Supplemental and Incidental Energy Fire Hazards in High-Rise, Mid-Rise and Large Footprint Structures: Part 2

Supplemental and incidental energy fire hazards in mid-rise, high-rise, and large-footprint structures present a growing risk that most fire departments have not yet fully accounted for. Part 1 of this series established the fundamentals: primary energy hazards — the purpose-built electrical rooms, utility vaults, and switchgear spaces that distribute power through a large building, and what the fire alarm panel reveals to a trained eye. But the hazard surface does not stop at the main electrical room door. Part 2 maps the energy that multiplies across the structure in forms no pre-incident plan marks: battery banks, backup generators, solar arrays, elevator equipment, server rooms, and the lithium-ion devices accumulating in every occupied space. These supplemental and incidental energy fire hazards turn routine alarms into crew-threatening events — and every one of them demands more air than a firefighter can carry.

But the energy inside a modern high-rise does not confine itself to dedicated electrical rooms. It multiplies across the structure in forms that are not marked on any pre-incident plan: battery banks in data closets, solar arrays radiating DC current on the roof, generators sitting in mechanical rooms with hundreds of gallons of diesel, and lithium-ion cells tucked into every apartment, every office, every hallway charging station.

These are the supplemental and incidental energy hazards — energy systems installed to support the building’s operations, and energy hazards that arise incidentally from what people bring inside and what technology accumulates. They are the subject of Part 2.

Chris G. Greene, a retired Seattle Fire Department captain and a leading voice on energy hazard response, dedicates the second half of his 2026 Fire in the Sky article to these hazards. What follows is an overview of his findings — not a reproduction of the article, but a summary of the threats every fire chief and company officer should understand before the next alarm sounds.

🔋 UPS Battery Rooms: The Energy You Don’t See Until It’s Burning

Uninterruptible power supply (UPS)

Every hospital, data center, financial trading floor, and modern commercial high-rise contains at least one Uninterruptible Power Supply room. In many buildings, every floor has one. A UPS room is essentially a room full of large-format batteries — increasingly lithium-ion — charged to capacity, waiting. Their job is to bridge the gap between utility power loss and generator startup, keeping critical systems online for the seconds or minutes before backup generation takes over.

For the fire service, the UPS room represents a hazard that is easy to overlook during pre-incident planning. The door may read nothing more than “UPS Room,” “Battery Room,” or simply “Electrical.” There may be no signage indicating what kind of batteries are inside or what condition they are in. The room will not smell like smoke until the batteries are already in thermal runaway. And when they are, the fire behaves exactly like every lithium-ion fire the fire service has learned to respect: self-sustaining, toxic, and resistant to water-based suppression. Temperatures exceeding 1,800°F are possible. The gases released include hydrogen fluoride and carbon monoxide — both lethal in concentrations far below what visible smoke would suggest.

Greene’s guidance on UPS rooms is practical and direct. Identify them during pre-incident planning. Know which floors they occupy. And if an alarm originates in or near a UPS room, treat it as a potential battery event — not a routine electrical alarm — until you have confirmed otherwise. The difference in tactics is the difference between an investigation and a hazmat response. A crew that opens that door expecting to find a tripped breaker may instead find the early stages of a fire that water alone cannot stop.

⛽ Backup Generators: The Fuel Load Nobody Discusses

The backup generator is the workhorse of building resiliency. When utility power fails, the generator starts — typically within ten to fifteen seconds — and runs on diesel, natural gas, or propane for as long as the fuel supply holds. In a high-rise, the generator is usually located in a dedicated mechanical room, on a setback floor, or occasionally on the roof. It is surrounded by fuel lines, day tanks, transfer switches, and high-amperage cabling.

Firefighters have been injured and killed by generators that activated during an incident without warning or that were not where the pre-incident plan said they would be. The generator room itself introduces a combustible fuel load — diesel in day tanks and supply lines — inside a structure that personnel may need to operate in or adjacent to. The electrical hazard is obvious. What is less obvious is that a generator fire can quickly become a fuel fire, and a fuel fire in a mechanical room at mid-rise or above is not a contents fire. It is a suppression problem with no exterior access and no margin for error.

The tactical lesson Greene emphasizes: locate the generator during size-up. Confirm with building engineering whether it is running, whether it attempted to start and failed, and whether the fuel supply has been isolated. A generator that is still running during fire operations is an active electrical hazard positioned inside the building you are trying to protect. A generator that has failed may indicate that the fire has already compromised the building’s power infrastructure — and the clues to that are on the alarm panel, as Part 1 covered in detail.

☀️ Solar Photovoltaic Arrays: DC Current That Does Not Shut Off

Rooftop solar photovoltaic arrays are now standard on new high-rise construction and are being retrofitted onto existing buildings at an accelerating pace. The operational challenge they present is unique among the hazards Greene surveys: the panels generate direct current electricity whenever light strikes them, and the conductors from the panels to the inverter remain energized even when the building’s main electrical service has been shut down.

For a crew operating on the roof — for ventilation, reconnaissance, or to access a bulkhead — the array turns the entire rooftop into an electrical hazard. Rapid shutdown requirements under the National Electrical Code have reduced the voltage between panels, but they have not eliminated the hazard. An axe, a rotary saw blade, or a firefighter themselves can still complete a circuit. And the rapid shutdown system itself can fail, especially if the fire has already damaged the wiring that controls it.

Beyond the electrical hazard, the array itself adds fire load. Panels, mounting rails, plastic junction boxes, wiring conduits — all of it burns. And the fire it produces is directly above the occupied floors, feeding smoke and heat into the very spaces crews are trying to protect from below. Greene calls attention to a gap in pre-incident planning that affects most departments: the solar array is rarely documented in a way that is operationally useful. Crews need to know where the array is, where the inverter is, where the DC disconnect is, and whether the array is integrated into the building’s fire alarm system. A rooftop that looks clear from the street may be covered in energized panels — and the time to learn that is not during vertical ventilation.

🛗 Incidental Energy Fire Hazards: When the Building’s Contents Become the Fuel

Energy Hazards

Beyond the dedicated energy rooms and engineered systems, modern buildings accumulate energy hazards incidentally — through the things people bring inside, the equipment they install, and the technology that multiplies faster than the fire code can track.

The most widespread of these is lithium-ion battery sprawl. E-bikes and electric scooters plugged into hallway outlets. Power tool batteries charging in maintenance closets. Laptops, tablets, and phones by the thousand in a single residential tower. Portable power banks stacked in retail storage. Hoverboards, drones, cordless vacuum batteries. None of these are individually remarkable. Collectively, they turn a 30-story residential building into a distributed battery storage facility — without any of the fire protection that a purpose-built BESS installation would require.

When one of these cells enters thermal runaway — and every firefighter has now seen the video footage of what that looks like — the fire it produces can overwhelm a compartment in under a minute. In a high-rise, that compartment is a single apartment. Below it are twenty floors of occupied units. Above it are ten more. The fire does not need to be large to be deadly. It needs to be in the wrong place at the wrong time, producing gases that migrate through the building faster than crews can climb toward the seat of the fire.

Elevator equipment rooms present a different kind of incidental hazard. Modern traction elevators use regenerative drives that feed energy back into the building’s electrical system. The motor-generator sets, the control cabinets, and the shaft itself concentrate electrical equipment in a vertical chase that runs the full height of the building. A fire in an elevator machine room — or a fire that breaches the shaft — has a ready-made path for smoke, heat, and toxic gases to reach every floor. Greene notes that these rooms are frequently locked, often lack adequate sprinkler coverage for the fire load they contain, and are almost never marked on the fire alarm panel in a way that tells the first-arriving company what is burning behind that door.

Server rooms and telecommunications closets add still more energy density. A single rack of network equipment can draw as much power as a small apartment, and a large commercial building may have dozens of these rooms distributed across its floors — often colocated with UPS batteries. They are rarely sprinklered at the density the fire load demands. They are often secured. And the first indication that a fire has started in one may be a smoke detector activation three floors away, carried through the cable trays and conduit penetrations that connect every telecom room in the building.

⚡ Distributed Energy Fire Resources: The Next Frontier

Greene closes the article with a look at what is coming. Distributed energy resources — microgrids, fuel cells, on-site battery storage at the multi-megawatt scale, and building-integrated wind turbines — are being incentivized by utility policy, sustainability mandates, and corporate ESG commitments. They are arriving without corresponding requirements for firefighter access, emergency shutdown procedures, or air replenishment infrastructure.

The pattern is consistent and, by now, familiar. The technology arrives first. The fire code follows — sometimes years later, sometimes never at the local level. The gap between what is installed and what firefighters are prepared to handle is not an accident of poor training. It is a structural consequence of a permitting process that treats energy systems primarily as engineering problems rather than life-safety problems. The building department reviews the electrical load calculation. Nobody reviews the air supply calculation for the crew that will respond when the system fails.

The Thread That Ties It All Together: Air

Every energy hazard covered in this two-part series — the primary hazards of vaults and switchgear rooms from Part 1, the supplemental hazards of UPS rooms, generators, and solar arrays, and the incidental hazards of lithium-ion sprawl, elevator equipment, and server rooms — shares a single common thread. They burn longer than a standard SCBA cylinder lasts. They produce environments that are toxic, superheated, and located in parts of the building where access, egress, and air resupply are at their hardest. And these hazards are being installed in buildings that were permitted without any requirement for the infrastructure that would allow firefighters to safely and sustainably operate against them.

A UPS battery fire in a mid-rise data closet requires a sustained interior response. A generator fuel fire in a mechanical room above the tenth floor requires suppression in a confined space with limited egress. A lithium-ion thermal runaway event in a high-rise apartment requires crews working in an immediately toxic atmosphere with zero visibility. A solar array fire on a 25-story rooftop requires operations at elevation with no margin for air supply error. Not one of these scenarios can be resolved within the fifteen to twenty minutes of working air a firefighter carries on their back.

Firefighter Air Replenishment Systems — FARS — address this gap directly. Pre-plumbed, pressurized air standpipe networks built into stairwells and operational areas allow crews to refill SCBA cylinders without leaving the building. They are the air equivalent of a water standpipe: infrastructure that extends the reach of firefighting operations into structures where portable supply alone cannot keep up with the demand. The International Fire Code Appendix L, Uniform Fire Code Appendix F, and NFPA all contain provisions for FARS. More than 400 jurisdictions across 26 states have adopted them.

But the energy systems are being installed faster than the code requirements that should accompany them. A building that contains UPS battery rooms, backup generators, rooftop solar, lithium-ion devices in every unit, and a distributed energy resource installation — and was permitted without FARS — is a building that was designed for a fireground that no longer exists.

What Fire Departments Can Do Now

The recommendations that emerge from Greene’s work are straightforward and actionable. Identify every UPS room, generator room, solar array, and battery storage location in the high-rises in your response area. Document them. Put them in the pre-incident plan in language a company officer can absorb on the way to the call.

Train every company officer to recognize the alarm panel signatures of an energy fire. If the building has lost power and the egress lights are glowing during the day, the incident is not a routine alarm — it is an energy event until proven otherwise. If a smoke detector activates in a UPS room, a generator room, or a telecom closet, the response posture needs to shift from investigation to cautious assessment, because what is behind that door may not respond to the tactics a contents fire would call for.

And most critically: assess whether your crews can sustain an interior attack against the worst energy fire any of these buildings could produce — on the air they carry. Because every energy fire, whether it starts in a UPS room, a generator housing, a solar array, or an e-bike in a studio apartment, places the same operational demand on the firefighter: it will burn longer than a single SCBA cylinder lasts. The countdown starts before the attack begins. The air has to last until the job is done. Where the answer is no — and for most departments, it will be — that gap needs to be documented, communicated to municipal leadership, and addressed at the permit counter. The window to require the infrastructure that makes these hazards survivable is the design phase. Once the building is occupied, the hazards are locked in.

The training gap Greene describes is real. But gaps can be filled. And the work begins with the first-due company, the first five minutes, and the first officer who knows what they are looking at.

Read the full article: Energy Hazard Fire Considerations for High-Rise and Large-Scale Buildings — Chris G. Greene, Fire Engineering, Firefighter Air Supplement, 2026.

The Fire in the Sky supplement is published annually by the Firefighter Air Coalition in partnership with Fire Engineering. The 2026 edition is now available.