Interconnection Standards

In 2007, the Washington Utilities and Transportation Commission (UTC) established interconnection standards for distributed generation (DG) systems up to 20 megawatts (MW), which were updated in July 2013. These standards are applicable to investor-owned utilities in Washington state, such as Avista, PacifiCorp, and Puget Sound Energy, but exclude municipal utilities, public utility districts, and cooperative electric utilities.

The updated standards introduce three levels of interconnection, categorized by system capacity and other criteria. Tier 1 is for systems up to 25 kilowatts (kW) that use inverter-based interconnection equipment. Tier 2 covers systems between 26 kW and 500 kW. Tier 3 is for all systems up to 20 MW that do not meet Tier 1 or Tier 2 criteria. These standards are specific to investor-owned electric utilities and do not apply to municipal utilities or electric cooperatives.

Tier 1 systems require an inverter compliant with the Underwriters Laboratories (UL) 1741 standard. Tier 2 systems necessitate the ability to disconnect the generation facilities from the electric grid as per the IEEE 1547 standard. Inverter-based systems in Tier 2 must adhere to both UL 1741 and IEEE 1547 standards. Utilities may suggest modifications to their electrical systems to accommodate the generation facility, but the customer is responsible for the costs of these minor changes. Systems larger than 50 kW might need a three-phase connection. A visible, lockable AC disconnect switch is usually mandatory for Tier 2 systems, although utilities can waive this if certain conditions are met.

Tier 3 systems must also isolate the generation facility from the electric grid following IEEE 1547 and have inverter-based systems that comply with IEEE 1547 and UL 1741. These systems should be designed to prevent loss of protective functions due to a single failure point and include ground fault protections and breaker failure detection. Utilities may request feasibility or impact studies for these systems.

All system interconnections must comply with safety and reliability standards, including the National Electric Code (NEC), National Electric Safety Code (NESC), IEEE standards, North American Electric Reliability Corporation (NERC) standards, Western Electricity Coordinating Council (WECC) standards, American National Standards Institute (ANSI), Underwriters Laboratories (UL) standards, and local, state, and federal building codes. Utilities must maintain a point of contact for customer inquiries on interconnection and provide standardized application forms and interconnection agreements as approved by the UTC. Application fees are capped at $100 for systems up to 25 kW, $500 for systems between 25 kW and 500 kW, and $1,000 for systems larger than 500 kW but not exceeding 20 MW. The UTC’s rules also include mechanisms for resolving disputes.