8/27/2013
Equipment Short-Circuit Current
Rating and Available Fault Current
Dan Neeser – Field Application Engineer
Eaton’s Bussmann Business
Dan.Neeser@CooperIndustries.com
© 2013 Eaton. All rights reserved.
Agenda
• Interrupting Rating & Series Rating
• Short-Circuit Current Ratings
• Maximum Fault Current Marking
• Fault Current Calculations
• UL 508A – Industrial Control Panels
Supplement SB - SCCR
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Interrupting Rating
• NEC® Article 100 Definition
• Highest current an OVERCURRENT DEVICE
(fuse or circuit breaker) is rated to safely interrupt.
• Self protection rating only
• NEC® 110.9 Interrupting Rating.
• Requires the overcurrent device to have an
interrupting rating not less than the maximum
available fault current.
• The maximum fault current must be calculated and
varies based on system size/location.
• Similar Requirements in OSHA 1910.303(b)(4)
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Interrupting Rating
• NEC 110.9: Device interrupting rating MUST
be equal to or greater than the maximum
available fault current.
• Chapter 1 Video Clips - Interrupting Rating
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Interrupting Rating - Proper Application
Isca 1
1st
Determine available short-circuit
current (Isca) at lineside
2nd
Apply OCPDs with adequate
Interrupting Rating.
terminals of each OCPD.
Isca 2
MSB
Isca 4
Isca 3
Isca 5
Isca 6
M
M
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Interrupting Rating - Proper Application
Must select circuit breaker with interrupting rating adequate for
point of application – varies by voltage and circuit breaker type.
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I.R. - Proper Application
High Interrupting rating of current-limiting fuses at full voltage reduces
concerns about proper interrupting rating at point of application
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Series Rated Systems
A combination of:
- Circuit breakers / circuit breakers
OR
- Fuses / circuit breakers …
that can be applied at available fault levels
above the interrupting rating of the load side
circuit breaker, but not above that of the main
or line side device.
Downstream device will ALWAYS be a circuit breaker
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Series Rated Systems: CB-CB
Series Rated
Combination
200A Circuit Breaker
65,000 A.I.R.
65,000 A.I.R.
20A Circuit Breaker
10,000 A.I.R.
ISC=65,000 A
ISC=65,000 A
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Series Rated Systems: Fuse-CB
Series Rated
Combination
LPJ 400 SP
200,000 A.I.R.
300,000 A.I.R.
20A Circuit Breaker
10,000 A.I.R.
ISC=300,000 A
© 2013 Eaton. All rights reserved.
ISC=200,000 A
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Series Rated Systems
• NEC® 240.86(A) Selected Under Engineering
Supervision in Existing Installations
• The series rated combination devices must be selected
by licensed, professional engineer.
• Requires Documentation and Stamp.
• Series combination rating and upstream device must be
field marked on end use equipment.
• Downstream circuit breaker must remain passive.
• NEC® 240.86(B) Tested Combinations
• The combination of line side and load side devices must
be tested and marked on the end-use equipment
(panelboards & switchboards).
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Series Rated Systems
• Additional Limitations and Requirements
• Labeling Requirements
• Manufacturer – NEC® 240.86(B)
• Field Marking – NEC® 110.22(B)&(C)
• Motor Contribution Limitations
• NEC 240.86(C)
• Motor FLA cannot exceed 1% of IR of protected CB.
• Lack of Selective Coordination
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Current Limitation
• A current limiting
fuse will clear a
fault within one
half of a cycle.
• Thermal energy is
proportionate to the
square of “RMS”
current multiplied by
the time (Irms2t)
• Mechanical stresses
are proportionate to
the square of “peak”
current multiplied by
the time (Ip2t)
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Current Limitation
• Conductor protection for fault of 45kA/480V (26kA with
conductor) with non-current-limiting device (1 cycle) vs.
current-limiting device (>1/2 cycle)
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Short-Circuit Current Ratings
• NEC® Article 100 Definition
• The highest current EQUIPMENT can withstand without extensive
damage (fire or shock hazard).
• Also known as component protection
• May be based on a specific type of overcurrent device
• NEC® 110.10 Circuit Impedance, Short-Circuit Current Ratings,
and Other Characteristics.
• Requires the equipment to have a short circuit current rating
not less than the maximum available fault current.
• The maximum fault current must be calculated and varies
based on system size/location.
• Similar Requirements in OSHA 1910.303(b)(5)
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Motor Starter - SCCR
• High Fault Test: starter
protected by an instantaneous
trip circuit breaker (MCP) that
only provides Type 1 protection
• High Fault Test: starter
protected by Low-Peak® fuses
that provides Type 2 (No
Damage) protection
480V - 22,000A
480V - 22,000A
Fault
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Fault
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SCCR Marking Requirements
• 430.8 – Marked on motor controllers
• 440.4(B) – Marked on HVAC equipment
• 409.110(3) – Marked on industrial control panels
• 409.22 – Fault current cannot exceed marked SCCR
• 670.3(A)(4) – Marked on industrial machinery
control panel
• 670.5 – Fault current cannot exceed marked SCCR
• UL 508A, Supplement SB is an approved method
to determine SCCR for industrial control panels
and industrial machinery control panels
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Fault Current Marking Requirement
• 110.24 Available Fault Current.
• (A) Service equipment must be marked with
the maximum available fault current and
date of calculation
• (B) If fault current increases due to system
modification, the marking must be updated.
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How to Comply?
Engineer – Calculate
All equipment must comply with:
NEC 110.9 (IR) &110.10 (SCCR)
Isc = 60,142 A
Contractor – Label
Max Avail. Fault Current = 58,524 A
Service Equipment
Date Determined/Calculated: 9/2010
Required per NEC 110.24
Isc = 38,525 A
HVAC
SCCR = 40kA
Isc = 42,153 A
Isc = 27,532 A
Industrial Machinery Panel
Industrial Control Panel
SCCR = 65kA
SCCR = 30kA
© 2013 Eaton. All rights reserved.
Isc = 18,752 A
Motor Controller
SCCR = 25kA
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IR/SCCR Inspection Check-List
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Fault Current Calculations
• To determine the fault current:
• Draw the one-line diagram
• Identify sources of short-circuit current (utility,
generation, motors)
• Include system component information
(tranformers, conductors, busway, overcurrent
devices are not considered)
• Use calculation method (ohmic, per unit, point-topoint, software)
• Use “typical” fault current values as a reference
only.
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Typical Short-Circuit Current Values
• Short-Circuit Current at service entrance equipment
varies by type/size of system:
• Small residential systems (100A to 200A) – 10,000A to
15,000A or less
• Small commercial building systems (400A to 800A) –
20,000A to 30,000A
• Larger commercial and manufacturing building systems
(2,000A to 3,000A) - 50,000A to 65,000A
• Higher short-circuit currents are possible where low
impedance (energy-efficient) transformers are used (or
where larger transformers (kVA) feed multiple services.
• Commercial buildings directly connected to utility “grid
system” – 200,000A or greater
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8/27/2013
Short-Circuit Current Factors
• Typically
highest at
service point
• Decreases in
downstream
equipment
(due to
impedance of
transformers
and
conductors)
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Calculation Example
• 480V/3000A Service supplied from 2500 kVA
Transformer
• Find Isc at Transformer Secondary & Service
Equipment
480V, 3000A Service Equipment
Service Transformer
2500 kVA
Isc = 58,524A
13.2kV – 480V
5% Impedance
Isc = 60,142 A
7 – 600 Cu kcmil/phase
25 Feet in PVC
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Calculation Example
• Step One: Calculate Secondary FLA of
Transformer (3 phase)
IFLA =
KVA X 1000
EL-L X 1.732
KVA = KVA of Transformer
EL-L = Secondary Phase to Phase voltage
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Calculation Example
ISC
2500 KVA Transformer
13.2kV - 480V, 3 Phase
Z = 5%
IFLA =
2500 X 1000
480 X 1.732
IFLA = 3007 A
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Calculation Example
• Step Two: Calculate Multiplier
• Z = Impedance of Transformer, refer to transformer
nameplate or manufacturer data
Mult =
Mult =
100
%Z
100
5
Mult = 20
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Calculation Example
• Step Three: Calculate Secondary Short
Circuit Current
ISC = IFLA X Mult
ISC = 3007 X 20
ISC = 60,140 A
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8/27/2013
Calculation Example
• Calculate Short Circuit Current at 480V
Switchgear
480V
Switchgear
7 – 600 Cu kcmil/phase
25 Feet in PVC
ISC = 60,140A
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Calculation Example
• Isc known from previous calculation
• Calculate multiplier for Cable
• Step One: Calculate f value
ISC 480V SWGR = ISC X Mult
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8/27/2013
Calculation Example
• Step One: Calculate f value
f =
1.732 X L X ISC
C X EL-L
L = Length of conductor
ISC = Short-circuit current at beginning of circuit
C = “C” Value Constant for Cable
From Table. Multiply by # of runs
EL-L = Phase to Phase Voltage
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Calculation Example
• C Value – 600 kcmil, CU, PVC = 28,033
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8/27/2013
Calculation Example
• Step One: Calculate f Value
• L = 25 Feet
• ISC = 60,140 A
• C = 7 X 28,033 - See “C” Value Table
• EL-L = 480V
f =
1.732 X 25 X 60,140
= 0.0276
7 X 28,033 X 480
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Calculation Example
• Step Two: Use Calculated f Value
• Calculate Multiplier
Mult =
Mult =
1
1+f
1
1 + 0.0276
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= 0.9731
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Calculation Example
• Calculate Short Circuit Current at 480V
Switchgear
ISC 480V SWGR = ISC X Mult
ISC 480V SWGR = 60,140 X .9731
ISC 480V SWGR = 58,522 A*
* Add motor contribution if present
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What if the System Changes?
Service Transformer
3000 kVA
480V, 4000A Service Equipment
Isc = 70,587A
13.2kV – 480V
5% Impedance
Isc = 72,171 A
9 – 600 Cu kcmil/phase
25 Feet in PVC
Service Transformer
2500 kVA
480V, 3000A Service Equipment
Isc = 72,667A
13.2kV – 480V
4% Impedance
Isc = 75,178 A
7 – 600 Cu kcmil/phase
25 Feet in PVC
© 2013 Eaton. All rights reserved.
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8/27/2013
Cooper Bussmann Short-Circuit Calculator
• New version
• Old version
• Apple or Android Apps
• Download for Window XP
• Web (run from homepage)
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Create a System
1
• Choose three phase or single phase system
2
• Click on Add to my System 3
• Select a component you want to add
Calculating available fault current and creating 110.24 labels has never been easier!
© 2013 Eaton. All rights reserved.
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Add a Transformer
1
• Select Add Transformer
2
• If you have a known primary fault current you can add it here. If not, select assume infinite
3
• Enter values into the appropriate fields
Calculating available fault current and creating 110.24 labels has never been easier!
© 2013 Eaton. All rights reserved.
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Add Motor Contribution
1
• After adding a Transformer, you will be asked if you want to add motor contribution
2
• You can complete a quick calculation by selecting a % of your transformer’s FLA or add it manually
3
• Add these calculations to your system
Calculating available fault current and creating 110.24 labels has never been easier!
© 2013 Eaton. All rights reserved.
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Add Other Components
1
• Select Add To My System
2
• Add Conductor Run or Buss Run
3
• Add the correct values and Add To System
Calculating available fault current and creating 110.24 labels has never been easier!
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System Summary
1
• View system one‐line diagram
2
• Email one‐line diagram
3
• Select a specific fault to create a label
Calculating available fault current and creating 110.24 labels has never been easier!
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Create a Label
1
2
• Select a fault and click on Create Label
• Enter the email address, project name, fault name, and label size, then Send Label
Calculating available fault current and creating 110.24 labels has never been easier!
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Fuse Sizing
1
2
3
• Tap on Fuse Sizing Diagram
• Tap on the circuit to protect • Review each section and the fuse suggestions for the selected location
Sizing fuses for mains, feeders, and branch circuits has never been easier!
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User Guide & Contact Us
• The User guide provides useful information about FC2
• There is a user guide selection at the bottom if each page
• Based on where you are in the app, you will be provided a specific overview of FC2’s functionality
• While the FC2 is very intuitive, there may be additional questions • There is a Contact Us selection at the bottom of each page
• From this location, you can select to send an email for technical assistance or customer service support
Calculating available fault current and creating 110.24 labels has never been easier!
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Cooper Bussmann FC2 Web Version - Example
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How to Determine SCCR?
• Short-Circuit Current Rating (SCCR)
• Can be established by testing during the listing
and labeling process
OR
• Can be determined using an approved
(calculation) method
•
UL 508A Supplement SB is an approved method
(industrial control panels and industrial machinery
control panel)
•
AHJ Approved Method
• NRTL field evaluation can also be used.
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Industrial Control Panel SCCR
• Industrial Control Panel.
An assembly of two or more components consisting of one of the
following:
1)power circuit components only, such as motor controllers,
overload relays, fused disconnect switches, and circuit breakers;
2)control circuit components only, such as push buttons, pilot
lights, selector switches, timers, switches, and control relays;
3)a combination of power and control circuit components. These
components, with associated wiring and terminals, are mounted
on, or contained within, an enclosure or mounted on a subpanel.
The industrial control panel does not include the controlled
equipment.
48
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UL 508A Supplement SB
• What Needs to be Analyzed per UL 508A,
Supplement SB?
• All power circuit components (SB 4.1)
• Feeder and branch circuit components that supply power to
loads (motors, lighting, heating and appliances)
• Includes disconnect switches, fuses, circuit breakers, load
controllers, overload relays, power distribution/terminal
blocks, bus bars, etc.
• Control circuit components are not required to be
analyzed
• Pushbuttons, pilot lights, selector switches, timers, control
relays, etc.
© 2013 Eaton. All rights reserved.
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Power vs. Control Circuits
Control circuit components
don’t have to be analyzed
*
Power
Transformer
*Control Circuit, but
affects SCCR
Control Transformer
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UL 508A Supplement SB
• How to Determine Component SCCR (SB4.2.2)?
• The SCCR marked on the component or on instructions.
• The SCCR determined by the voltage rating of the
component and the assumed short circuit current from
Table SB4.1.
• or
• The SCCR for a load controller, motor overload relay, or
combination motor controller that has been investigated
in accordance with the performance requirements,
including short circuit test requirements for standard fault
currents or high fault currents specified in the Standard
for Industrial Control Equipment, UL 508, and described
in the manufacturer’s procedure.
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UL 508A Supplement SB
• What are the Rules (Sweep 1)?
• Lowest component SCCR limits
assembly SCCR unless:
• Combination ratings can be used to
increase branch circuit component ratings
(SB 4.2.2)
• Component with marked/specified OCPD
• Check with component manufacturer for
combination ratings
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Combination Ratings - Branch
Component SCCR
OCPD I.R.
Nameplate
Voltage:
480
SCCR:
100kA
• Use combination
ratings with
specified/marked
OCPD to fix low rated
branch components
Combination ratings of overcurrent protective devices and components can be used
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Combination Ratings - Contactor
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Combination Ratings - MMP
• OL Protection Only
• Fuse or Circuit Breaker Required for
SC Protection
• Suitable for Group Installations
• Straight Ratings (480V)
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Combination Ratings – Type E/F
• Combination Motor Controller
• SC & OL Protection
• Line Side Adapters Required
• Slash Ratings (480/277V)
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Combination Ratings - ASD
• 200kA with Class CC/J/T Fuse
• Suitable for Group Installations
• 65kA with Type E CMC (480/277V)
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Combination Ratings - Feeder
Component SCCR
OCPD I.R.
Nameplate
Voltage:
480
SCCR:
100kA
• Use UL Listed power
distribution block with high
SCCR when used with
specified fuses or circuit
breakers and required load
side conductors
• Must have feeder circuit
spacing (Listed PDB) if in
feeder circuit
Combination ratings of PDBs is cost effective fix
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Combination Ratings - PDBs
PDBFS220 (4 – 14 Load Side
Conductors) – 100kA with 175A
Class J fuse or less
59
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UL 508A Supplement SB
• What are the Rules (Sweep 1)?
• Lowest component SCCR limits assembly
SCCR unless:
• Feeder components are used that limit the shortcircuit current reducing the need for higher branch
circuit component SCCR (SB 4.3)
• Current limiting overcurrent protective devices
• Transformers rated 10kVA or less
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Feeder vs. Branch Circuits
Feeder Circuit
Branch
Circuit
Branch
Circuit
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Use of CL Devices - OCPD
• CL Device must be in the feeder circuit
• For CL Fuses or Circuit Breakers
• Use PEAK Let-through, Not RMS Let-Through (more
conservative)
• Fuses - cannot use manufacturer specific fuse data –
only tables based on performance requirements from
UL 248 (more conservative - apply to all
manufacturers)
• Circuit Breakers - must be Listed and marked “currentlimiting” and use published let-thru curves (most circuit
breakers are not CL)
• Can only raise downstream BRANCH circuit
components (not overcurrent protective devices –
fuses/circuit breaker IR or Combination motor
controllers SCCR)
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Fuse LT – UL vs Manufacturer
• LPJ-100SP
• UL Limits
• 50kA = 12kA
• 100kA = 14kA
• 200kA = 20kA
• Manufacturer Let
Through Curves
• Ip LT:
• 50kA = 9kA
• 100kA = 12kA
• 200kA = 14kA
• Irms LT:
• 50kA = 4kA
• 100kA = 5kA
• 200kA = 6kA
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Use of CL Devices - OCPD
Component SCCR
OCPD I.R.
Nameplate
Voltage:
480
SCCR:
14kA
F
• Current limiting
feeder OCPD
can be used to
increase branch
component
SCCR.
B
Use of CL OCPD can only fix components
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Use of CL Devices - Transformers
• CL Device must be in the feeder circuit
• For Transformers
• If 10kVA with secondary devices (components and
overcurrent devices) of 5kA or higher, apply the IR
of the primary overcurrent protective device
• If 5kVA (120V Secondary) with secondary devices
(Components and overcurrent devices) of 2kA or
higher, apply the IR of the primary overcurrent
protective device
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Use of CL Devices - Transformers
Component SCCR
OCPD I.R.
Nameplate
Voltage:
480
SCCR:
200kA
10 kVA
• Transformers can be used to
limit the short-circuit current
available
• If 10kVA & sec comp = 5kA+
• If 5kVA/120V & sec comp = 2kA+
• Then assign entire circuit I.R. of
primary overcurrent device
Use of Small Transformers (10kVA or less) can increase secondary component/OCPD ratings
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UL 508A Supplement SB
• What are the Rules (Sweep 2)?
• Lowest overcurrent protective device interrupting
rating (or SCCR for some devices) always limits
assembly SCCR
• All Feeder and branch circuit overcurrent devices
• Tested series combination ratings or series ratings
(fuse-circuit breaker or circuit breaker-circuit breaker)
are NOT allowed.
• Branch circuit overcurrent devices tapped from the feeder
circuit supplying a control circuit.
• Supplemental protective devices protecting the control
circuit transformer in a motor branch circuit.
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Overcurrent Protection Devices
• Branch Circuit Protective Devices
• Fuses
• Circuit Breakers
• Application Limited Devices
• Supplemental Protectors/Fuses
• Not suitable for branch circuit
protection (protecting a load)
• Can only be used in control circuits
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Why High SCCR?
• NEC & Equipment standards only require the
SCCR to be marked
• NEC requires the marked value to be adequate
for the maximum available fault current
• Although not required, high SCCR is often
preferred
• Flexibility of application
• Multiple installations
• Fault current unknown
• Insufficient SCCR can delay installation
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Fix It - Solutions
• High I.R. Protective
Devices
• Class R
• Class J
• Class J Drive Fuse
• Class T
• Class CC
• CubeFuse
• High Speed
• High IR/CL CBs
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Fix It - Solutions
• High SCCR Components
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Questions?
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