The roof is not the only ceiling
Commercial solar discussions usually begin with roof area. How many modules fit, how much of the roof is usable, how much shading is there. Those are real constraints, but they are not the only ones. Behind the wall in the electrical room sits equipment with its own ratings, and a system that fits the roof can still be too large for the service that has to carry it.
The earlier this is known, the cheaper it is. A designer who learns the limit at the end redraws the array. A designer who learns it at the start sizes the array to the limit and spends effort on the options that remain.
The pieces that set the limit
Service size and voltage
The utility service has an ampere rating and a voltage configuration, for example three-phase at a given voltage. The nameplate on the main switchgear or the service entrance equipment shows these. The inverter configuration must match the building's voltage, and the size of the service sets the context for everything downstream.
Main breaker and bus rating
The main breaker protects the bus, and the bus has its own rating. Where solar connects to a busbar, the National Electrical Code limits the combined overcurrent protection feeding that bus relative to the bus rating. The rule is often summarized as the 120 percent rule for load-side connections. The practical effect is that a bus that is already heavily loaded by the main breaker leaves limited room for a solar breaker.
Physical space in the gear
Even where the ratings work, there may be no open space for another breaker, no room for a new section, or no clear working space in front of the equipment. Space is a survey finding, not a calculation, which is why the photographs matter.
Available fault current and equipment ratings
Adding a source changes the fault current the equipment must withstand. The engineer will want the existing equipment's short-circuit ratings and any available utility fault data, so the survey should capture the nameplate details rather than assume them.
Two ways to connect, and why the survey must support both
A load-side connection ties the solar into the existing bus through a breaker. A supply-side connection ties in ahead of the main breaker, typically with a tap or a new enclosure. Which one is possible depends on the gear, the space and the ratings. If the survey records enough for the engineer to evaluate both, the project is not stalled when the first option fails. The note on electrical service and interconnection survey items goes through the capture list in detail.
What to photograph and write down
- Nameplates on the main gear, the main breaker and any panelboards, readable in the frame, and also transcribed.
- The interior of the gear with the dead front removed where the facility allows it, showing the bus and available spaces.
- Working clearances in front of the equipment, with a tape measure in the shot.
- The route from the proposed array location to the gear, including walls, floors and obstructions the conduit would cross.
- Existing labels and single-line diagrams posted in the room, even if they look out of date.
Our guide on main switchgear photos and documentation explains the shot list and the transcription habit that prevent the most common callbacks.
What to do with the answer
If the electrical limit is below the size the roof could support, there are only a few honest options: size the array to the limit, change the point of connection, or have the owner and the utility discuss equipment upgrades. The survey does not choose among them. It makes sure the choice is made on the real equipment and not on a guess. The licensed engineer then confirms what the equipment can support.
Questions to put to the owner before the visit
A few questions asked early save a trip. Is there a recent single-line diagram, and who maintains the electrical room? Has the service been upgraded or has equipment been added since the building opened? Is more than one meter or service on the property? Are there plans to add load, such as vehicle charging or new equipment, that would change how the building uses its service? The owner's answers do not replace the survey, but they tell the crew what to look for and who to ask for access to locked rooms and enclosures.
Where Solar Survey AI fits
Our commercial on-site survey covers the roof, the structure and the electrical room in one visit, with turnaround confirmed when we scope the job. The electrical record goes to engineering stamped by a licensed professional engineer where required, so the size of the system is checked against the building as measured. Use us when you need the electrical room documented before the layout is fixed.