Maximum demand is the highest amount of electricity that an electrical system is expected to need at any time. There are four main ways to figure out maximum demand for EV chargers:
1. Assessment
One way to assess maximum demand is to list all the electrical loads. According to the Wiring Rules, maximum demand can be assessed when the electrical equipment has fluctuating or intermittent use:
Primary load management
This controls the system all the time and ensures a good user experience.
Backup load management
This kicks in if the primary system fails, ensuring that EV charging doesn’t affect the building’s overall power supply.
2. Measurement
To use the measurement method, you need data from smart meters over at least a year. In most of Australia, the highest demand usually occurs in summer, while in places like Tasmania, it can happen in winter. For example, if an apartment building has a power supply rated at 500A and historical usage shows a peak of 300A, and they plan to install three 25kW chargers (which add 100A), the total demand would be 400A.
3. Limitation
The limitation method relies on circuit breakers and is discussed more in detail in the guidelines for EV charger installations. However, in more complex buildings, relying only on this method to manage maximum demand for EV chargers isn’t enough.
Application
This is the most commonly used method for the calculation of maximum demand at the time of design in the electrical industry today. In this method, the maximum demand is calculated by simply summing up all the loads concerned. AS/NZS 3000:2018 cl.2.2.2 (a) refers to Appendix C, which is informative and not normative.
In the case of EV charging, the calculation method requires for installations of up to 5 dwellings, that for such installations, the EV charger shall be assumed to contribute 100% of rated load. So a 32A EV charger in a house will add 32A to the maximum demand calculation.
For a single dwelling, with a supply of 63A, if a 32A (7kW) charger is intended to be installed, calculation of maximum demand will probably provide a result requiring an upgrade on supply to the building from the network, since the addition of 32A to the existing loads in the house (oven, aircon, hot water service, etc) will exceed 63A.
This approach will also ensure the higher availability of charging, which may be of sufficient value to the consumer that it’s worth the extra cost.