Topic guide
Protective-device coordination, time-current curves, and NEC selective-coordination requirements.
Selective coordination means only the overcurrent device closest to a fault operates. Everything upstream stays closed, and loads that had nothing to do with the fault keep running. It is verified by plotting time-current characteristics for every device in a path on the same log-log axes and checking that the curves do not overlap anywhere in the fault current range that device will actually see. Where curves cross, two devices can operate on the same fault, and which one wins is a coin toss.
For some systems this is a code requirement, not a preference. The NEC requires selective coordination of overcurrent devices in emergency systems (Article 700), legally required standby systems (Article 701), and critical operations power systems (Article 708). The reasoning is straightforward: an egress lighting fault should not take out the whole emergency panel. Section numbers within those articles have moved between code cycles, so what matters on a given project is the edition your AHJ enforces.
Two things make coordination harder than it looks on paper. The first is instantaneous tripping. A molded-case breaker's instantaneous element operates in a few cycles regardless of what is downstream, so two breakers in series can both see enough current to trip instantly, and no amount of dial-turning separates them. The second is arc flash. Faster clearing lowers incident energy, and slower clearing buys coordination margin, so the two goals pull against each other on the same settings sheet. Solving one and ignoring the other is how a facility ends up with either a cascading outage or a 40 cal/cm² label.
The other thing a coordination study fixes is nuisance tripping — the main that drops when a large motor starts, or the feeder breaker that trips on transformer inrush. That is usually a pickup set too low for legitimate inrush current rather than a failing device, and the curves show it plainly. The articles below cover reading time-current curves, NEC requirements, and the coordination-versus-arc-flash trade-off. Our selective coordination service page covers what a study includes and what it costs to run alongside an arc flash analysis.
Selective coordination means only the device nearest a fault opens. What that takes, where NEC Articles 700, 701, and 708 require it, and why it fights your arc flash numbers.
More on Selective Coordination →Why healthcare essential electrical systems carry a selective coordination mandate, why the generator source changes the analysis, and what accreditation surveyors ask to see.
More on Selective Coordination →Ready for the service itself? Selective Coordination
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