EV Charger Power Distribution Design
In the previous section, different types of power distribution equipment was introduced. This section aims to introduce basic power distribution through riser diagrams and provide typical configuration of distribution equipment.
In the following example of a riser diagram, a new utility service is established by the utility company from overhead lines and brought to a pad mounted transformer. The transformer then steps medium voltage down to 208V which then serves a distribution board that has breakers in it that serve circuits powering Level 2 EV chargers.
What if 480V Level 3 chargers are to be added in this system? Then the utility transformer would be designed to step the voltage down to 480V rather than 208V as shown in the example below. Another transformer would be added to the system to step the 480V down to 208V in order to serve the array of Level 2 chargers from the original design as shown.
What if there is an existing building? Then the point of connection would be preferably in a location close the service as shown below.
For large charging systems across a wide area, strategically locating panels in areas to consolidate cabling and loads will improve costs of the project.
One scenario for consideration to take into- if the utility power is down for an extended amount of time, how critical would charging be at a property?Depending on the critical nature of the charging requirement, integrating a generator into the distribution system can be strategically designed to avoid chaos in the event of a disaster. Property owners may consider adding a generator for a majority of the system or for a select number depending on the system. It is mindful to also note the gas storage needed and the time it takes for a charge to occur. Most common diesel standby generators have a standard 8-hour run-time tank (at full load). That may only allow for one complete charge for vehicles if powering Level 2 chargers. So it would be wise to specify a 24 or even 48 hour fuel tank if the generator is diesel and being used to back up Level 2 chargers. However, the most effective use of generator back up would be with Level 3 chargers as cars would be able to be rotated out quickly and serve more vehicles in its operational time.
The examples presented above are typical, however, each project must be individually assessed in electrical design by industry professional as two projects are rarely identical.
The above mentions configurations for equipment, but now we go into system sizing. Per NEC 625.42, EV’s are considered “continuous loads” and circuits must be sized 125% of their Full Load Amps or their Rated Load Amps. This means is if a Level 2 charger is charging at a maximum of 30A, the circuit (wire and breaker) must be sized 40A. To size a panelboard and services with chargers, the cumulative Rated Load Amps of each charger must be summed, then multiplied by the Continuous Load factor of 125%. However, there is an exception in feeder/ circuit sizing if power management software or other amperage limiting methods are used.
