Selective Coordination

Selective Coordination
In-Depth Presentation of Short Circuit
Selective Coordination with Low-Voltage
Circuit Breakers
IEEE IAS San Francisco
and Santa Clara Chapter
Ed Larsen, Senior Member
Industry Standards Manager
Square D/Schneider Electric

Introduction
Definition
NEC Selective Coordination Requirements
The Challenge
Circuit Breaker Principles
Resources from the Manufacturers
Challenges Meeting the NEC
Design Guidelines
Example
Summary
SF IAS Sel Coord.ppt 2

Definition
The Challenge
Design Guidelines
Example
Summary

Definition: What is selective coordination?
NEC 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 choice
of overcurrent protective devices and their ratings or settings.
In other words…
Only the overcurrent protective device (OCPD) nearest to a
fault should clear the fault

Definition
The Challenge
Design Guidelines
Example
Summary

Article 240 Overcurrent Protection
240.12 Electrical System Coordination.
Where an orderly shutdown is required to minimize the
hazard(s) to personnel and equipment, a system of
coordination based on the following two conditions shall be
permitted:
(1) Coordinated short-circuit protection
(2) Overload indication based on monitoring systems or
devices
FPN: The monitoring system may cause the condition to go to alarm,
allowing corrective action or an orderly shutdown, thereby
minimizing personnel hazard and equipment damage.
Unchanged

Article 517 Health Care Facilities
517.17 Ground-Fault Protection.
(C) Selectivity. Ground-fault protection for operation of the
service and feeder disconnecting means shall be fully
selective such that the feeder device, but not the service
device, shall open on ground faults on the load side of the
feeder device. A six-cycle minimum separation between the
service and feeder ground-fault tripping bands shall be
provided. Operating time of the disconnecting devices shall
be considered in selecting the time spread between these two
bands to achieve 100 percent selectivity.
FPN: See 230.95, fine print note, for transfer of alternate source
where ground-fault protection is applied.
Unchanged

Article 620 Elevators, Dumbwaiters, Escalators, Moving
Sidewalks, Wheelchair Lifts, and Stairway Lift Chairs
620.62 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.
Unchanged

2005 NEC Selective Coordination
Requirements
Article 700 Emergency Systems
700.27 Coordination.
Emergency system(s) overcurrent devices shall be
selectively coordinated with all supply side overcurrent
protective devices.
Legally required standby system(s) overcurrent devices
shall be selectively coordinated with all supply side
overcurrent protective devices.
New
New

Requirements
Article 517 Health Care Facilities
517.26 Application of Other Articles.
The essential electrical system shall meet the requirements of
Article 700, except as amended by Article 517.
New

Typical Health-Care Facility Electrical System
(2005 NEC FPN Figure 517.30)

Requirements
Article 700 Emergency Systems
II. Circuit Wiring
700.9 Wiring, Emergency System.
(B) Wiring.
Exception to (5)(b): Overcurrent protection shall be permitted
at the source or for the equipment, provided the overcurrent
protection is selectively coordinated with the downstream
overcurrent protection.
New

Requirements
Articles 700 Emergency Systems and 701 Legally
Required Standby Systems
The following exception was added to sections 700.27 and
701.18
Exception: Selective coordination shall not required in the
following circuits:
(1) Between transformer primary and secondary overcurrent
protective devices, where only one overcurrent protective
device or set of overcurrent protective devices exist(s) on the
transformer secondary, or
(2) Between overcurrent protective devices of the same size
(ampere rating) in series.
New

Requirements
SECONDARY CB
PRIMARY CB
LV TRANSFORMER
Making these
two breakers
coordinate with
one another
does not
enhance
system
selectivity!

Requirements
PANEL 1
PANEL 2
CB 1
CB 2
G
CB 1
CB 1
ENGINE-GENERATOR SET
SWITCHBOARD
Making these
breakers
coordinate with
one another does
not enhance
system
selectivity!

Requirements
Article 708 Critical Operations Power Systems
708.1 Scope. The provisions of this article apply to the installation,
operation, monitoring, control, and maintenance of the portions of
the premises wiring system intended to supply, distribute and control
electricity to designated critical operations areas (DCOA) in the
event of disruption to elements of the normal system.
Critical operations power systems are those systems so classed by
municipal, state, federal, or other codes, by any governmental
agency having jurisdiction, or by facility engineering documentation
establishing the necessity for such a system. These systems include
but are not limited to power systems. HVAC. fire alarm. security.
communications and signaling for designated critical operations
areas.
Note: This NEC article was renumbered from 585 to 708
New

Requirements
708.1 Scope.
FPN No. 1: Critical Operations Power Systems are generally
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.
New
•Air traffic control centers
•Central station service facilities (fire and security system
monitoring)
•Chemical, petrochemical, and hazardous material (including
biohazard) handling facilities
•Communications centers, telephone exchanges, cellular
tower sites
•Emergency evacuation centers
•Financial, banking, business data processing facilities
Note: This list was in an early draft of the article but is
not be in the final text
•Fuel supply pumping stations (i.e. natural gas distribution and
delivery infrastructure)
•Hospitals and associated support facilities
•Municipal infrastructure – water and sewer treatment facilities
•911 centers
•Offices and facilities deemed critical to continuity of government
•Police, fire, civil defense facilities including power for radio repeater
operations
•Radio and television stations
•Transportation infrastructure – airports, rail stations, seaports

Requirements
Critical operations power system(s) overcurrent devices shall
be selectively coordinated with all supply side overcurrent
protective devices.
Note: No exceptions!
New

Definition
The Challenge
Design Guidelines
Example
Summary

Selectivity is typically achieved
with circuit breakers by
coordinating the time-current curve
(TCC) characteristics of the
devices to be coordinated
Coordinated in the overload
zone
Seemingly not coordinated in
the short circuit zone
Can circuit breakers comply
with the Code?
10
10
100100
1K1K
10K10K
100K100K
0.01 0.01
0.10 0.10
1 1
10 10
100 100
1000 1000
CURRENT IN AMPERES
TIMEINSECONDS
CB M1
CB F1
CB PM1
CB B1
CB M1
CB F1
CB PM1
CB B1
The Challenge

Definition
The Challenge
Design Guidelines
Example
Summary

How the Instantaneous Trip Function Works
T-M Breakers
– Fixed instantaneous
– Factory set
– Must hold/trip values in the Digest reflect the
TCC tolerance
– Adjustable instantaneous
– Factory set low
– Final adjustment subject to +30%/-20%
tolerance per UL 489

How the Instantaneous Trip Function Works
Electronic Trip Breakers
– Adjustable instantaneous
– Factory set low
– Final adjustment subject to +/-10% tolerance
– Selective override
– Factory set for breaker self-protection
– Usually +/-10% tolerance
– Making current release (discriminator)*
– Factory set for breaker self-protection
– +/-10% tolerance
– Turned off by a timer or switch after mechanism
is latched* Typically LVPCBs only

Selective Override
5. For a withstand circuit breaker,
instantaneous can be turned OFF.
See 613-7 for instantaneous trip
curve. See 613-10 for
instantaneous override values.
10
10
100100
1K1K
10K10K
100K100K
0.01 0.01
0.10 0.10
1 1
10 10
100 100
1000 1000
CURRENT IN AMPERES
TIMEINSECONDS
Current Scale X 10^0
Reference Voltage: 480
TCC view

Making Current Release
The MCR results in a close and latch rating

Factors Impacting Short Circuit Selective Coordination
Using time-current curves alone sometimes leads to the
determination of a short circuit selective coordination level
that is lower than can actually be achieved. Factors to
consider…
How Time-Current Curves are Developed
Current Limiting Properties of Circuit Breakers
Dynamic Characteristics of Circuit Breakers

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 short circuit 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
10
10
100100
1K1K
10K10K
100K100K
0.01 0.01
0.10 0.10
1 1
10 10
100 100
1000 1000
CURRENT IN AMPERES
TIMEINSECONDS

What is Needed for Good Selectivity
Withstand Capability
– Ability to withstand a high level of current
– Dependent on many factors, such as...
– Current path geometry
– Contact pressure springs
– Mechanism
Trip System
– Ability to utilize the withstand capability of the
breaker
– Dependent on many factors, such as...
– Accuracy of the sensors
– Adjustments available to the user
Upstream
breakers must
have both for
the best
selective
coordination!

Definition
The Challenge
Design Guidelines
Example
Summary

Short Circuit Selective Coordination Tables
Present short circuit selective coordination data for
various tested combinations of low-voltage circuit
breakers
May yield higher levels of short circuit selective
coordination than the TCCs indicate
TCC studies still have to be made in order address
overload and other protection issues

Eaton
– Selective Coordination Industry Application (IA01200002E)
– Selective Coordination Breaker Application Chart For
Molded Case Circuit Breakers fed by Distribution
Transformers
General Electric
– GE Overcurrent Device Instantaneous Selectivity Tables
(DET-537)
Siemens
– Selective Trip Coordination with Molded Case Circuit
Breakers
– Selective Trip Coordination with Molded Case Circuit
Breakers

Square D
– Short Circuit Selective Coordination for Low Voltage
Circuit Breakers (0100DB0501)
– Selectivity Guidelines for Square D Panelboards
(0100DB0604)
– Enhancing Short Circuit Selective Coordination with
Low Voltage Circuit Breakers (011DB0403)

Design and Application Guides
Eaton
– Selective Coordination
Square D
– Guide to Overcurrent Coordination 600V and
Below (0100DB0603)

Other Resources
General Electric
– InstaPlan™ 2.0, a tool for planning power system
instantaneous selective coordination
Eaton
– 2005 NEC Selective Coordination Design Issues
– Selective Coordination Calculator

Square D Assumptions
Circuit Breaker Contact Position
All upstream breakers are in the closed (ON) position
when the fault occurs
Instantaneous Trip Setting
The instantaneous trip setting on all upstream breakers, if
adjustable, will be set to the highest position (OFF if
available – select LSI or LSIG trip units)

Using Short Circuit Selective Coordination for
Low Voltage Circuit Breakers (0100DB0501)

Basic Information Needed
System One-line Diagram
System Voltage
Circuit Ampacity
Available Short Circuit Current
Add motor contribution (if necessary)
Adjust for X/R (if necessary)
From the normal source to the lowest point in the
system
From the alternate source to the first bus below the
ATS

Available Short Circuit Current
G
CB 1
G
CB 2
AUTOXFER
SW
CB 4
TO NORMAL SOURCE
E N
CB 3
AUTOXFER
SW
E N
AUTOXFER
SW
CB 5
E N
CB 6
Normal
source
SCA
Alternate
source
SCA
This assumes the alternate
source SCA < the normal
source SCA

Example #1 - System Description
System
208Y/120 Vac system with 10 kSCA available at the lighting
panelboard
Equipment
NQOD 225A main lugs lighting panelboard with single pole
20A QOB 10 kAIR rated circuit breakers fed from a
QDA32225 circuit breaker with a 25 kA interrupting rating
located in an upstream I-Line power panelboard

Example #1 - Determining the Level of Short
Circuit Selective Coordination
1. Find the 240Vac table listing QO
downstream circuit breakers in
Appendix A Table 1 on page 6
2. Find the column for the QO
downstream circuit breaker to be
studied
3. Go down the column until the row
listing the 225 A QD upstream
circuit breaker is found
4. Read the selective coordination
level at the intersection of the
column and row, namely 2.0 kA.
This means that the QD upstream
circuit breaker is selectively
coordinated with downstream QO
circuit breakers up to 2,000 amps.

Example #1 - Improving the Level of Short
Using the same example, determine if a
higher level of selective coordination can
be achieved by following these steps:
1. Move down the column for the QO
studied, looking for upstream
breakers that will yield a higher level
of selective coordination
2. When the desired level of selective
coordination is found, read across
the row to find the upstream breaker
that will yield this level. In this case,
an LA-MC circuit breaker will yield a
level of selective coordination of up
to 18,000 amps. Footnote 10 refers
to Appendix B.

Referring to Appendix B Table 12 on
page 38
1. Find the column for the QO
studied
listing the 225 A LA-MC upstream
circuit breaker is found
3. Read the selective coordination level at
the intersection of the column and row,
namely 18 kA for a series rated
application
4. Footnote 2 indicates that for a fully
rated application the level of selective
coordination is 10 kA. This means
that the LA-MC upstream circuit
breaker is fully selectively coordinated
with downstream QO circuit breakers
up to 10,000 amps

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       
 

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
Voltage: 240 V
Settings: Phase
Fixed (730-3)
Name: PD-0001
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
Voltage: 240 V
Settings: Phase
Fixed (730-3)
Coordinates to 18kA per Data
Bulletin 0100DB0501
Coordinates to 18kA per Data
Bulletin 0100DB0501
Current Scale x 1
There is an overlap on the TCC
However, these two circuit breakers
coordinate to 10 kA, the level of short-
circuit current at the downstream
circuit breaker
They are fully coordinated
A text note is used to delineate that
this breaker combination coordinates
above the level shown on the TCC

Example #2 - System Description
System
480Y/277 Vac system with 25 kSCA available at the lighting
panelboard
Equipment
NF 250A main lugs lighting panelboard with single pole EG
35 kAIR rated circuit breakers fed from a JGA36250 circuit
breaker with a 35 kA interrupting rating located in an I-Line
power panelboard

Example #2 - Determining the Level of Short
1. Find the 480Vac table listing EG
downstream circuit breakers in
Appendix A, Table 6: on page 20
2. Find the column for the EG
studied
listing the 250 A JG upstream circuit
breaker is found
4. Read the selective coordination level
at the intersection of the column and
row, namely 2.4 kA. This means that
the JG upstream circuit breaker is
selectively coordinated with
downstream EG circuit breakers up
to 2,400 amps.

Using the same example, determine if a
higher level of selective coordination
can be achieved by following these
steps:
1. Move down the column for the EG
studied, looking for upstream
breakers that will yield a higher
level of selective coordination
2. When the desired level of selective
coordination is found, read across
the row to find the upstream
breaker that will yield this level. In
this case, a PG circuit breaker will
yield a level of selective
coordination of 35,000 amps. This
means that the upstream PG
circuit breaker is fully selective with
downstream EG circuit breakers.

  
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 
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



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       
       
 
   
     
       
 

Name: PD-0006
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
Voltage: 480 V
Settings: Phase
LTPU/LTD (A 0.4-1.0 x S) 1 (250A); 0.5
INST PG 250-1200 (1.5-12 X S) 6 (1500A)
Name: PD-0006
Type: EG
Frame/Model: 20A
Trip: 20 A
Voltage: 480 V
Settings: Phase
Fixed
Name: PD-0001
Frame/Model: PG
Trip: 250 A
Voltage: 480 V
Settings: Phase
LTPU/LTD (A 0.4-1.0 x S) 1 (250A); 0.5
INST PG 250-1200 (1.5-12 X S) 6 (1500A)
Coordinates to 35ka per Data
Bulletin 0100DB0501
Coordinates to 35ka per Data
Bulletin 0100DB0501
Current Scale x 1
There is an overlap on the TCC
However, these two circuit breakers
coordinate to 35 kA, above the level
of short-circuit current at the
downstream circuit breaker
They are fully coordinated
A text note is used to delineate that
this breaker combination coordinates
above the level shown on the TCC

Using Enhancing Short Circuit Selective
Coordination with Low Voltage Circuit
Breakers (011DB0403)

Coordination
How to Use Appendix A
Find the table with the correct system
voltage
Find the breaker to be studied
Find the maximum instantaneous or
selective override trip level
Suggestion
Use this data bulletin when the desired
combination cannot be found in Short
Circuit Selective Coordination for Low
Voltage Circuit Breakers

Coordination
Example – Coordination Across a LV Transformer
To select a primary circuit breaker that will coordinate with
secondary branch circuit breakers:
1. Determine the available SCA downstream (assume 1 kA)
2. Calculate the fault current that will be seen on the
primary: 208 V / 480 V = 0.433 x 1000 A = 433 A
3. Find a circuit breaker with the proper voltage, interrupting
and continuous current ratings in Appendix A that also
has an instantaneous trip of at least 433 A
4. Check to make sure that the primary circuit breaker
meets the NEC transformer protection requirements
Enhancing Short Circuit Selective Coordination does not
present new information, it just compiles it in one easy to use
location!

Definition
2005 NEC Selective Coordination Requirements
The Challenge
Design Guidelines
Example
Summary

What to do?
Understand the issues with the NEC requirements
Know the AHJ’s interpretation of NEC 700.27 and
701.18
Know how NEC 700.27 and 701.18 is enforced in
your area (or if it will be enforced, state or local
exceptions, etc.)
– Will selective coordination be required up to both
the normal and alternate sources, or only up to
the alternate source?
– What about existing equipment (such as an old
switchboard feeding a new panelboard)?

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

alternate source?
Appendix B.1 of NFPA 110 also seems to imply only
up to the alternate source

alternate source?
But read 700.27 carefully
Emergency system(s) overcurrent devices shall be
selectively coordinated with all supply side
overcurrent protective devices.
Normal system OCPDs are on the supply side of the
emergency system, and thus are included (Square D
interpretation)

Examples of Code Modifications
State of Washington
– 027 Coordination.
The requirements for selective coordination described in
NEC 700.27 are not required where the emergency system
was installed prior to June 1, 2006. For new emergency
systems that are supplied from an existing emergency
system installed prior to June 1, 2006, the new portion of
the emergency system must comply with NEC 700.27. The
ground fault sensing function of overcurrent protective
devices will only be required to selectively coordinate with
the ground fault sensing functions of other overcurrent
protective devices.
– A similar modification was made to 701.18

State of Massachusetts 2008 MEC
Added the following to 700.27 and 701.18
Exception No. 2: Where the emergency system design is under
the control of a licensed professional engineer engaged in the
design or maintenance of electrical installations, the selection of
overcurrent protective devices shall be permitted to coordinate
to the extent practicable. The design shall be documented,
stamped by the professional engineer, and made available for
review by the authority having jurisdiction.
FPN: Overcurrent protective devices used for emergency circuit
protection, where coordinated to optimize selective operation of
the circuit overcurrent protective devices when a short circuit or
ground fault occurs, increase overall reliability of the system.

City of Tucson
– Section 700.27 Coordination. DELETE this section
in its entirety.
– Section 701.18 Coordination. DELETE this section
in its entirety.
State of Florida
– Healthcare facilities come under the jurisdiction of
AHCA for plan check and inspection
– Requires selective coordination to 0.1 sec.

State of California
– Healthcare facilities come under the jurisdiction of
OSHPD for plan check and inspection
– Proposals
– 700.27 Coordination. Emergency system(s)
overcurrent devices shall be selectively
coordinated with all supply side overcurrent
protective devices. [Not permitted for OSHPD 1, 2,
3, & 4]
– 701.18 Coordination. Legally required standby
system(s) overcurrent devices shall be selectively
coordinated with all supply side overcurrent
protective devices. [Not permitted for OSHPD 1, 2,
3, & 4]

State of Wisconsin
– Proposals
– 517.26 Application of Other Articles.
The essential electrical system shall meet the requirements
of Article 700, except as amended by Article 517. Essential
electrical system(s) overcurrent devices shall be selectively
coordinated with all supply side overcurrent protective
devices for faults with a duration of 0.1 seconds and longer.
The selection and coordination of the overcurrent devices
shall be documented and stamped by a professional
engineer and approved by the engineer of record for the
project. This study and all associated documentation shall
be made available for review by the authority having
jurisdiction.

State of Wisconsin
– Proposals
– 700.27 Coordination.
– Emergency system(s) overcurrent devices shall be selectively coordinated
with all supply side overcurrent protective devices for faults with a duration of
0.1 seconds and longer.
– Legally required standby system(s) overcurrent devices shall be selectively
coordinated with all supply side overcurrent protective devices for faults with a
duration of 0.1 seconds and longer.
– Critical operations power system(s) overcurrent devices shall be selectively
coordinated with all supply side overcurrent protective devices for faults with a
duration of 0.1 seconds and longer.
– Exceptions:
– (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,
– (2) Between overcurrent protective devices of the same size (ampere
rating) in series.

New York City
– Question (from a consulting engineer dated 9/26/07)
“The NEC sections 700.27 and 701.18 require that
‘Emergency System(s) overcurrent devices shall be
selectively coordinated with all supply side overcurrent
protective devices.’ Does this mean that all the emergency
system overcurrent devices, from the smallest branch to the
power supply, must be selectively coordinated?”
– Answer (from the NYC Electrical Code Revision and
Interpretation Committee dated 9/15/07)
“Selective coordination requirements for short-circuit
conditions are defined in section 240.12. Emergency
system(s) overcurrent devices, per section 700.27, shall be
selectively coordinated for overcurrent conditions only.”

New York City
NYC Electrical Code (based on the 2005 NEC)
Section 240.12 - Revise to read as follows:
240.12 Electrical System Coordination. Rating and
arrangement of service overcurrent devices, which have a
rating above 601 amperes, shall be selectively coordinated.
Such coordination shall provide a system of selective short
circuit and overload protection between the service overcurrent
protection and the second level overcurrent protection point.
Where an orderly shutdown is required to minimize the
hazard(s) to personnel and equipment, an additional
overcurrent protection level is permitted. A system of
coordination based on the following two conditions shall be
permitted:
(1) Coordinated short-circuit protection
(2) Overload indication based on monitoring systems or
devices.

New York City
New York City Electrical Code (based on the 2005 NEC)
Section 240.12 - Revise to read as follows:
240.12 Electrical System Coordination.
Exception No. 1: Service overcurrent devices which supply
single loads (i.e., motors) shall not require coordination.
Exception No. 2: Coordination between the service overcurrent
device and distribution main shall not be required where the
service disconnecting means supplies a single main overcurrent
device for a single distribution panel or switchboard. However,
selective coordination shall be required between distribution
branch devices, and between the service equipment and the
main panel.
Exception No. 3: The provisions of this Section shall not apply
to the operation of ground fault protection equipment.

Examples of Enforcement
State of Washington
– Selective coordination must be assured by a state
licensed PE
– The Department of Labor and Industries is interpreting
the Code to mean that selective coordination is
required only to the alternate source
City of Tacoma
– Selective coordination must be assured by a state
licensed PE. A coordination study need not be
submitted. (unconfirmed)
City of Denver
– Selective coordination required only to the alternate
source (unconfirmed)

Cautions
Make sure automatic transfer switches have
adequate withstand ratings
– May need to relocate the switch, or
– May need to increase the frame size of the
switch
Make sure busway has adequate withstand ratings
Make sure the generator protection devices
coordinate with the downstream circuit breakers
Total ground fault selective coordination may not be
possible, or may be difficult, due to other Code
requirements [517.17(B)(1) and (2)]

Cautions
Mixing Overcurrent Protective Devices
– Mixing OCPDs from different manufacturers or
mixing fuses and circuit breakers requires using
TCCs only
– Fuse ratio or circuit breaker tables cannot be
used
Arc Flash
– Selective coordination impact on arc flash PPE
levels needs to be considered
– May be possible to reduce with Zone Selective
Interlocking

Definition
The Challenge
Design Guidelines
Example
Summary

Design Guidelines
Conduct a Selective Coordination Study First
Before letting a job out for bid, conduct a selective
coordination study first as it may affect the system design
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

Design Guidelines
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
Name: PD-0006
Type: EG
Frame/Model: 20A
Trip: 20 A
Voltage: 480 V
Settings: Phase
Fixed
Name: PD-0001
Frame/Model: PG
Trip: 250 A
Voltage: 480 V
Settings: Phase
LTPU/LTD (A 0.4-1.0 x S) 1 (250A); 0.5
INST PG 250-1200 (1.5-12 X S) 6 (1500A)
Name: PD-0006
Type: EG
Frame/Model: 20A
Trip: 20 A
Voltage: 480 V
Settings: Phase
Fixed
Name: PD-0001
Frame/Model: PG
Trip: 250 A
Voltage: 480 V
Settings: Phase
LTPU/LTD (A 0.4-1.0 x S) 1 (250A); 0.5
INST PG 250-1200 (1.5-12 X S) 6 (1500A)

Design Guidelines
Use the Short Circuit Selective Coordination Tables
Rather than the Time-Current Curves
Feeding Lighting Panelboards
Don’t feed lighting panelboards from lighting panelboards
unless there is a transformer in between
Other Lighting Panelboard Recommendations
Better levels of selective coordination are available with
225A and larger panelboards
Consider using main lugs panels

Design Guidelines
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

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/277Vac panelboard, consider feeding
a 208Y/120Vac 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

Design Guidelines
Generator Protection
Selective coordination is sometimes difficult or
impossible while providing adequate generator
protection
Be wary of circuit breakers supplied with engine-
generator sets
– They are often thermal-magnetic or electronic trip
with LI protection
– They may need to be electronic trip with LS
protection and high withstand if possible, or ANSI
LV power circuit breakers
Make sure generator protective relays or controls will
coordinate with the downstream devices

Design Guidelines
Challenge: CB1 and CB2 must both be selective with
CB3, CB4, CB5 and all downstream breakers (CB6…)
G
CB 1
G
CB 2
AUTOXFER
SW
CB 4
TO NORMAL SOURCE
E N
CB 3
AUTOXFER
SW
E N
AUTOXFER
SW
CB 5
E N
CB 6

Design Guidelines
One solution: More, smaller generators without paralleling
G
CB 1
G
AUTOXFER
SW
TO NORMAL SOURCE
E NAUTOXFER
SW
E N AUTOXFER
SW
E N
CB 6
G Expensive!
Decreases reliability
Not always practical

Design Guidelines
Better solution:
Allow paralleling switchgear feeders to provide short-
circuit protection
Supplement with bus-differential protection for the
generator paralleling bus
Not a “cure-all”, but it does often help

G
CB 1
G
CB 2
AUTOXFER
SW
CB 4
TO NORMAL SOURCE
E N
CB 3
AUTOXFER
SW
E N AUTOXFER
SW
CB 5
E N
CB 6
87B PROTECTIVE ZONE
CB1 and CB2 set to
provide overload, but
not short-circuit,
protection for the
generators
These settings allow
coordination with CBs
on the level of CB3
Bus
differential
protection
provides
short circuit
protection
for the
generators
for faults on
generator
paralleling
bus
CBs on CB3
level provide
short circuit
protection for
generators
Design Guidelines

Design Guidelines
Selective coordination requires an extremely high level of
analysis
Often not possible to achieve on conventional designs without
major reconfiguration (cannot succeed with device selections
alone)
Expect significantly higher design time, space requirements,
and equipment costs
Vendor-Specific Design
Difficult to delegate the design to a vendor by specifying
“vendor shall provide fully selective equipment” because of
effects on equipment sizes, room sizes, system layout
strategy, feeder sizes, etc.
Bogue Waller, P.E., Principal Electrical Engineer, Nash Lipsey
Burch, LLC, Nashville, TN

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

Definition
Design Guidelines
Example
Summary

Example
Which of these circuit breakers need to be selectively
coordinated?

Example
coordinated?
#3 must coordinate with #4 because it is a supply side
device
Yes

Example
Which of these circuit breakers need to be
selectively coordinated?
#1 does not need to coordinate with #3,
assuming that they are both the same
size, in accordance with Exception 2 in
the 2008 NEC which states, "Exception:
Selective coordination shall not be
required in (1) or (2): (2) Between
overcurrent protective devices of the
same size (ampere rating) in series."
#1 does need to coordinate with #4 and
the other breakers on that bus. (Note:
While this exception does not exist in the
2005 NEC, most engineers would come
to this same conclusion, and most AHJs
would probably agree.)
No
Yes

Example
coordinated?
#A does not need to coordinate with #1 or 3, however, it
does need to coordinate with #4 because it is a supply
side device
No
Yes

Example
coordinated?
#2 does not need to coordinate with #3, assuming that
they are both the same size, but it does need to
coordinate with #4 and the other breakers on that bus
No
YesYes

Example
coordinated?
#A, #B and #C are not required to be coordinated in
accordance with the scope of Article 700 and drawing B.1
in NFPA 110 Annex B
This illustrates the
problem with requiring
coordination up to the
normal source – if the
normal loads are not
coordinated, has the
emergency system
reliability really been
improved?
No
No

Definition
Design Guidelines
Example
Summary

Summary
Understand the NEC
Selective coordination requirements
How the AHJ will interpret and enforce them
Understand the Operating Characteristics of Circuit
Breakers
Limitations of the TCCs in the short circuit zone

Summary
Understand the Design Characteristics of Circuit
Breakers
Withstand Capability
Instantaneous Trip Setting
Field adjustable instantaneous adjustment can be
turned OFF on LSI and LSIG trip units
Continuous Current Rating Overlap
Allows for selecting a larger frame size breaker,
particularly on electronic trip circuit breakers

Summary
Evaluation Methodology
Conduct a short circuit study (from both sources)
Make initial circuit breaker selections
Determine the selective coordination levels
Optimizing Techniques
Use the short circuit selective coordination tables
rather than the TCCs if there is an overlap of the
TCCs in the short circuit zone
Consider better system configurations
Increase the frame size of the upstream breakers
Change the upstream breaker type
Add impedance to reduce short circuit current levels

Summary
Low voltage circuit breaker based systems can be
selectively coordinated!
Thank You!
Questions?

Selective Coordination

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