Open Compute Project’s Storage Paper Imports FM Global’s Six-Foot Indoor Rack Clearance, With a Fire Test as the Only Route to Tighter Back-to-Back Spacing

Six feet of clear air between an indoor lithium-ion battery rack and the non-combustible construction around it, and the same six feet between the accessible faces of adjacent racks. Nine feet to anything combustible. A battery room rated for two hours, as a minimum.

Those figures come from FM Global Data Sheet 5-33, a property insurance document. They appear, imported wholesale, in the Open Compute Project’s “OCP Ready” requirements white paper for data center energy storage. The paper circulates into OCP Global Summit week in San Jose, which runs October 13 to 15.

The significance is not the numbers themselves. It is who is now quoting them. The Open Compute Project is the specification body for hyperscale infrastructure, and its members are the largest private buyers of indoor energy storage in the world. When that constituency writes an insurer’s geometry into its own procurement requirements, the geometry stops being advisory.

The code baseline. The paper names NFPA 855 (2026) as the consensus bellwether document regarding the installation of energy storage systems in both occupied and unoccupied locations. It requires explosion control for every electrochemical chemistry except lead-acid and other aqueous systems. Lithium-ion indoors, in other words, carries an explosion control obligation by default under the specification, not by exception.

The one escape hatch. Tighter back-to-back spacing between racks is available on a single condition: an installation-level fire test demonstrating that thermal runaway cannot propagate between racks. That is the whole of the exception as the paper frames it. The six-foot accessible-face clearance and the nine-foot clearance to combustibles are not what the test buys. What it buys is the ability to put the non-accessible sides of two racks closer together than the data sheet would otherwise permit.

A single sentence in a specification document has therefore converted a test report into the controlling commercial variable for indoor battery density. Floor area in a data hall or a commercial electrical room is the scarce input. The test report is what unlocks it.

Fire authorities are moving faster than the code cycle. The San Diego County Fire Authority published battery energy storage system guidelines dated September 17, 2026. The document states that formal local code adoption of NFPA 855 can take a year or more, and sets the authority’s practice of applying the most recently published edition as soon as it is released, rather than waiting for the adoption cycle to catch up.

That practice matters because the 2026 edition changed what an installation has to demonstrate. It routes explosion protection through NFPA 69 gas concentration control, adds active thermal runaway propagation prevention, and introduces large-scale fire testing with suppression disabled.

The last of those is the one that reaches back into existing paperwork. A test conducted with suppression systems active is a different test, measuring a different thing, than a test conducted with suppression disabled. An authority having jurisdiction that applies the newest edition on publication compresses the window in which an older test package answers the question being asked.

Two clocks, two tolerances. The result is that the geometry of an indoor battery room is now set by documents running on different schedules. A property insurance data sheet updates at the carrier’s discretion, with no public comment period and no adoption lag. A consensus standard updates on a multi-year cycle, and then waits on state and local code bodies that may take another year or more to reference the new edition. A test procedure sits underneath both, and changes when the testing body revises it.

The three do not move together, and they do not resolve to the same answer. The insurer’s rule is the tighter of the two live constraints, and it is the one that binds a building owner whose property carrier follows FM Global, regardless of what the fire code in that jurisdiction currently references. Code compliance establishes the floor. Insurer acceptance establishes what actually gets built.

Where this lands outside the data hall. The OCP document is addressed to hyperscale operators, but the constraint it describes is not specific to them. Any commercial landlord whose property carrier follows FM Global 5-33 faces the same two-hour room and the same multi-foot clearances, and faces them without the floor plan of a purpose-built data center to absorb the cost.

For a commercial electrical room, six feet of clearance to adjacent racks and non-combustible construction, plus nine feet to combustibles, rules out most compact indoor configurations on geometry alone. The square footage does not exist. That makes the installation-level fire test the only documented path, and it makes the specificity of the test report the variable that determines whether the path is open.

The limit of a test report. A fire test demonstrates the behavior of the configuration that was tested. The further a shipped installation drifts from the tested arrangement, in ancillary equipment, in enclosure, in the surrounding structure, the weaker the inference from the report to the installation. SEAC made the same point from the code side last week: a UL 9540A report covers the installation it tested.

This is the practical consequence for manufacturers of indoor systems. The value of a test report is not binary. It is a function of how closely the tested configuration matches the configuration a customer intends to install, and that match is something a manufacturer controls through product and documentation decisions rather than something it acquires once and holds.

Two parallel developments at the same OCP summit suggest where the broader market is heading. QuantumScape announced an in-rack energy storage product built on solid-state lithium-metal cells on October 8, aimed at AI data centers, and will show it during summit week. Nickel-zinc vendors are demonstrating on the same agenda. The OCP specification itself assumes indoor battery rooms as a baseline condition rather than treating them as an edge case to be argued for.

Taken together, the direction is consistent. The question facing building owners is no longer whether lithium goes indoors. It is which evidence package permits it, and at what spacing. The documents answering that question are an insurance data sheet, a consensus standard most fire codes have yet to reference, and a test procedure. Only one of the three is under a manufacturer’s influence.

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