Utility service and medium voltage.
Everything downstream inherits its fault current from this room. It is also where the table method is most likely to run out on you, and where the question of whose electrical safety program governs has to be answered before anyone opens a door.
The boundaries change above 5 kV
At 480 V the limited approach boundary for an exposed fixed circuit part is 3 ft 6 in. Step up to the 5.1 kV–15 kV band — where most data center primary distribution lives — and it grows to 5 ft 0 in., with a restricted approach boundary of 2 ft 2 in. per Table 130.4(E)(a). Crews who move between the 480 V house and the medium-voltage room all day carry the wrong numbers in their heads more often than any other population we teach.
The boundaries apply to exposed energized conductors and circuit parts. Metal-clad gear with doors closed and secured does not put you inside a shock boundary — but interaction with it can still be an arc flash trigger under 110.2(B)(2), which is a separate question from shock entirely. The shock boundaries and the arc flash boundary are independent of each other.
Category 4 is the floor here, and the table has exits
Under the PPE category method, metal-clad switchgear from 1 kV to 15 kV is Category 4 with an arc flash boundary of 40 ft — subject to the table's own parameters: 35 kA available fault current, 0.24 s clearing, 910 mm working distance. Exceed any parameter and 130.7(C)(15)(a)(2) does not soften the category — it forbids the table method entirely and requires an incident energy analysis under 130.5(G).
At the service entrance, available fault current is set by the utility, and it is precisely where those parameters get exceeded. This is why the medium-voltage room is the strongest argument in the building for a real arc flash study: the table method was never designed to cover the stiffest bus in the facility.
Above 15 kV the calculation methods themselves thin out — IEEE 1584's model stops at 15 kV, and the remaining methods in Informative Annex D are conservative open-air approximations. If your service is 34.5 kV, your numbers come from an engineering study, not from any table or app.
Whose program governs at the demarcation
The utility's crews work to their own rules under a different regulatory regime; your crews and your contractors work to NFPA 70E and your electrical safety program. The line between those two worlds is a physical point in this room, and 110.5's host and contract employer duties are the mechanism for making it explicit. If the switchgear nameplate says the utility owns the primary compartment and you own the secondary, that sentence belongs in writing, in the ESP, and in the job briefing — not in folklore.
Racking is a Yes, with the doors open or closed
Table 130.5(C)(3) is unambiguous: racking a circuit breaker in or out is likely to produce an arc flash event whether the doors are open or closed. Remote racking exists precisely to move the operator outside the arc flash boundary during the highest-likelihood task in this room. If your gear has it, your procedures should require it; if it doesn't, that 40 ft boundary is the working reality during every rack-out.
A medium-voltage room where the arc flash labels are five years old, the utility has since reconductored the feeder, and nobody has recalculated. The label says 8 cal/cm²; the bus no longer agrees. Condition of maintenance gets audited — available fault current almost never does, and 130.5(B) requires the estimate to account for both.
Six of the nine steps in the energized-work walk get answered in this room before a tool comes out. Run it against the actual task.
This is the material of the 8- and 16-hour NFPA 70E classes — taught on the 2027 edition, against data center topology, about 55 times a year. Private onsite days run against your one-lines.