With the National Electrical Code® (NEC®) putting greater emphasis on both power continuity and selective coordination, today’s engineers need to know how to meet both these requirements in their electrical power system designs. Despite the efforts of manufacturers to aid engineers’ efforts with technical literature and other tools, I have found there are still a lot of misconceptions on these topics among electrical professionals. This is the first in a series of blog posts I’ll be writing to help you understand how circuit breakers work and how they can be used in the design of selectively coordinated systems. I’ll also be suggesting some system-design guidelines that have proven helpful.
Defining the requirements
Understanding what’s required in selective coordination means first understanding what the NEC requirements are. This post looks at updates in the 2011 and 2014 editions of the Code, beginning with an important revised definition initiated in Article 100 of the 2014 edition, shown here with new text underlined:
“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.”
From all the reading I’ve done on the topic, I’ve come to the conclusion that all of us in the industry have been pretty loose in our use of the terms “coordination” and “selective coordination.” However, the 2014 NEC has clearly defined the difference: “coordination” is partial, while “selective coordination” is total. Moving forward, we need to recognize this difference in conversations and specifications. Please pay attention and use the correct terms to communicate what you mean – and, most importantly, what you want for your projects.
Other recent selective coordination updates include:
- Article 645. This article covering Information Technology Equipment included a new definition in the 2011 NEC, in Section 645.2:
“Critical Operations Data System. An information technology equipment system that requires continuous operation for reasons of public safety, emergency management, national security, or business continuity.”
The 2014 NEC added a requirement for these systems in Section 645.27:
“645.27 Selective Coordination. Critical operations data system(s) overcurrent protective devices shall be selectively coordinated with all supply-side overcurrent protective devices.”
- Article 695. A requirement for selective coordination was added to Article 695 for Fire Pumps in the 2011 edition.
695.3 Power Source(s) for Electric Motor-Driven Fire Pumps.
(C) Multibuilding Campus-Style Complexes.
(3) Selective Coordination. The overcurrent protective device(s) in each disconnecting means shall be selectively coordinated with any other supply-side overcurrent protective device(s).
Finally, Article 620 (for elevators, dumbwaiters, escalators, moving walks, platform lifts and stairway chairlifts), Article 700 (emergency systems), Article 701 (legally required standby systems) and Article 708 (critical operation power systems) all saw the following language added regarding qualifications for those designing selectively coordinated systems and how those systems should be documented:
Selective coordination shall be selected by a licensed professional engineer or other qualified person engaged primarily in the design, installation, or maintenance of electrical systems. The selection shall be documented and made available to those authorized to design, install, inspect, maintain, and operate the system.
My next post will cover coordination requirements for health care facilities, as defined in Article 517. In the meantime, should you have questions regarding selective coordination in any current or future project, please register for Schneider Electric’s Consulting Engineer portal for a wide range of resources, including access to one-on-one support from expert staff engineers.