Microgrid Interconnect Devices in the National Electrical Code

The Microgrid Interconnect Device (MID) has had a significant impact on the National Electrical Code (NEC), particularly in the context of distributed energy resources (DERs) like solar photovoltaic systems, battery storage, and microgrids. Major changes in the 2020 NEC have caused some confusion about the applicability of certain code sections.

The MID is a device or system that allows for the safe and seamless connection of a microgrid to the main power grid. It ensures that the microgrid can operate in both grid-connected and islanded modes while maintaining the safety and reliability of the electrical system. It was first defined in Section 705.170 of the 2017 code cycle.

Key NEC Sections Applicable To Microgrids

The NEC, which is revised every three years, has incorporated changes to accommodate the integration of DERs and microgrids. Some of the key changes related to the MID and microgrids include:

  1. Article 705 (Interconnected Electric Power Production Sources): This article has been updated to address the interconnection of DERs, including microgrids, to the main grid. It includes requirements for the MID to ensure safe disconnection and reconnection of the microgrid.
  2. Article 706 (Energy Storage Systems): With the growing use of battery storage in microgrids, this article provides guidelines for the safe installation and operation of energy storage systems, which are crucial components of many microgrids.
  3. Article 710 (Stand-Alone Systems): This article covers the requirements for stand-alone systems, including microgrids that can operate independently of the main grid. It addresses the safety aspects of operating in islanded mode.
  4. Article 712 (Direct Current Microgrids): As microgrids can operate on both alternating current (AC) and direct current (DC), this article provides specific requirements for DC microgrids, including the use of MIDs for interconnection.

By incorporating these changes, the NEC ensures that microgrids and DERs are safely integrated into the electrical grid, providing flexibility and resilience to the overall power system. The MID plays a crucial role in this integration, ensuring safe and seamless transitions between operating modes.

Microgrids Are Not Standby Systems

In the context of the National Electrical Code (NEC), a Microgrid Interconnect Device (MID) is not directly classified as a standby system. Instead, it is a component that facilitates the connection and disconnection of a microgrid (which may include standby systems) from the main grid. The MID could be a dedicated “smart switch” or a hybrid or multimode inverter that allows two-way power flow under specific circumstances.

A standby system, as defined by the NEC, is a backup power system used to supply power in the event of a primary power failure. It’s typically used for emergency or backup power and is not intended for continuous operation. Microgrids, on the other hand, interact with the utility grid and are not only active when there is a grid failure. As a result, Article 702 of the National Electrical Code does not apply to Microgrid systems.

A microgrid, on the other hand, is a localized group of electricity sources and loads that can operate independently from the main grid (islanded mode) or in conjunction with it (grid-connected mode). A microgrid can contain a variety of energy sources, including renewables, and can serve as an “islanded” system when it provides backup power. Islanding is the ability to disconnect from the power source of power in the event of an outage while retaining the ability to manage multiple sources of power and loads.

The MID’s role is to safely manage the transition between these modes, ensuring that the microgrid can seamlessly connect to or disconnect from the main grid while maintaining safety and reliability standards. Therefore, while the MID itself is not considered a standby system, it is a crucial component in the operation of microgrids that may include standby systems.

Why The Distinction Is Important

Microgrids turn into standalone systems when operating in island mode (as opposed to a standby system). Therefore, Section 710.15(A) applies. This section allows the output of a microgrid system to be less than the connected load.

This is different from generators and uninterruptible power supplies (UPS), which are governed by Section 702 and must be sized to handle the connected load.

This is critical because it allows smaller batteries, generators, and solar resources to supply power to entire residences without any source being required to meet the entire demand of the building. The sources can combine to meet load demands, and the microgrid will collapse (shut down) when the loads exceed the available power resources. Most Microgrid Interconnect Devices will automatically restart when resources are once again available to meet demand.

This greatly simplifies installations, reducing load management requirements, and minimizing costs.

Microgrids Are Different

Because microgrids incorporate multiple power sources, the rules are different. These devices are purpose-built to safely disconnect excessive loads and reconnect loads based on logic built into the listed device. They also incorporate a transfer switch or switches to isolate sources as required, and do so automatically.

But they are different than a transfer switch both functionally and with respect to the NEC, mainly because they support bi-directional power flow to an from a primary source of power (usually the utility grid). A Microgrid Interconnect Device can also operate in Standalone (island) mode indefinitely, as is the case in some off-grid systems.

Who Makes Microgrid Interconnect Devices?

Common examples of Microgrid Interconnect Devices for the residential market include:

  • Enphase’s System Controller (original, 2, 3, and 3G)
  • Tesla’s Gateway used for the Powerwall series of batteries.
  • SolarEdge’s Backup Interface
  • FranklinWH aGate X (part of the Franklin Home Power system)
  • Sol-Ark 12K/15K Multimode inverters

There are many more hybrid/multimode inverters that can serve as MIDs. The above listed brands are the most popular used in our service area (in no specific order).

Changing World, Charging World

As more and more battery backup systems are installed, and new players come to market, the Microgrid technology will improve and expand rapidly. Until recently, for example, we never really dreamed of a compliant setup where a generator could be AC coupled to PV output and battery output simultaneously (Enphase’s System Controller 3G now does that).

Even more exciting will be the integration of these devices into simpler to install. more elegant, and consumer-friendly options. Already we are seeing many players in the market sell good looking battery backup products. This shows that consumer demand is driving the market.

We all want energy freedom, and charging batteries from multiple available sources for backup purposes and grid independence is key to the long-term success of distributed energy resources. Microgrid Interconnect Devices are what pull it all together.


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