Selective Coordination Requirements NEC 700,701 and 702 Systems

Last updated: July 12^th, 2024

SELECTIVE COORDINATION

What is selective coordination? Selective coordination is a design factor of a power system for protective devices. Protective devices are used in the system which keeps the system safe during fault condition. While designing a power system, our duty is to ensure that the protective devices are adequately sized to protect their downstream system which includes loads, cables, protective devices, panel boards and other useful equipment of power system. To achieve this, we must selectively coordinate PDs. By achieving selective coordination, it allows the nearest upstream Protective device to operate under fault condition, avoids unnecessary tripping of downstream devices, achieve continuity of electric feed for critical loads.

IMPORTANCE OF SELECTIVE COORDINATION

1. Compliance of code
   Selective coordination is mandatory in some critical or emergency systems by NEC. To ensure the uninterrupted power to the loads that are critical and required by the state or government for the continuous operations.
2. Safety
   It keeps the system isolated from the fault current by operating the protective device when it is needed.
3. Reliability
   In the case of power outages where electric supply to loads is mandatory from an alternative source of power, selective coordination please an important role to the system.

NEC REQUIREMENTS FOR SELECTIVE COORDINATION

There are some key NEC sections which are listed below.

1. NEC 700.28 (Emergency systems)
NEC Article 700.28 states: “Emergency system(s) overcurrent devices shall be selectively coordinated with all supply-side OCPDs. Selective coordination shall be selected by a licensed professional engineer or other qualified persons 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.”
In case of any short circuit or abnormal condition in the room of hospital, it should only trip 30 amps feeder breaker of that room in the panelboards rather than tripping the main breaker 200 amps in the panel board protecting the entire board to trip. Poor coordination can lead to the power outage if the main breaker of that panelboard serving critical operations of different facilities that includes patients being treated is tripped due to the miss coordination of protective devices, patients can be led to risk. NEC ensures the reliability of the system to avoid these risks.

2. NEC 620.62 (Elevator circuits)
When more than one driving machine is fed from a single feeder, selective coordination is required between the overcurrent protective device in each disconnecting means and any other supply side overcurrent protective devices. Selective coordination for elevator overcurrent protective devices is critical. For example, in Figure below, if a fault were to occur that would cause overcurrent protective device M1 to open, all the elevators in the building would lose power. One of the reasons that coordination is so important is because firefighters commonly use the elevator to get closer to a fire during fire-fighting operations. (Neeser, 2017). Refer to the single line diagram below.If elevators are supplied by emergency, legally required standby or critical operation power systems, then selective coordination is required per the 2017 NEC in 700.32, 701.27 or 708.54. Selective coordination is also required for multiple elevators per NEC 620.62. Another important electrical system design consideration for supplying power to multiple elevators is NEC 620.62. NEC 620.62 requires the elevator disconnecting means (fused switch or circuit breaker) to selectively coordinate with all supply-side overcurrent devices. (Neeser, 2017) . Refer to the single line diagram below.

3. NEC 517.26 (Health care essential system)

“This code is specially for health care facilities. The life safety branch of the essential electrical system shall meet the requirements of article 700, except as amended by article 517.

1. Section 700.4
   shall not apply.
2. Section 700.10(D)
   shall not apply.
3. Section 700.17
   shall be replaced with the following: branch circuits that supply emergency lighting shall be installed to provide service from a source complying with 700.12 when normal supply for lighting is interrupted or where single circuits supply luminaires containing second batteries.
4. Section 700.32
   shall not apply.” (Health Care Facilities, n.d.)

Article 700 in the NEC® is all about emergency systems. Those systems consist of circuits and equipment intended to supply, distribute, and control electricity for illumination, power, or both, to required facilities when the normal electrical supply or system is interrupted. (Application of other Articles, n.d.)

Emergency systems include things like emergency egress lighting, exit signs, some elevators, fire protection and alarms, public safety communication systems and the like in the event of failure of the normal power supply or in the event of accident to elements of a system intended to supply, distribute, and control power and illumination essential for safety to human life.

In the 2011 NEC®, section 517.26 required the entire essential electrical system in a health care facility to comply with the rules in Article 700 for emergency systems. This did not make sense because the essential electrical system includes the equipment system which does not generally supply life safety loads.

In the 2014 NEC®, the revised text makes it clear that only the life safety branch of the essential electrical system must comply with Article 700 which makes more sense because the life safety branch supplies all of the life safety systems needed for safely egressing the building. (Application of other Articles, n.d.). Refer to the single line diagram below.

517.26 Application of Other Articles (health Care Facilities)

Life safely branch of the essential elecrical system shall meet the requirements of Article 700, except as amended by Article 517

Branch circuits that supply emergency lighting shall be installed to provide service from a source complying with 700.12 when normal supply for lighting is interrupted or where single circuits supply luminaries containing secondary batteries (amended from 700.17)

4. NEC 708.54 (critical operational power systems)

In a critical operations power system, it is crucial that the electrical system remains functioning. According to informational note 1 in 708.1, critical operations power systems are usually installed in vital infrastructure facilities that, if destroyed or incapacitated, would disrupt national security, the economy, public health, or safety; and where enhanced electrical infrastructure for continuity of operation has been deemed necessary by governmental authority.

In those facilities, the circuit breaker closest to a fault condition at the load end should clear the fault before the main breaker protecting the entire panel board does. It is required by the NEC that all the overcurrent protective devices should be selectively coordinated to ensure that the remainder of the branch circuits in the same panel remain energized and the system is still operational during fault. (Selective Coordination, n.d.).

For example, if a building is caught with a fire caused by the short circuit, and due to the miscoordination the breakers feeding the water sprinklers is also operated to close the circuit, the building will not be able to extinguish the fire and will lead to an unwanted result. NEC codes are there to protect the facilities and critical operational areas to remain operational during a natural or human caused disasters.

Examples of facilities using Critical operational power systems:

• Police stations.
• Fire stations.
• Emergency management centers.
• National security facilities.
• Hospitals.
• National economic security (financial facilities).

5. NEC 701.27 (legally required standby systems)

NEC Article 701.27 states: “Legally required standby system(s) overcurrent devices shall be selectively coordinated with all supply-side OCPDs.” The protective device upstream of the automatic-transfer device is also part of these “supply-side OCPDs.” As such, it should be coordinated with the emergency system protective devices downstream of the automatic-transfer device. (PACUKU, 2016). Refer to the single line diagram below.

How to achieve Selective Coordination

System Design

1. Breaker Selection
   Selection of breaker plays a very important role at the beginning stage, either it should be a thermal magnetic breaker or LSI breaker or any other breaker which is suitable for our desired system.
2. Calculation of available fault current
   Knowing the fault current prior selecting the Circuit Breaker at various points is very important in order to reduce the chances and magnitude of arc flash
3. Adjusting Breaker Settings
   Adjusting the breaker trip settings to achieve the coordination between the Circuit Breakers.
4. On-site commissioning
   On-site commissioning is the last stage to verify the studies during a live system.

Selective coordination is more than just technical requirements, it’s a pillar of electrical safety and reliability. Engineers must understand the importance of selective coordination and how to implement it more effectively. Achieving selective coordination in the system allows us to achieve continuous power supply to our critical loads even during fault condition.

Remember to keep track of code of records during design and always seek professional help while designing or performing studies.

Contact AllumiaX today to let us know about the safety, reliability, and protection requirements in your facility. Our team of expert engineers and technicians will get in touch with you.

AllumiaX is a licensed Engineering company headquartered in Seattle, Washington USA. We work with general-contractors, electrical contractors, facility owners, gear manufacturers and EPC firms to deliver various Power System Engineering studies and consulting including Arc Flash, Short circuit , Protection Coordination , Load Flow, Transient Stability, Thermography and more. Get in touch with us or Request a Quote.