2020 NEC, Section 712.2 DC | Microgrids and System Directories
By: Robert Key | Aug 31, 2022
Article 712, Direct Current Microgrids, was introduced to NEC 70 in 2017. What is a DC microgrid? A DC microgrid contains a DC bus, which in turn feeds DC loads connected to it. The most common use for these systems is low-power usage electronic devices such as laptops, cell phones, and routers. For 2020 NEC, Article 712 has been updated, with changes to the identification requirements for ungrounded conductors and other changes to wording for purposes of clarity. It has also been expanded.
The definition of a DC microgrid can be found in Section 712.2. It states:
A direct current microgrid is a power distribution system consisting of more than one interconnected dc power source, supplying dc-dc converter(s), dc load(s), and/or ac load(s) powered by dc-ac inverter(s). A dc microgrid is typically not directly connected to an ac primary source of electricity, but some dc microgrids interconnect via one or more dc-ac bidirectional converters or dc-ac inverters.
So then, a DC microgrid is a DC power distribution system that can supply AC or DC loads but is not directly connected to an AC power source.
Section 712.10, Directory, gives expanded direction regarding identifying all interconnected power production sources.
Section 712.10 Directory
(A) Source Directory.
A permanent directory denoting all dc electric power sources operating to supply the dc microgrid shall be installed at each source location capable of acting as the primary dc source.
(B) Building Directory.
A building supplied by a dc microgrid system shall have a permanent plaque or directory installed outside the building at each service equipment location or at an approved readily visible location. The plaque or directory shall denote the location of each power source disconnecting means on or in the building or be grouped with other plaques or directories for other on-site sources.
Exception: Multiple power production sources that are grouped at one location shall be permitted to be identified as a group.
The source and building directories allow for safe disconnection of power sources so that maintenance and repair can be performed, and emergency responders can quickly shut off power in an emergency. Note what is required:
- A “Source Directory,” i.e., a permanent plaque or directory identifying any and all DC power sources at the primary DC source.
- A “Building Directory” at each service equipment location (or other approved location). It must identify the location of other power sources for the building unless it is grouped with other power source directories.
- The marking must also comply with Section 110.21(B), Field-Applied Hazard Markings, and be able to endure in its environment. A permanent marker for example, would not suffice.
DC microgrids are especially useful as a more efficient way to use power produced via on-site power generation, such as residential photovoltaic systems. These systems produce DC power, which then must be converted into AC in order to connect to the premises wiring, only then to be converted back to DC to power a device such as a laptop. The DC microgrid eliminates this step, allowing the user to run devices without any connection to the local electric utility.
As the use of newer technologies for power production increases, such as the use of PV systems, wind power, and our reliance on computers and other DC powered devices, the use of DC microgrids will likely increase proportionally. This places a responsibility on Code professionals to keep up with changes so that they can properly install, inspect, and maintain these systems.
