Selective Coordination

What’s New

NEC Emergency System Generators and Circuit Wiring
The NEC provides specific requirements in order to ensure the safe installation, operation, and maintenance of emergency systems intended to provide continuity of power to required facilities and equipment when there is a loss of the normal electrical supply. Equipment arrangement and circuit wiring are keys to meeting the NEC requirements for emergency systems and Chad Kennedy explains how to accomplish this task in this Electrical Shortz data bulletin from the Schneider Electric North America Codes & Standards group.
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Guide to Low Voltage Transformer Protection and Selective Coordination
These guidelines serve as a reference for selectively coordinating your electrical system using molded case and miniature circuit breakers that are designed to the UL 489 Standard.
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- Makes it easier to design to the most recent NEC selective coordination requirements.
- Identifies the primary, secondary and branch breakers to properly protect the transformer and also determines the selective coordination levels of the breaker combinations. (table on page 3)
- Removes the time needed to do a system study for each combination by documenting each combination with the specific TCC details.

NEC Selective Coordination – Up to Which Source
Since the requirements for selective coordination were added to the 2005 and 2008 NEC there has been a question regarding what the word “all” means in the requirements. Code Making Panel 13 has answered the question and Ed Larsen explains the answer in this Electrical Shortz data bulletin from the Schneider Electric North America Codes & Standards group.
View or download (1.2MBpdf)

What is selective coordination?

The term selective coordination refers to the selection and setting of protective devices in an electric power system in such a manner as to cause the smallest possible portion of the system to be de-energized due to an abnormal condition. This ensures any over-current event is first tripped by the smallest breaker in the system before allowing a larger main breaker to intercept the fault. This limits the service interruption to only the circuit experiencing the problem and does not shut down a larger portion of the facility.

How is selective coordination achieved?

In most cases selective coordination is achieved via the timing characteristics of the devices to be coordinated. For example, each of the circuit breakers in system has its own time-current characteristic; by coordinating these, selective coordination may be achieved. This is usually accomplished by comparing the device time-current characteristics to ensure there is no overlap in protection between the upstream and downstream circuits. For more information regarding selective coordination system design view the Selective Coordination Streamlining System Design brochure.

How can I optimize my design for selective coordination?

1. Likely the biggest benefit is to perform a study to determine the short circuit current available at various points in the system. This may allow the selection of circuit breakers with a lower interrupting rating and a higher instantaneous trip level.
2. Work from the bottom up, starting from bottom of the system and coordinate the branch lighting panels first, then the power distribution panels, then the switchboard or switchgear.
3. Use the short circuit selective coordination tables. They may be found in the Short Circuit Selective Coordination for Low Voltage Circuit Breakers data bulletin.
4. You may also increase the frame size of main or feeder breakers which increases the instantaneous trip level of these breakers.
5. Incorporating ground fault protection into feeder and branch circuits so that low level ground faults will be cleared before they escalated into high level phase-to-phase faults.

Additional Resources

Selectivity Guidelines For Square D® Panelboards

The natural advantages of circuit breakers make them the logical choice for overcurrent protection. New requirements in the National Electrical Code for emergency and legally required standby systems make it advantageous to consider selective coordination at the beginning of the design process.
View or download (104Kpdf)

Guide to Power System Selective Coordination 600V and Below

With the inclusion of new language in the 2005 National Electrical Code® (NEC®), the requirements for selective coordination of electrical power systems are, at present, more stringent than ever before.
View or download (962Kpdf)

Enhancing Short Circuit Selective Coordination with Low Voltage Circuit Breakers

Presents techniques for improving the short circuit selective coordination of low voltage circuit breakers used in electrical distribution systems.
View or download (1.2MBpdf)