its rated load.
• – In multi-dwelling sites, diversity may be applied if justified with load control or usage patterns.

Example:
House with a 63A single-phase supply
Existing load (lights, oven, A/C): 38A
New EV charger: 32A
Total: 38A + 32A = 70A → exceeds main supply → may require:

• – Load control
• – Smaller charger
• – Supply upgrade

Clause Reference: AS/NZS 3000:2018 Cl. 2.2.2(a) + Appendix C

Applying the Right Method in Practice

Here’s a simple decision-making guide:

Additional Considerations

1. Coordination with the DNSP

Distribution Network Service Providers (DNSPs) might have specific requirements for the installation of EV chargers. These are:

• – Limiting load export/import
• – Pre-approval for charger ratings over 32A
• – Maximum allowed demand on the service

2. Voltage Rise Calculations

EV chargers are able to supply extremely high current via submains or last subcircuits. Use AS/NZS 3008 to limit voltage drop/rise—particularly where there are long cable runs or communal installations.

Calculating maximum demand is not merely a compliance exercise—it future-proofs the installations and maximizes customer satisfaction. It’s all about selecting the method best suited to the project size, amount of existing infrastructure, and degree of control available.

Always consult:

• – AS/NZS 3000:2018, particularly Clause 2.2 and Appendix C
• – AS/NZS 4777 (for inverter-based systems)
• – Local DNSP rules