Clean Energy SF: San Francisco, fog then sun, est. 2026

Vol. 1 - No. 14 - Thursday, September 24, 2026 - Free to readers

Grid ยท Analysis

The Trans Bay Cable: the city's undersea power lifeline

A buried direct current line carries hundreds of megawatts under the bay to San Francisco. Here is what it does, why the city leans on it, and how a reader can watch its flows.

Filed October 13, 2026 By the Clean Energy SF editors Reading time: 6 minutes

I. How a cable took a power plant's place

Flip a switch in a Noe Valley flat and the power that answers has probably traveled under the bay. San Francisco sits at the tip of a crowded peninsula, with little room for the kind of large power plant that once stood at its edge, so most of what lights the city now arrives over wires from somewhere else. One of the biggest of those wires runs underwater, and it is easy to miss precisely because it works.

That wire is the Trans Bay Cable, a buried high voltage direct current line (a cable that carries power as steady one-direction current rather than the back-and-forth alternating current in your walls) running under San Francisco Bay from Pittsburg, in the East Bay, to a converter station near Potrero Hill. It can carry roughly 400 megawatts at its rating, per its operator, which is a large share of what the city draws on an ordinary day. It came online around 2010, and the old gas-fired Potrero plant on the southeast waterfront, which had run for decades, shut down around 2011 once the import line and other upgrades were in place.

II. Why a peninsula imports its power

Geography does most of the explaining. The city is dense, land is expensive, and the neighborhoods closest to the old waterfront plants, Bayview and Hunters Point, carried the air-quality burden of generation for years. Building new local plants here is hard and unpopular, so the practical answer has been to import power over a mix of overhead lines down the Peninsula and this one direct current cable under the bay.

Direct current is the right tool for a long underwater run. It loses less energy over distance than alternating current, needs fewer cables in the trench, and the converter stations at each end let operators dial the flow up or down on command, which helps the grid operator manage a tight coastal load pocket. The California Independent System Operator, the nonprofit that runs the state's high-voltage grid and balances supply against demand second by second, treats the cable as a controllable resource rather than just a passive wire.

III. How to watch the flows yourself

You do not need a utility badge to see how hard the grid is working. The California Independent System Operator publishes a public Today's Outlook page that updates through the day with system demand, the supply mix, and net imports into California, and it runs a more technical data system called OASIS for market and transmission information. Neither gives you a single meter labeled Trans Bay Cable, so be honest about the limit: what you can read is the whole picture the cable feeds into, not an isolated readout of that one line.

For the cable's own ratings and design, the operator's project pages and filings with the Federal Energy Regulatory Commission, the federal body that oversees interstate transmission and wholesale power, are the accountable record. If a number you see online lacks a source you can open in a public document, treat it as roughly right at best, and check it against the operator or the agency before you trust the decimal places.

IV. Worth watching this month

1. The California Independent System Operator's Today's Outlook page updates through each day and is the quickest routine read on how much power the Bay Area is importing when demand climbs.

2. The grid operator's annual transmission planning process, usually finalized in the spring, is where any proposed change to Bay Area import capacity would surface, and that one actually matters.

3. A late-season heat spell, when the fog pulls back and the city warms in September or October, is when import lines like this one carry the most weight, so watch for any Flex Alert conservation notices.

4. Any planned maintenance outage on the cable should appear in the grid operator's public outage data, which is routine but worth a glance if reliability is on your mind.

Frequently asked questions

How much of the city's power does the cable carry?

Its rating is roughly 400 megawatts, per the operator. That is a large share of San Francisco's demand on a normal day, but not all of it, and the exact amount flowing at any moment changes constantly.

Why direct current instead of ordinary alternating current?

Direct current loses less energy over a long underwater run and lets operators control the flow precisely. Those traits make it well suited to feeding a dense coastal city through a single buried line.

Did the cable really replace a power plant?

It helped. The old Potrero waterfront gas plant closed around 2011, after the cable and other grid upgrades gave the city enough imported supply to let it retire.

Can I see the cable's live output?

Not as a single labeled figure. The grid operator's public pages show system demand and net imports for the region, which is the picture the cable feeds into, rather than one isolated meter.

References