Transmission Voltage and Data Centers: 69 kV to 765 kV

Electrical power is the product of voltage and current. For a given conductor, raising the voltage lets a line carry more power with lower losses over longer distances. That is why utilities move bulk power at high voltage and step it down near the customer.

For site selection, voltage is a quick proxy for scale. A site next to a 12.47 kV distribution feeder and a site next to a 345 kV line are in different categories before anything else is considered. Voltage tells you what class of load the nearby system was built to move. It does not tell you how much spare room it has.

Distribution systems typically operate below about 35 kV. Common distribution voltages include 12.47 kV, 13.2 kV, 13.8 kV, 25 kV and 34.5 kV. They serve homes, businesses and smaller industrial customers through feeders out of a distribution substation.

Transmission starts above that, generally at 69 kV, though some utilities treat 69 kV and 115 kV as sub-transmission. Transmission lines connect generating plants, substations and neighboring systems, and they are planned to meet NERC reliability standards for the bulk electric system.

Edge facilities can often be served from distribution. Above that, the size of the load usually pushes the project to transmission service with its own substation. Once a project takes transmission service, the developer typically pays for, and sometimes owns, the substation that steps power down to a medium voltage such as 34.5 kV or 13.8 kV for distribution across the campus.

The table below gives rough planning roles for each voltage class. Treat the load ranges as indicative only. A short, lightly loaded 138 kV line between two strong substations can outperform a long, heavily loaded 230 kV line.

For the load side of this comparison, see our guide on how much power a data center needs.

Two lines at the same voltage can have very different capacity to serve a new load. Utilities and grid operators look at several factors.

• Thermal rating. Conductor size and type, and how hot the line is allowed to run, set how much current it can carry.
• Existing flows. A line already carrying power across a region has less room for a new load tapped off it.
• Contingency limits. Under NERC planning standards, the system has to stay within limits when a major element such as a line or transformer is out of service. A new load often hits these limits before it hits a line’s normal rating.
• System strength and voltage support. A large load at the end of a long line can cause voltage problems even when thermal capacity looks adequate.
• Upstream constraints. The binding limit may be a transformer or line miles away, not the one next to the site.

This is why utilities run load studies before committing to serve a large customer, and why a site next to a high-voltage line can still face major network upgrades.

There are a few common ways to bring transmission service to a site. The utility decides which it will accept, based on reliability and its own planning rules.

• Radial tap. A single line is extended from an existing line to the customer substation. It is the simplest connection but gives the site only one path, and many utilities limit how much load they will serve this way.
• Loop-in (or “loop-through”). The existing line is cut and routed into a new switching station, so the site is fed from two directions on the same line.
• New switching station. A new utility-owned station with breakers is built on or near the site, often designed for future expansion.
• Two independent sources. Lines from two different substations or two separate circuits feed the campus, which improves resilience to a single outage.

Each option has a land footprint, and each needs easements if the line crosses other property. Our guide to substation proximity and capacity covers the substation side in more detail.

Public and commercial transmission maps are a useful starting point for screening. They are also approximate. Line routes can be misplaced, voltages mislabeled, and recently built or retired facilities missing. Maps rarely show circuit count, ratings or how heavily a line is used.

• Confirm the voltage and number of circuits on the structures in the field when you can.
• Identify which substations each line runs between.
• Check who owns and operates the line; it may not be the local retail utility.
• Treat any capacity estimate drawn from maps alone as a hypothesis for the utility to confirm through the large-load interconnection process.