1. Selective Coordination
Justin Schroeder, P.E. 1

2. Introduction Definition NEC coordination requirements
2011 and 2014 editions
NFPA requirements
NFPA requirements
Circuit breaker principles
Resources and product solutions
Challenges meeting the NEC requirements
Design guidelines
Summary 2

3. Definition: What is selective coordination?
Article 100 defines selective coordination as…
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.
In other words…
Only the overcurrent protective device (OCPD) nearest to a fault should clear the fault
Change

4. Selective Coordination Definition
The concept of protective zones is a useful tool to visualize this.
Fault in this zone
 CB M1 Trips
 CB F1 Trips
 CB PM1 Trips
 CB B1 Trips

5. NEC Selective Coordination Requirements
Article 517 Health Care Facilities
517.17(B) Feeders.
Deleted the prohibition of a second step of GFPE on the generator feeder
2 levels of GFPE when main has GFPE per (C)
No GFPE on load side of ATS
517.17(C) Selectivity.
Deleted the requirement for 6 cycles of separation
GFPE must be selectively coordinated.
These changes should make coordination easier to achieve
Change

6. NEC Selective Coordination Requirements
Article 517 Health Care Facilities
Application of Other Articles.
The life safety branch of the essential electrical system shall meet the requirements of Article 700, except as amended by Article 517 and NFPA 99 Chapter 6.
Informational Note No. 1: For additional information see NFPA , Standard for Emergency and Standby Power Systems.
Informational Note No. 2: For additional information see and NFPA 99 Chapter 6.
Change

7. Typical Health-Care Facility Electrical System (Informational Note Figure 517.30)
Change
Must meet Article 700 per 2014 edition
Changed to “branch” in 2014 edition
Deleted in 2014 edition
Must meet Article 700 per editions

8. Definitions
The equipment system (Equipment Branch) – Meant for essential equipment loads
Feeds pumps, control systems, vacuums, essential heating, one elevator.
Can be fed by an alternate source at appropriate time-lag intervals.
The emergency system
Life safety loads (Life safety Branch) – Meant for egress
Exit signs
Elevators
Automatic doors
Critical loads (Critical Branch) – Meant for patient care
Select receptacles, power circuits related to patient care, task illumination
Change

9. NEC Selective Coordination Requirements
Article 517 Health Care Facilities
Essential Electrical Systems for Hospitals.
(G) Coordination. Overcurrent protective devices serving the essential electrical system shall be coordinated for the period of time that a fault’s duration extends beyond 0.1 second.
Exception No. 1: Between transformer primary and secondary overcurrent protective devices, where only one overcurrent protective device or set of overcurrent protective devices exists on the transformer secondary.
Exception No. 2: Between overcurrent protective devices of the same size (ampere rating) in series.
Informational Note: The terms coordination and coordinated as used in this section do not cover the full range of overcurrent conditions.
New

10. Exception No 1

11. Exception No. 2

12. Cable Riser Example

13. NEC Selective Coordination Requirements
Article 620 Elevators, Dumbwaiters, Escalators, Moving Sidewalks, Wheelchair Lifts, and Stairway Lift Chairs
Selective Coordination.
Where more than one driving machine disconnecting means is supplied by a single feeder, the overcurrent protective devices in each disconnecting means shall be selectively coordinated with any other supply side overcurrent protective devices. 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.
Change

14. Inspection Form
Inspection form helps assure the AHJ that the equipment supplied and installed meets code requirements.

15. NEC Selective Coordination Requirements
New
Article 645 Information Technology Equipment
645.2 Definitions.
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.
Selective Coordination. Critical operations data system(s) overcurrent protective devices shall be selectively coordinated with all supply-side overcurrent protective devices.

16. NEC Selective Coordination Requirements
Article 695 Fire Pumps
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).
New

17. NEC Selective Coordination Requirements
Article 700 Emergency Systems
Selective Coordination. Emergency system(s) overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices.
Article 701 Legally Required Standby Systems
Selective Coordination. Legally required standby
system(s) overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices.
Exception to Both
Exception: Selective coordination shall not be required between two overcurrent devices located in series if no loads are connected in parallel with the downstream device.

18. NEC Selective Coordination Requirements
Articles 700 Emergency Systems and 701 Legally Required Standby Systems
Added to and
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.
Change

19. NEC Selective Coordination Requirements
Article 708 Critical Operations Power Systems
Added to (D) GFPE selectivity
Separation of ground-fault protection time-current characteristics shall conform to the manufacturer’s recommendations and shall consider all required tolerances and disconnect operating time to achieve 100 percent selectivity.
Same text as (C) GFPE selectivity for health care facilities
Change

20. NEC Selective Coordination Requirements
Article 708 Critical Operations Power Systems
Selective Coordination. Critical operations power
system(s) overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices.
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.
Exception: Selective coordination shall not be required between two overcurrent devices located in series if no loads are connected in parallel with the downstream device.
Change

21. Where is it required again?
Article 620 Elevators, Dumbwaiters, Escalators, Moving Sidewalks, Wheelchair Lifts, and Stairway Lift Chairs
Article 645 Information Technology Equipment (Critical Operation Data Systems)
Article 695 Fire Pumps – campus style building layouts
Article 700 Emergency Systems
Article 701 Legally Required Standby Systems
Article 708 Critical Operations Power Systems (COPS)

22. 2014 NEC Errata: Deleted Figures
Oops!

23. State Adoptions of NFPA -70 (NEC)
GA adopted the 2014 edition at the beginning of 2015
FL, SC, NC, AL are all still on 2011
TN 2008 with 2009 amendments
How does this affect you?
Different states will have different requirements and amendments to different code cycles so be aware! Currently in GA, the 2014 code has been adopted.
NEC-2014 went to great lengths to standardize their terminology and definitions which may mean slightly different things to older code cycles.

24. NFPA 99-2012 Health Care Facilities Code
Added three new sections* to read:
Overcurrent protective devices serving the essential electrical system shall be selectively coordinated down to 0.1 seconds.
Added three new annex sections** to read:
It is important that the various overcurrent devices be coordinated, as far as practicable, to isolate faulted circuits and to protect against cascading operation on short circuit faults. In many systems, however, full coordination could compromise safety and system reliability…
* , , ** A , A , A
Change

25. NFPA 110-2013 Standard for Emergency and Standby Power Systems
6.5 Protection General. The overcurrent protective devices in the EPSS shall be coordinated to optimize selective tripping of the circuit overcurrent protective devices when a short circuit occurs Short Circuit Current. The maximum available short circuit current from both the utility source and the emergency energy source shall be evaluated for the ability to satisfy this coordination capability. A It is important that the various overcurrent devices be coordinated, as far as practicable, to isolate faulted circuits and to protect against cascading operation on short circuit faults. In many systems, however, full coordination is not practicable without using equipment that could be prohibitively costly or undesirable for other reasons…

26. Do NFPA 99-2012 and NFPA 110-2013 conflict with the NEC? No!
NFPA 70 (NEC)
“This Code covers the installation of electrical conductors, equipment,…”
NFPA 99
“Chapter 6 covers the performance, maintenance and testing of electrical systems…”
NFPA 110
“This standard covers performance requirements for emergency and standby power systems…”
Standards Council
“NFPA 99 is considered the performance requirement and the NEC is considered the installation requirement.” (Similar statement made regarding NFPA 110)

27. Circuit Breaker Principles
Time-Current Curves (TCCs)
Developed by testing a single circuit breaker by itself
Two regions
Overload region: where the circuit breaker has an inverse time characteristic
Instantaneous region: where the circuit breaker operates “instantaneously”
“The instantaneous region is typically difficult to coordinate for two OCPDs connected in series”
Source: IEC/TR
CURRENT
TIME
Instantaneous Region
Overload Region

28. Circuit Breaker Principles
Selective Coordination with 2 Circuit Breakers in Series: Time-Current Curves (TCCs) vs. Tested Levels
TCCs
Developed by testing a circuit breaker by itself
In the instantaneous region may not be valid for two circuit breakers connected in series
Tested Levels
Take into account the current limiting properties and dynamic impedance of circuit breakers
Developed by comparing the actual let-through current of the downstream circuit breaker with minimum instantaneous trip of the upstream circuit breaker
Same principle as fuse ratio tables

29. Resources Manufacturer references?
Comparable selective coordination tables and tools are available from Schneider Electric, Eaton, GE and Siemens.
3rd Party System Analysis Software
Selective coordination information available in SKM, ETAP and Easypower products
NEMA ABP Selective Coordination White Paper
Provides guidance on how to comply with selective coordination requirements for design engineers and AHJs.

30. Challenges Meeting the NEC
Is selective coordination required up to both the normal and alternate sources, or only up to the alternate source?
700.1 Scope.
The provisions of this article apply to…emergency systems…intended to supply, distribute, and control electricity…when the normal electrical supply or system is interrupted…
These systems are intended to automatically supply illumination, power, or both…in the event of failure of the normal supply...
The scope seems to imply only up to the alternate source

31. Challenges Meeting the NEC
Is selective coordination required up to both the normal and alternate sources, or only up to the alternate source?
Appendix B.1 of NFPA 110 seems to imply only up to the alternate source

32. Challenges Meeting the NEC
Is selective coordination required up to both the normal and alternate sources, or only up to the alternate source?
Coordination.
Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.
Some interpret “all” to mean up to both sources

33. Challenges Meeting the NEC
Is selective coordination required up to both the normal and alternate sources, or only up to the alternate source?
2011 NEC proposal panel statement:
“…devices in the normal source…are outside the scope of Article 700.”
Use NEC Selective Coordination—Up to Which Source? (0600DB0902) to convince the AHJ that total coordination is required only up to the alternate source
Level of coordination up to the normal source is a matter of engineering judgment
In the 2014 NEC revision cycle CMP13 reversed itself, stating that selective coordination is required up to both sources (ROP & ROC 13-83) March 8, 2012

34. Selective Coordination Update
Common misunderstandings and pitfalls
Terminology review – 0.1s versus “total”
We don’t say 0.01s, 0.05s, 0.001s, etc.
0.1s – selectivity determined by TCCs – accounts for all initial arcing and ground faults
Total – selectivity determined by tables comparing tested load-side let-throughs and line-side instantaneous trips* (AND TCCs) – “selective for the entire operating ranges of the protective devices and for all levels of available fault current”
0.01s – terminology used, we believe mistakenly, intended for total coordination
* - SC selectivity based on test information

35. Selective Coordination Update
Common misunderstandings and pitfalls
Why we don’t say 0.01s, 0.05s, 0.001s, etc.
If the fault current exceeds their current limiting point, current limiting fuses operate in less than 0.01s
Current limiting fuses that coordinate down to 0.01s may not be coordinated at higher fault currents
Current limiting fuse TCCs cut off at 0.01s because only the ratio tables can be used to evaluate total coordination below that point

36. Challenges Meeting the NEC
Cautions
Make sure automatic transfer switches have an adequate withstand rating (30 cycle ratings vs. 3 cycle ratings)
May need to move the switch away from the source, or
May need to increase the frame size of the switch, or
May need to specify a switch with a higher withstand rating (more than 3 cycles)
Make sure busway has adequate withstand ratings

37. Challenges Meeting the NEC
Short-circuit Decrement Curve of 250 kW Generator Plotted on 300 A LA curve
Cautions
Make sure the generator protection devices coordinate with the downstream circuit breakers
Some manufacturers provide a time delay
Generator fault current
SCAgen = Gen FLC / x”d
x’’d => generator subtransient reactance
Gen FLC = kVA / (kVL-L * √3)

38. Challenges Meeting the NEC
Cautions
Mixing Overcurrent Protective Devices
TCCs may be used so long as the fault current does not exceed the instantaneous trip point of the upstream circuit breaker or current limiting point of the upstream fuse
OCPDs from different manufacturers or of different types usually cannot be mixed if the fault current is outside the parameters described above

39. Challenges Meeting the NEC
Cautions
Arc Flash
Selective coordination may impact arc flash incident energy levels
May be possible to reduce with…
Zone Selective Interlocking
Use of breakers LSI rather than the traditional thermal magnetic breaker.
Other means

40. Design Guidelines
Conduct Preliminary Short-Circuit & Selective Coordination Studies First
Reduce the number of levels (buses) that need to be coordinated to 3 or 4 if possible
Before letting a job out for bid, conduct preliminary short circuit and selective coordination studies first as they may affect the system design
Consider 3-phase and ground fault currents
If a lack of coordination seems to exist using the TCCs, use the Short Circuit Selective Coordination Tables
Work from the Bottom Up
Starting from the bottom of the system, coordinate the branch lighting panels first, then the power distribution panels, then the switchboard or switchgear

41. Downstream Available Short Circuit Current
Design Guidelines
Name: PD-0001
Manufacturer: *SQUARE D
Type: LA, LH/MC
Frame/Model: 250A
Trip: 225 A
Voltage: 240 V
Settings: Phase
Fixed
Name: PD-0006
Manufacturer: *SQUARED
Type: QO, 1P
Frame/Model: 20A
Trip: 20 A
Fixed (730-3)
Overlapping Curves
If there is no overlap of the curves at any point below the available short circuit current at the downstream panel, the circuit breakers are totally coordinated
If there is overlap in the short circuit region, use the short circuit selective coordination tables
Downstream Available Short Circuit Current

42. Downstream Available Short Circuit Current
Design Guidelines
Name: PD-0006
Manufacturer: *SQUARE D
Type: EG
Frame/Model: 20A
Trip: 20 A
Voltage: 480 V
Settings: Phase
Fixed
Name: PD-0001
Manufacturer: SQUARED
Type: POWERPACT P-Frame, 3.0 & 3.0A
Frame/Model: PG
Trip: 250 A
LTPU/LTD (A x S) 1 (250A); 0.5
INST PG ( X S) 6 (1500A)
Nest Curves
The time-current curve of a thermal-magnetic circuit breaker can sometimes be nested underneath the time-current curve of an upstream electronic trip circuit breaker
Downstream Available Short Circuit Current

43. Design Guidelines “Lighting” Panelboard Recommendations
Don’t feed “lighting” panelboards from “lighting” panelboards unless there is a transformer in between
Better levels of selective coordination are available with 225 A and larger panelboards, (larger selection of breakers)
Consider using main lugs panels, particularly at 277 V
Don’t daisy chain panel risers

44. Design Guidelines Ground Fault
Make sure system is selectively coordinated for ground faults
Make sure the GFPE also coordinates with the downstream phase overcurrent devices, not just the downstream GFPE

45. Design Guidelines Transformers
Use the higher protection limits in Article 450 (e.g. 250% instead of 125% FLA on the primary)
Consider 30 to 75 KVA transformers…100 to 225 amp panelboards with Mission Critical Mains
Increase the Frame Size of the Upstream Circuit Breaker
The upstream circuit breaker should be at least one frame size larger than the downstream circuit breaker. This may necessitate increasing the size of panelboards and feeder conductors.
Very high levels of short circuit selective coordination may be achieved by using high amp frame electronic trip circuit breakers with low amp sensors and/or lower ampere rating adjustments

46. Design Guidelines Rarely needed, but as a last resort...
Change the Upstream Circuit Breaker Type
Insulated case circuit breakers or low voltage power circuit breakers
Reduce the Voltage
If the desired level of selective coordination cannot be achieved using a 480Y/277 Vac panelboard, consider feeding a 208Y/120 Vac panelboard through a transformer
Split Up Some of the Loads
Multiple smaller transformers
Insert Impedance
Longer run of wire, 1:1 or higher impedance transformer or reactors

47. Design Guidelines
What if the AHJ requires selective coordination up to both the alternate and normal sources?
Conduct a preliminary short circuit study from the source that can potentially produce the highest SCA to the bottom of the system
Conduct a preliminary selective coordination study on that part of the system
Conduct a preliminary short circuit study from the other source to the first bus on the load side of the transfer switch(es)
Conduct a preliminary selective coordination study from the other source to the transfer switch feeder(s)

48. Design Guidelines Field Adjustment
Don’t neglect to properly adjust circuit breakers in the field as they are often shipped from the factory with all but the ampere-rating switch in the lowest position

49. Summary Use the correct terminology! Terms to use Coordination
“System to be coordinated to 0.1 s”
Selective coordination
“System to be selectively coordinated”
DO NOT use “coordination to 0.01s” when selective coordination is desired

50. Summary System Coordination to Reference(s) Which Source What Level
Healthcare essential electrical
Alternate
Optimize as far as practicable
0.1s
NEC , ; NFPA
NEC (G), NFPA
Healthcare GFPE
Normal & alternate
Total
NEC (C)
Elevator
Normal
Unspecified
1. NEC
2. NEC
Fire pump feeders in multi-building campus
1. NEC (C)(3)
2. NEC (C)(3)
Emergency
NEC
Legally required
NEC , ; NFPA
NEC ,
COPS
NEC , ; NFPA
2. NEC ,

51. Questions. Thank you. Justin Schroeder, P. E
Questions??? Thank you!!! Justin Schroeder, P.E. Schneider Electric USA 2979 Pacific Drive, Suite E
Norcross, GA Phone: