Industrial Control Panel Short Circuit
Current Ratings
Presented by Peter Walsh, PE
Rev: PRW 3-19
Overview of SCCR Requirements
Introduction
Requirements of NEC
UL 508a and the NEC
Branch and Feeder Analysis
Current-Limiting Affects
Increasing Panel SCCR
2
Presenter
Peter Walsh, PE
Senior Field Engineer-Mersen
•Member – National Electrical Code Making Panel 4 2008 Cycle
•Registered Professional Engineer
•Member of Arc Flash & PPE Research Testing Team in Newburyport Lab
•Participant in Annual 4-Day IEEE Safety Workshops
•Member of NFPA and IEEE IAS, Assoc Member of IAEI
•NFPA Technical Committee Member for NFPA 110 and NFPA 111
•IEEE Technical Committee Member for IEEE 1584 and IEEE 1814
3
Codes and Standards are Changing
NEC® 409 – National Electrical Code
– New article beginning in the 2005 NEC that covers “Industrial
Control Panels” and requires panels to be clearly Marked with a
Short Circuit Current Rating (SCCR).
4
5
Why the New Requirements?
Why the New Requirements? - Safety
People were not understanding
equipment Ratings, and
People were not applying Panels safely.
6
National Electrical Code Requirements
Overview of National Electrical Code
NEC 90.3 Code Arrangement-Chapters 1-4 apply generally.
90.4 Enforcement-The Authority Having Jurisdiction (AHD) has
responsibility for enforcement and interpretation.
90.7 Examination of Equipment for Safety- Listed equipment is
only examined to detect field modifications or damage
7
National Electrical Code Requirements
NEC 100 (Definitions)
Listed-Equipment that has been investigated and found safe
for specific conditions by a nationally recognized testing firm
which performs periodic inspections of production products and
labels the listed products.
8
National Electrical Code Requirements
NEC 110.3(A)(1) Examination. Identification, Installation, and
use of Equipment.
– (A) Examination-In judging equipment, considerations such as the
following shall be evaluated:
• (1) Suitability for installation and use with conformity with the provisions
of this code
FPN-Suitability of equipment may be evidenced by listing or labeling
9
National Electrical Code Requirements
NEC 110.10
(Paraphrased)-The overcurrent devices and downstream
components shall be selected and coordinated to prevent
extensive damage.
Listed products applied in accordance with their rating shall be
considered to meet the requirements of this section.
10
National Electrical Code Requirements
NEC 100 (Definitions)
Interrupting Rating- The highest current at rated
voltage that a device is intended to interrupt under
standard test conditions.
This Definition was frequently incorrectly used as an
panel rating instead of a device rating.
11
Short Circuit
100 A
FUSE
L
SOURCE
N
Up to 1,000 times rated current!
14
LOAD
15
Definition of SCCR
From UL-508A
2.40 SHORT CIRCUIT CURRENT RATING – The prospective
symmetrical fault current at a nominal voltage to which an
apparatus or system is able to be connected without sustaining
damage exceeding the defined acceptance criteria.
16
17
NEC and SCCR
What are the key references of Short Circuit Current
Rating (SCCR) in the National Electrical Code?
18
National Electrical Code Requirements
New Section Beginning in 2005
409 Industrial Control Panels
– 409.1 Scope- This article covers industrial control panels intended
for general use and operating at 600 volts or less.
20
National Electrical Code Requirements
409.110 Marking-An industrial control panel shall be marked
with the following information that is plainly visible after
installation:
– (3) Short circuit current rating (SCCR) of the industrial control panel
based on one of the following:
21
National Electrical Code Requirements
409.110(3) Markings-Continued
– a. Short-circuit current rating of a listed and labeled assembly
– b. Short-circuit current rating established using an approved
method
– FPN: UL 508A-2001, Supplement SB, is an example of an
approved method.
22
National Electrical Code Requirements
670 Industrial Machinery
670.3 Machine Nameplate Data
670.3 (A) Permanent Nameplate
25
National Electrical Code Requirements
670.3(A)(4) Short-circuit current rating (SCCR) of the machine
industrial control panel based upon one of the following:
– a. Short-circuit current rating of a listed and labeled machine control
enclosure or assembly
– b. Short-circuit current rating established using an approved
method
– FPN: UL 508A-2001, Supplement SB, is an example of an
approved method.
26
National Electrical Code Revision-2011
409.22 Short-Circuit Current Rating.
An industrial panel shall not be installed where the
available fault current exceeds its short-circuit current
rating as marked in accordance with 670.3(A)(4).
670.5 Short-Circuit Current Rating.
Industrial machinery shall not be installed where the
available fault current exceeds its short-circuit current
rating as marked in accordance with 670.3(A)(4).
27
SCCR Determination?
How are Short-Circuit Current
Ratings Determined?
28
What Drove The Change
UL508A – Industrial Control Panels
– Listed panels required the SCCR to be clearly
MARKED on the panel effective April 25, 2006.
29
Determining Panel SCCR
Let’s look at an example of a panel to determine its SCCR.
We’ll consider strategies for increasing panel SCCR.
30
Key Definitions From UL508A
Short Circuit Current Rating (SCCR)

The prospective symmetrical fault current at a nominal voltage to which
an apparatus or system is able to be connected without sustaining
damage exceeding the defined acceptance criteria.
Branch Circuit:
– The conductors and components following the last overcurrent protective
device protecting a load.
Feeder Circuit:
– The conductors and circuitry on the supply side of the branch circuit
overcurrent protective device.
Interrupting Rating (NEC definition)

The highest current at rated voltage that a device is intended to interrupt
under standard test conditions.
31
Determining Panel SCCR
Step 1: Assign a SCCR to each branch circuit.
Step 2: Assign a SCCR to the feeder circuit.
Step 3: Consider the current-limiting effects of the Feeder
OCPD (IF current-limiting).
32
Step 1: Consider Each Branch
Step 1: Make a pass of each branch circuit to determine the SCCR
value of each branch circuit in the panel.
– A) Check each component for SCCR markings:
•
•
If marked - use that value
If not marked - check instruction sheets provided with component to determine if
there is a value assigned and under what conditions.
– B) If no SCCR value is marked on the component or on instruction /
installation sheets:
•
Refer to Table SB4.1 to determine the default SCCR for the component.
– C) Check if “type testing” was done for a motor controller per UL 508 for
high fault currents.
•
The tested value may be used as the SCCR.
33
UL 508A Supplement SB4.1
34
60A Class
J-TD
What is the SCCR of
this panel?
15A
Fuse
FSPDB
Size 2A
175A
20A
MCCB
15A
Contactor
Overload
Relay
35
Integral
Motor
Controller
15A
Fuse
15A
Contactor
Overload
Relay
Step 1: consider each
branch circuit.
First Pass – Branch 1
15A
Fuse
60A Class
J-TD
FSPDB
Size 2A
175A
20A
MCCB
15A
Fuse
200KA
SCCR Marked
on Product
15A
Contactor
10KA
Overload
Relay
10KA
36
Integral
Motor
Controller
15A
Contactor
Overload
Relay
Step 1: consider each
branch circuit.
First Pass – Branch 1
60A Class
J-TD
FSPDB
Size 2A
175A
Branch 1
SCCR
10KA
15A
Fuse
20A
MCCB
15A
Fuse
200KA
SCCR Marked
on Product
15A
Contactor
10KA
Overload
Relay
10KA
37
Integral
Motor
Controller
15A
Contactor
Overload
Relay
Step 1: consider each
branch circuit.
Branch 1
SCCR
10KA
First Pass – Branch 2
15A
Fuse
60A Class
J-TD
FSPDB
Size 2A
175A
20A
MCCB
200KA
15A
Fuse
65KA
SCCR Marked
on Product
15A
Contactor
10KA
Overload
Relay
10KA
38
Integral
Motor
Controller
15A
Contactor
10KA
Overload
Relay
60A Class
J-TD
Step 1: consider each
branch circuit.
Branch 1
SCCR
10KA
First Pass – Branch 2
Branch 2
SCCR
10KA
15A
Fuse
200KA
FSPDB
Size 2A
175A
20A
MCCB
15A
Fuse
65KA
SCCR Marked
on Product
15A
Contactor
10KA
Overload
Relay
10KA
39
Integral
Motor
Controller
15A
Contactor
10KA
Overload
Relay
60A Class
J-TD
Step 1: consider each
branch circuit.
FSPDB
Size 2A
175A
Branch 1
SCCR
10KA
Branch 2
SCCR
10KA
15A
Fuse
200KA
First Pass – Branch 3
20A
MCCB
15A
Fuse
65KA
200KA
SCCR Not
Marked on
Product
15A
Contactor
10KA
Overload
Relay
10KA
40
Integral
Motor
Controller
15A
Contactor
?
10KA
Overload
Relay
?
UL 508A Supplement SB4.1
41
60A Class
J-TD
Step 1: consider each
branch circuit.
FSPDB
Size 2A
175A
Branch 1
SCCR
10KA
Branch 2
SCCR
10KA
15A
Fuse
200KA
First Pass – Branch 3
20A
MCCB
15A
Fuse
65KA
200KA
Table SB 4.1
15A
Contactor
10KA
Overload
Relay
10KA
42
Integral
Motor
Controller
15A
Contactor
SB4.1 5KA
10KA
Overload
Relay
SB4.1 5KA
60A Class
J-TD
Step 1: consider each
branch circuit.
FSPDB
Size 2A
175A
Branch 1
SCCR
10KA
Branch 2
SCCR
10KA
15A
Fuse
200KA
First Pass – Branch 3
20A
MCCB
65KA
Branch 3
SCCR
5KA
15A
Fuse
200KA
Table SB 4.1
15A
Contactor
10KA
Overload
Relay
10KA
43
Integral
Motor
Controller
15A
Contactor
SB4.1 5KA
10KA
Overload
Relay
SB4.1 5KA
Step 2: Consider Feeder Circuit
Step 2: Make a pass of the feeder circuit and determine the
SCCR of the feeder circuit.
44
Step 2: what’s the SCCR
of the feeder circuit ?
Second Pass – Feeder
15A
Fuse
60A Class
J-TD
FSPDB
Size 2A
175A
200KA
?
20A
MCCB
200KA
15A
Contactor
10KA
Overload
Relay
10KA
45
SCCR Not
Marked on
Product
15A
Fuse
65KA
Integral
Motor
Controller
200KA
15A
Contactor
SB4.1 5KA
10KA
Overload
Relay
SB4.1 5KA
Step 2: what’s the SCCR
of the feeder circuit ?
Second Pass – Feeder
15A
Fuse
60A Class
J-TD
FSPDB
Size 2A
175A
200KA
100KA
20A
MCCB
200KA
15A
Contactor
10KA
Overload
Relay
10KA
46
65KA
Integral
Motor
Controller
From Table SB4.1
the default SCCR is
10KA unless
Tested
15A
Fuse
200KA
15A
Contactor
SB4.1 5KA
10KA
Overload
Relay
SB4.1 5KA
FSPDB Ratings
FSPDB Ratings
47
FSPDB Ratings
48
Step 2: what’s the SCCR
of the feeder circuit ?
Second Pass – Feeder
15A
Fuse
60A Class
J-TD
FSPDB
Size 2A
175A
200KA
100KA
20A
MCCB
200KA
15A
Contactor
10KA
Overload
Relay
10KA
49
Feeder
SCCR
100KA
15A
Fuse
65KA
Integral
Motor
Controller
200KA
15A
Contactor
SB4.1 5KA
10KA
Overload
Relay
SB4.1 5KA
Feeder & Branch SCCRs
after Step 2.
Branch 1
SCCR
10KA
Branch 2
SCCR
10KA
15A
Fuse
200KA
60A Class
J-TD
FSPDB
Size 2A
175A
200KA
Feeder
SCCR
100KA
100KA
Branch 3
SCCR
5KA
20A
MCCB
65KA
15A
Fuse
200KA
Table SB 4.1
15A
Contactor
10KA
Overload
Relay
10KA
50
Integral
Motor
Controller
15A
Contactor
SB4.1 5KA
10KA
Overload
Relay
SB4.1 5KA
Step 3: Effects of Feeder OCPD
Step 3: Consider the current-limiting effects of the feeder
OCPD.
– If a FUSE, refer to Table SB4.2 to determine the Ip for the fuse and
compare the Ip to the SCCR for each branch component.
•
If the Ip is equal to or lower than each downstream component, then
you are allowed to mark the panel with the SCCR value for the
appropriate Ip column (50kA, 100kA or 200kA). But the Ip is not
allowed to increase the SCCR value of the downstream fuse or circuit
breaker, just the SCCR value of other components.
– If a circuit breaker, and if labeled “Current-limiting” , the CB
manufacturer’s published Ip curves may be used to determine CB
Ip.
51
Fuse Basics - Ip
FAULT CURRENT
WITHOUT FUSE
IN CIRCUIT
NORMAL LOAD
CURRENT
52
FUSE OPERATION
Fuse Basics - Ip
(IRMS )2 t ~ Thermal Energy
Ip
MELTING
TIME
ARCING
TIME
CLEARING TIME
53
Step 3: If current-limiting,
consider feeder OCPD
effects
From Table SB4.2 (100KA): Ip = 10KA
Branch 1
SCCR
10KA
Branch 2
SCCR
10KA
15A
Fuse
200KA
60A Class
J-TD
FSPDB
Size 2A
175A
200KA
Feeder
SCCR
100KA
100KA
Branch 3
SCCR
5KA
20A
MCCB
15A
Fuse
65KA
200KA
Table SB 4.1
15A
Contactor
10KA
Overload
Relay
10KA
54
Integral
Motor
Controller
15A
Contactor
SB4.1 5KA
10KA
Overload
Relay
SB4.1 5KA
UL 508A Supplement SB Updates
55
60A Class
J-TD
Step 3: Feeder OCPD
effects on branch #1
From Table SB4.2 (100KA): Ip = 10KA
Branch
Branch11SCCR
increases
from 10KA to
SCCR
100KA
100KA
Branch 2
SCCR
10KA
15A
Fuse
200KA
FSPDB
Size 2A
175A
200KA
100KA
Branch 3
SCCR
5KA
20A
MCCB
65KA
15A
Fuse
200KA
Table SB 4.1
15A
Contactor
10KA
Overload
Relay
10KA
56
Integral
Motor
Controller
15A
Contactor
SB4.1 5KA
10KA
Overload
Relay
SB4.1 5KA
Step 3: Feeder OCPD
effects on branch #2
Ip = 10KA allows increase in SCCR of
Branch 2
Branch 1
SCCR
100KA
Branch 2
SCCR
increases from
10KA to
65KA
15A
Fuse
200KA
60A Class
J-TD
FSPDB
Size 2A
175A
200KA
Feeder
SCCR
100KA
100KA
Branch 3
SCCR
5KA
20A
MCCB
65KA
15A
Fuse
200KA
Table SB 4.1
15A
Contactor
10KA
Overload
Relay
10KA
57
Integral
Motor
Controller
15A
Contactor
SB4.1 5KA
10KA
Overload
Relay
SB4.1 5KA
60A Class
J-TD
Step 3: Feeder OCPD
effects on branch 3
From Table SB4.2 (100KA): Ip = 10KA
Branch 1
SCCR
100KA
Branch 2
SCCR
65KA
15A
Fuse
200KA
FSPDB
Size 2A
175A
200KA
Feeder
SCCR
100KA
100KA
Branch 3 SCCR
remains 5KA
20A
MCCB
65KA
15A
Fuse
200KA
Table SB 4.1
15A
Contactor
10KA
Overload
Relay
10KA
58
Integral
Motor
Controller
15A
Contactor
SB4.1 5KA
10KA
Overload
Relay
SB4.1 5KA
Question: What’s the panel
SCCR?
Answer: 5KA due to branch
3
Branch 1
SCCR
100KA
Branch 2
SCCR
65KA
15A
Fuse
200KA
60A Class
J-TD
FSPDB
Size 2A
175A
200KA
Feeder
SCCR
100KA
100KA
Branch 3 SCCR
remains 5KA
20A
MCCB
65KA
15A
Fuse
200KA
Table SB 4.1
15A
Contactor
10KA
Overload
Relay
10KA
59
Integral
Motor
Controller
15A
Contactor
SB4.1 5KA
10KA
Overload
Relay
SB4.1 5KA
SB4.3 and SB4.4 Require
Panel SCCR cannot be higher than the feeder circuit SCCR
and cannot be higher than the lowest branch circuit SCCR.
This results in this panel being limited to installations with
available fault current of less than 5,000A! Most industrial
facilities have 20,000 to 65,000 of available fault current.
60
Increasing Panel SCCR
Use current-limiting feeder fuses to protect weak components.
Upgrade to components with higher SCCR.
Add sub-feeder fuses.
Consider type tested components, i.e., components that have
been tested by the manufacturer under specific conditions and
witnessed by UL.
61
How can we increase this panel SCCR ?
1) Upgrade your components in Branch 3
15A CC
Fuse
200KA
15A
Contactor
Originally
5KA
Upgraded Overload
Relay
62
Upgrade to
component with
10KA SCCR
How can we increase this panel SCCR ?
Option 1) Upgrade your components in Branch 3
FEEDER FUSE:
60A J – TD
10KA Ip @ at 100KA
from Table SB4.2
15A CC
Fuse
200KA
Ip of 60A J fuse at
100KA is equal to or
lower than branch
rating.
Component upgrade
allows the branch 3
SCCR to be increased
to 100KA after
considering effects of
60A feeder fuse.
SCCR is now marked
on Product
15A Contactor
New - 10KA
Upgraded Overload
Relay
New - 10KA
63
How can we increase this panel SCCR ?
Option 2) Add a sub feeder fuse in Branch 3
20A CC
200KA
Fuse
By adding a 20A Class CC
fuse as a sub feeder, you are
now allowed to use this fuse
as your current limiting
feeder fuse.
15A CC
Fuse
200KA
Table SB 4.1
15A
Contactor
Sub Feeder Fuse
20A Class CC
4KA Ip @ 100KA from Table
SB4.2
64
SB4.1 5KA
Overload
Relay
SB4.1 5KA
SCCR Not
Marked on
Product
65
How can we increase this panel SCCR ?
Option 2) Add a sub feeder fuse in Branch 3
20A CC
200KA
Fuse
By adding a 20A Class CC
fuse as a sub feeder, you are
now allowed to use this fuse
as a current limiting feeder
fuse.
15A CC
Fuse
Adding a sub feeder
fuse allows the
branch 3 SCCR
to be increased to
- 100KA -
200KA
Table SB 4.1
15A
Contactor
Sub Feeder Fuse
20A Class CC
4KA Ip @ 100KA from Table
SB4.2
66
SB4.1 5KA
Overload
Relay
SB4.1 5KA
SCCR Not
Marked on
Product
How can we increase this panel SCCR ?
Option 3) Use type tested combination components

Components that have been tested by the manufacturer under specific conditions and
witnessed by UL.
15A CC
Fuse
200KA
15A Contactor
5KA
Overload Relay
5KA
67
SB4.2.2 c)
SB4.2.3
How can we increase this panel SCCR ?
3) Use “type tested” combination components

Components that have been tested by the manufacturer under specific conditions and
witnessed by UL.
15A CC
Fuse
200KA
SB4.2.2 c)
SB4.2.3
This
particular
Controller
SCCR is
100kA when
protected by
branch 15A
Class CC
fuses due UL
“type testing”
68
How can we increase this panel SCCR ?
Option 3) Use type tested combination components

Components that have been tested by the manufacturer under specific conditions and
witnessed by UL.
It’s a little work to get the
information and many
combinations have not
been tested but it can
save some time and
money.
15A CC
Fuse
200KA
Using type tested
components allows
branch 3 SCCR
to be increased to
- 100KA -
Combination “type test”
increased SCCR rating.
100KA
69
Example Supplier Data Sheet
Data for A-B 100C09D10 Motor Controller
(A) Signifies amp rating of OCPD
Data Courtesy Rockwell Automation
70
What’s the short circuit
rating of the panel ?
Answer: 65KA
Branch 1
SCCR
100KA
PDB
175A
Branch 2
SCCR
65KA
15A
Fuse
200KA
60A Class
J-TD
200KA
Feeder
SCCR
100KA
100KA
Branch 3
SCCR
100KA
20A
MCCB
65KA
15A
Fuse
200KA
Table SB 4.1
15A
Contactor
Marked
10KA
Overload
Relay
Marked
10KA
72
Integral
Motor
Controller
Marked
10KA
15A
Contactor
100KA type
tested
Overload
Relay
100KA type
tested
The Control Circuit - SCCR
See SB3.2 and SB4.4
For control circuits tapped from the feeder, just provide a branch
circuit protective device with an I.R. that is not less than the
panel SCCR. The control circuit components are considered
protected with the above procedure.
Similarly using a control transformer the primary protection I.R.
is used for SCCR calculations.
Control components are never exposed to the fault current as
result of the shorting out of the load current of the panel,
therefore they have no SCCR.
73
The Control Circuit - SCCR
74
Power Transformers
Power Transformers 5kVA Max
– Secondary components must have 2kA SCCR Min
– 120 V secondary Max
– Must have the primary OCPD with rating at least the same as the
panel
Power Transformers 10kVA Max
– Secondary components must have 5kA SCCR Min
– Must have the primary OCPD with rating at least the same as the
panel
Other Power Transformer circuits must use the lowest rated
SCCR component rating of the secondary
75
In summary: How can we increase the panel SCCR ?
Select components with high SCCR.
1.
–
Select components with higher SCCRs marked on them. No
markings means using the default values.
Use current-limiting fuses to protect weak components.
2.
–
Feeder or sub-feeder fuse allows you to take advantage of the
lower Ip a smaller fuse provides.
Consider use of type tested combination components.
3.
–
Components that have been tested by the manufacturer under
specific conditions and witnessed by UL.
76
How Can We Help?
Provide COMPONENTS with high SCCR values marked on
them.
–
–
–
–
–
USFM - 200kA
UltraSafe – 200kA
Open style fuse blocks – up to 200kA
SurgeTrap – 200kA
FS UL-98 disconnects – 50 to 200kA
– FSPDBs – 100kA
– Open style PDBs – 100kA
77
Instructions allow high
SCCR of 100k. Can be
directly marked on product.
How Can We Help?
Provide COMPONENTS with high SCCR values as a result of
Special Testing
Mersen MPDB protected by a specific Circuit Breaker
– Mersen will test popular MPDBs with specific CBs to obtain UL
certification
– Customer will provide 3 circuit breakers for the testing
– Timing is dependent on available lab schedule but is estimated to
be about 2 months
78
80
Example 100k SCCR
81
FuserBloc Switch and FSPDB
82
Surge Trap and UltraSafes
83
Summary
Significant changes to the 2005 NEC and UL508A require
SCCR values to be MARKED on Industrial Control Panels to
ensure their safe use in end user installations.
– In many industrial facilities 5KA and 10KA SCCRs will not be
enough based on the fault levels available. The higher the safer.
Methods to achieve higher SCCR values is eased by the use
of FUSES.
– 50KA is pretty easy to obtain
– With a little review 100KA is attainable
– 200KA is achievable with properly selected current limiting fuses
when using higher rated components.
Literature is available at the Mersen Booth
and from the EP.Mersen.com website
85
Presented by
Peter Walsh, PE
86