Article 100 Definitions. Coordination (Selective).
Code Change Summary: The existing definition was expanded.
The phrase “selective coordination” sounds complex but the actual concept is quite simple. Circuit breakers, for example, are selectively coordinated when the circuit breaker closest to a fault trips rather than circuit breakers further upstream, or ahead of the closest circuit breaker.
If a fault condition on the load side of a 20-amp branch circuit breaker causes the 200-amp main breaker in the same panelboard to trip before the 20-amp branch circuit breaker trips, then the breakers are NOT selectively coordinated. This can be dangerous for panelboards controlling emergency loads.
If a fire in one part of the building melts through an emergency circuit causing a short circuit, only the circuit breaker ahead of the melted circuit should trip, rather than the main breaker ahead of all of the other emergency circuits in the same panel.
The code change makes it clear that selective coordination means more than just choosing the right overcurrent devices. Selecting and installing them correctly is key. Part of installing them correctly is ensuring that adjustable settings are properly set. Most adjustable overcurrent devices come from the factory with the settings dialed to the most restrictive condition.
Adjustable trip circuit breakers for example, must be set in the field according to the coordination study performed by the electrical engineer. The engineer has ultimate responsibility to ensure that selective coordination has been achieved and that all adjustible overcurrent devices have been properly adjusted.
Below is a preview of the NEC®. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).
2011 Code Language:
Coordination, (Selective). Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings.
2014 Code Language:
Coordination, (Selective). Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the selection and installation of overcurrent protective devices and their ratings or settings for the full range of available overcurrents, from overload to the maximum available fault current, and for the full range of overcurrent protective device opening times associated with those overcurrents.
