ISOs and RTOs: which grid operator serves a datacenter site?

What an ISO or RTO does

An independent system operator (ISO) or regional transmission organization (RTO) runs the high-voltage grid across many utilities' service areas without owning it. FERC introduced the ISO concept in Orders No. 888 and 889 as a way to give all generators non-discriminatory access to transmission, and in Order No. 2000 encouraged utilities to form RTOs to administer the grid regionally.

In practice each one:

• dispatches power plants across its footprint using bid-based energy and ancillary service markets
• manages congestion on the transmission system, which shows up as different prices at different locations
• runs the interconnection queue for new generation and plans new transmission

According to FERC, about two-thirds of US electricity load is served in RTO regions.

The seven grid operators and where they run

FERC lists seven ISOs and RTOs in the US. Footprints follow utility service areas, not state lines, so several states are split between operators.

• **PJM Interconnection**: all or part of 13 states (Delaware, Illinois, Indiana, Kentucky, Maryland, Michigan, New Jersey, North Carolina, Ohio, Pennsylvania, Tennessee, Virginia and West Virginia) plus the District of Columbia.
• **MISO**: portions of 15 states from Michigan and Indiana to Montana, and south to Louisiana and Mississippi.
• **SPP**: portions of 14 central states in FERC's listing. On April 1, 2026 SPP extended its RTO into the Western Interconnection, adding utilities in Arizona, Colorado, Montana, Nebraska, New Mexico, Utah and Wyoming, which makes it the first RTO spanning two interconnections.
• **ERCOT**: most of Texas, about 90% of the state's electric load. Its market is overseen by the Public Utility Commission of Texas and the Texas Legislature.
• **CAISO**: most of California and a portion of Nevada. CAISO also runs the Western Energy Imbalance Market, and its Extended Day-Ahead Market went live with PacifiCorp on May 1, 2026.
• **NYISO**: all of New York State.
• **ISO New England**: Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island and Vermont.

The rest of the map

Large parts of the country have no ISO or RTO. FERC describes traditional wholesale markets in the Southeast, Southwest and Northwest, where vertically integrated utilities own generation, transmission and distribution, operate their own systems, and buy and sell power mostly through bilateral contracts. Federal systems such as the Tennessee Valley Authority, the Bonneville Power Administration and the Western Area Power Administration also operate in these regions.

• Southeast: all or parts of Florida, Georgia, Alabama, Mississippi, North Carolina, South Carolina, Missouri and Tennessee, per FERC.
• West outside CAISO: many separate balancing authorities. FERC counts 20 in the Northwest Power Pool area alone. Some of these utilities now take part in CAISO's western markets or have joined SPP's western RTO, so the map is changing year to year.

For a datacenter in one of these areas, the serving utility is both the grid operator and the counterparty for the connection request.

Is WECC an ISO?

No. The Western Electricity Coordinating Council (WECC) is one of six Regional Entities to which the North American Electric Reliability Corporation (NERC) delegates authority to monitor and enforce mandatory reliability standards. The six are the Midwest Reliability Organization, NPCC, ReliabilityFirst, SERC, Texas RE and WECC.

Regional Entities audit and assess grid reliability. They do not dispatch power plants, run markets or process interconnection requests. That is why "WECC" and "SERC" are useful as names for broad regions without an ISO, but not as answers to "who will study my load?"

The naming can be confusing in Texas: ERCOT (the Electric Reliability Council of Texas) is the grid operator, while Texas RE is the separate Regional Entity that enforces reliability standards there.

Why the operator matters for a datacenter

The grid operator shapes three things a site selector cares about.

1. Connection rules. In June 2026 FERC ordered the six operators under its jurisdiction (all of the above except ERCOT) to justify or revise how their tariffs handle large loads such as datacenters. ERCOT is moving large loads to a batch study approved by the PUCT. The rules you face depend on which side of that line the site falls.
2. Congestion and price. Organized markets publish binding transmission constraints and locational prices, which show where the grid is already strained. The grid congestion screen summarises binding constraints in ERCOT, PJM, MISO, SPP, CAISO and NYISO.
3. Queue context. Each operator runs its own generator interconnection queue. Deep queues near a site signal both competition for grid capacity and new supply that may arrive.

The operator is rarely your only counterparty, though. In most cases you request service from the local utility, which then works through the operator's processes. Utility terms shows the large-load posture of the utilities GridCensus tracks.

How GridCensus labels regions

GridCensus assigns every candidate site to one of nine regions: the seven operators above, plus WECC for the western grid outside CAISO and SERC for the Southeast. The last two are region labels for areas served by vertically integrated utilities, not grid operators.

Candidate sites by region, from the September 2026 refresh:

• PJM: 44,364
• WECC: 21,799
• MISO: 19,709
• SPP: 14,428
• SERC: 14,283
• CAISO: 8,603
• ERCOT: 6,817
• ISO-NE: 5,727
• NYISO: 2,881

Region labels are a screening shortcut. Because footprints split states and are still moving (SPP's western expansion is recent), confirm the serving utility for any specific site with Vet a site, which looks it up from EIA service-territory boundaries. Compare regions on the ISO overview or the queue-depth ranking.