A fault on a receptacle circuit in an ordinary office building that takes out a feeder is an inconvenience. The same cascade on a hospital's critical branch is an event with patients attached to it. That is the reasoning behind one of the strictest protective-device requirements in the NEC, and why coordination work in healthcare is its own discipline.
Why do hospitals carry a selective coordination mandate?
Healthcare facilities run an essential electrical system — the separately-fed portion of the distribution that keeps operating when normal power fails. NEC Article 517 organizes it into branches: the life safety branch (egress lighting, alarms), the critical branch (patient care spaces), and the equipment branch (the mechanical loads that keep the building survivable). Each is supplied through its own automatic transfer switches from both the normal source and the generator plant.
Those branch classifications tie the essential system into the emergency-system rules of Articles 700 and 701 — and with them, the selective coordination requirement: a fault anywhere in those paths must be cleared by the nearest device, leaving the rest of the branch energized. The general mechanics of that requirement — curves, tables, and the instantaneous-region problem — are covered in What Is Selective Coordination?; this article is about what changes in a hospital.
Why does the generator make coordination harder?
The essential system runs from two sources, and they behave nothing alike.
The utility can deliver tens of thousands of amps into a fault. The generator plant delivers a fraction of that — a generator's fault contribution is limited by its own reactances, and it decays as the fault persists. The consequence is that every device in the essential system has two operating scenarios:
- On utility power, fault currents are high, devices operate in the instantaneous region, and the coordination problem is the usual race between fast devices.
- On generator power, the same fault may produce so little current that downstream breakers operate slowly — or a device sized around utility-level faults never reaches its instantaneous pickup at all. Curves that cleared each other comfortably at 30 kA can overlap at 3 kA.
Selectivity demonstrated only on the utility-source plot is half an analysis. The study has to run both scenarios, which also feeds the arc flash side: generator-source faults that clear slowly are precisely the ones that accumulate incident energy, which is why our arc flash and coordination analyses run on the same model with the same source scenarios.
What about the transfer switches?
Automatic transfer switches complicate the topology in a way ordinary buildings never see. Each ATS creates a junction where the same downstream distribution can be fed from either source, so the device coordination upstream of the ATS differs by source while everything downstream must stay selective in both cases. ATS withstand and closing ratings also have to be checked against the calculated fault current — a transfer switch is equipment under NEC 110.10 like everything else, and it is routinely the piece nobody has looked at since commissioning.
What do surveyors and AHJs actually ask for?
Hospitals answer to more inspectors than almost any other facility type: the AHJ enforcing the NEC edition adopted locally, CMS conditions of participation, and accreditation surveys. On the electrical side, what gets requested is consistent:
- The study. Time-current curve plots covering the essential-system paths, under both sources, with the study date visible.
- The settings schedule. Every adjustable device's as-set values — and they must match what is physically dialed in the field. A study that says short-time delay 0.3 s while the trip unit in the gear says 0.1 s is worse than no study, because it documents that nobody closed the loop.
- Professional responsibility. The analysis performed and sealed by a licensed engineer. Every PowerSafe study ships that way — it is the difference between an opinion and a defensible document.
When should an existing hospital re-run coordination?
The triggers are the same ones that invalidate an arc flash study, because they move the same inputs: a generator addition or replacement, new or re-rated transfer switches, a utility service change, renovations that added load to a branch, or protective device replacements where the new trip units never got set to the study values. NFPA 70E's five-year review cycle for the arc flash risk assessment is a natural point to revalidate coordination on the same model — one field effort, one model, both deliverables.
The selective coordination service page covers scope and turnaround. For healthcare work, ask specifically that both source scenarios appear in the deliverable — if the proposal doesn't say "generator source," the plot set you receive probably won't either.