Electric vehicles are quickly becoming our most power-hungry residential appliance; one that needs to be thoroughly understood in order to safely charge at home.
This isn’t a problem when charging at public EV charging stations designed for high energy draw, but most charging sessions are done overnight in the comfort of EV owners' homes; where supply and infrastructure to handle high-energy demand is limited.
With the addition of such a power-hungry appliance to your ecosystem, it's important to consider charging methods, safety features, and good charging habits designed to keep you and your home safe for years of charging to come.
Balancing your power draw
In New Zealand, houses commonly have a single-phase power supply with a maximum current rating of 63 Amps (A). This is the maximum amount of current that can be drawn at any given time. For reference, running your washing machine may draw up to 10A, a heat pump as much as 20A, or an electric induction hob even 32A.
Compare these appliances with the most common wall-mounted AC charger in NZ; a 7.4kW unit on a single-phase which can draw an additional 32A by itself. You can see with just a few of these high-energy appliances running in tandem, you’ll quickly approach the maximum current load your home can handle.
What happens if you reach or exceed your home’s maximum rated load? You'll likely blow the main protective service fuse at your boundary box or street pole. Not only is this a huge inconvenience waiting without power until a service person arrives to replace it, but your main circuit board may suffer from overheating before the fuse is overloaded.
Home Overload Protection
Thankfully there are steps you can take to avoid this issue. The first option is to have a professional electrician install a 60A circuit breaker to protect the pole fuse from blowing if your main circuit does overload. This is an affordable measure to take to avoid a service call out, however it doesn't avoid the potential of overloading your circuit if your appliance load isn't managed.
Another, is to have your electrician set a lower limit for the total draw of your EV charger during installation. This will help minimise the problem but can impact your overall charging time depending on the limit configured.
Lastly, to avoid manually balancing your high-power appliance load, you can consider installing a smart charger that offers automatic home overload protection. This feature will slow or stop a charging session if your total power usage nears your building’s maximum draw by autonomously monitoring your home’s total power draw via a power sensor; balancing the quickest rate of charge with the power available to safely use.
It's a simple yet important electrical safety feature that’s part of all Evnex home charging installations.
How does it work?
During installation, an electrician will set a maximum current limit for the EV charger and fit a power sensor at the building’s switchboard to monitor the main house circuit; linked via a data cable.
The power sensor will read the total energy draw of the building 50 times a second to ensure it is not within 10% of its maximum rated limit. If the overall power demand of the home reaches this threshold while the charger is active, it will dynamically lower the rate of charge.
As your household power demand fluctuates, your charger will continue to adapt its rate of charge until either stopping the charge completely or returning to maximum speed when safe to do so. It’s a handy tool that provides automated protection for EV drivers from accidentally overloading their main circuit.
Safe EV Charging Methods at Home
Mode 3 chargers are a permanently installed means of AC charging for EVs and are the preferred safe charging method for residential use due to its additional IEC 61851-1:2017 specified safety features. These include the requirement for a dedicated sub-circuit and RCD protection. Alongside this, some EV home chargers also include additional protections preventing over-voltage, under-voltage, over-current, and over-temperature.
Mode 2 chargers (IC-CPD) on the other hand, come in the form of 3-pin plug-in portable charging cables. These are often conveniently included with the purchase of an EV but provide less protections when used as a main method of home charging.
Although compliant Mode 2 chargers do include internal RCD protection devices to prevent accidental shock, many still rely on a standard household (AS/NZS 3112) flat pin plug for connection to the mains supply. This places more dependence on the integrity of a building’s wiring, electrical outlets, and good user practices for safe operation.
It’s important to understand that standard sockets are not designed to carry high currents over long periods of time; such as required for overnight EV charging. If used as a regular method of charging, the repeated connection and disconnection cycles of the plug can also result in the socket resistance increasing. This can cause heat build-up between the contacts and presents a risk of melting or fire during heavy power use.
These are key reasons why Mode 2 is not a permitted charging method for public charging use in New Zealand or recommended by Worksafe for employer-owned EV charging at home.
Safe Charging Habits
If you are using a Mode 2 charging cable as your main method of home charging, be sure to consider these Worksafe guidelines listing unsafe EV charging habits.
Unsafe practices for EV charging include...
- Using a socket adaptor.
- Using a charging adapter, not from the vehicle manufacturer or electric vehicle supply equipment provider.
- Using an extension cord to increase charging cable length.
- Using a multi-box or portable socket.
- Charging more than one electric vehicle at a time on a single power outlet.
- Using two or more cascading supply leads.
- Using modified charging equipment not originally designed for the NZ market.
You can ensure your home charger is built for use in NZ by requesting a copy of an SDoC (Supplier declaration of compliance) from the manufacturer.
Peace of Mind
Ultimately, how and when you top up your EV will change with your daily drive. Sometimes you'll charge at home, at work, or at a public charging station. But wherever you are, having peace of mind that your charging session meets safety standards to protect both you and your home gives you one less thing to worry about.
Thanks for reading and helping power the transition to sustainable electric mobility in New Zealand.
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