Spacing Requirements for
Power Distribution and
Terminal Blocks
Spacing Requirements for Power Distribution
and Terminal Blocks
New requirements in the
2005 NEC
®
for a marked
short-circuit current rating on
industrial control panels,
industrial machinery, and
commercial/industrial HVAC
equipment have resulted in
considerable attention being
placed upon the proper
selection and application of
Power Distribution Blocks (PDBs) and Terminal Blocks. It is
fairly well understood that if an assembly short-circuit current
rating above 10,000 amperes is desired, a Power Distribution
Block or a Terminal Block with a high short-circuit current
rating must be utilized. However, there is great confusion
concerning the spacing requirements of these devices. This
paper focuses on proper applications, based upon these
spacing requirements.
Proper Application
When applying Power Distribution Blocks (PDBs), there are
various requirements that shall be satisfied, based upon
different UL Standards, the NEC
®
, and the specific
application. Some of the requirements and ratings include:
voltage, continuous current, wire range (load and line side),
short-circuit current rating or withstand rating (SCCR), type of
upstream overcurrent protective device (fuse or circuit
breaker) and spacing (between uninsulated live part and
uninsulated grounded part or uninsulated live parts of
opposite polarity). The requirements vary based on the
application for industrial control panels (feeder circuit or
branch circuit), field applications, or HVAC applications.
PDB Spacing Requirements for Equipment:
Depending upon the specific application and the standard to
which the equipment is being designed, there are certain
PDBs that can meet these spacing requirements, some with a
high SCCR. The table below details the spacing
requirements at 600V based upon the equipment standards.
Spacing between
Spacing between
live parts of
live parts and
opposite polarity
grounded parts
or enclosure
Through air Over surface
@600V
@600V
@600V
3
inspector checks, among other things, to make sure there is
adequate spacing for the OEM application in which they are
being applied.
Power Distribution blocks are evaluated to UL1953, the Power
Distribution Block standard and are listed for general
installation, meaning they have adequate spacing for most
OEM and field applications. These Power Distribution Blocks
are marked with a listing mark, which means that the
inspector does not need to check ”conditions of acceptability”
as would be needed with recognized products. As listed
products, they are suitable for use in the field. For example,
an electrical contractor may install them in a wireway,
something that could not be done with a recognized product
such as a terminal block.
Industrial Control Panels (UL508A):
UL508A contains two important requirements to consider
when applying power distribution blocks.
• Spacing of 1˝ through air, 2˝ over surface (at 600V) is
required when used in a feeder circuit (that’s everything
ahead of or on the line side of the final branch circuit
overcurrent protective device).
• An assumed short-circuit current rating of 10kA for blocks
not marked with a higher short-circuit current rating, per
UL508A Table SB4.1.
The following details the requirements regarding the use of
power distribution blocks in industrial control panels.
Use of Power Distribution Blocks in Industrial Control
Panel Feeder Circuits
• A Listed PDB (UL1953) can be used “as is” since it
meets the 2˝ and 1˝ spacing requirements for feeder
circuits in UL508A section 10.2 (see table 10.2) & 28.2.4.
• A Recognized Terminal Blocks (UL1059) can only be
used if it meets the spacing requirements in 10.2 &
28.2.1 (see Table 10.1) and it is suitable for field wiring
(use Group A, C or D of UL 1059).
• Use Group A: Service — including dead-front
switchboards, panelboards, service equipment, and
the like.
• Use Group B: Commercial appliances, including
business equipment, electronic data processing
equipment, and the like.
• Use Group C: Industrial, general.
• Use Group D: Industrial, devices having limited ratings.
Such as where the load on any single circuit of the
terminal block does not exceed 15 amperes at 51 —
150 volts, 10 amperes at 151 — 300 volts, 5 amperes
at 301 — 600 volts, or the maximum ampere rating,
whichever is less.
Listed PDBs have adequate spacing for feeder circuit
applications but most recognized terminal blocks don’t
have the spacing required for use in feeder circuits
UL Standard
508A Branch
Circuits
508A Feeder
Circuits
1995 HVAC
⁄
8
˝
⁄
8
˝
1
⁄
2
˝
⁄
2
˝
1
⁄
2
˝
⁄
2
˝
1˝
3
2˝
1
1˝
1
Note: Refer to Specific UL standards for complete spacing details.
UL1953 vs. UL1059
Most power distribution blocks available today are actually
terminal blocks, and are recognized to UL 1059, the Terminal
Block standard. Terminal blocks may or may not meet the
spacing needed for OEM applications. Terminal blocks are
marked with a UL recognition mark to ensure that the UL
© 2011 Cooper Bussman, Inc.
2
Spacing Requirements for Power Distribution
and Terminal Blocks
Use of Power Distribution Blocks in Industrial Control
Panel Branch Circuits
• A Listed PDB (UL1953) can be used “as is” since it
exceeds the spacing requirements for branch circuits in
UL508A section 10.2 (see table 10.1) & 28.2.4.
• A Recognized Terminal Block (UL1059) can only be used
if it meets the spacing requirements at the required
voltage in UL 508A, section 10.2 & 28.2.1 (see Table
10.1) and it is suitable for field wiring (Use Group A, C or
D of UL 1059).
Listed PDBs have adequate spacing for branch
circuit applications and most recognized PDBs are
also adequate for branch circuit applications
Power Distribution Block Application Examples –
UL 508A
HVAC Equipment (UL1995):
The larger spacing that is mandated in UL508A is not present
in UL1995. However, HVAC equipment utilizes control panels
that are UL Listed 508A control panels. If the control panel for
the HVAC equipment is a listed UL508A panel, the larger
UL508A spacing is required.
NEW 2005 NEC
®
Requirements For Wireways:
Listed power distribution blocks can be used to meet the new
2005 NEC
®
requirement in section 376.56(B) for wireways.
Since terminal blocks are recognized products to UL1059,
they are not allowed for use for field wiring in metal wireways
NATIONAL ELECTRICAL CODE
®
:
• 376.56(B) Power Distribution Blocks.
• (1) Installation. Power distribution blocks installed in metal
wireways shall be listed.
• (4) Live Parts. Power distribution blocks shall not have
exposed live parts in the wireway after installation.
The NEC
®
now requires a PDB listed to UL 1953,
equipped with a cover to prevent exposure of live
parts when installed in metal wireway
Power Distribution Block Application Examples –
UL 1995 and NEC
®
376.56(B)
PDB in UL 508A, 480V Feeder Circuit must have spacing of
1˝ through air and 2˝ over surface. PDBs listed to UL 1953
meet these spacing requirements. Terminal Blocks recog-
nized to UL1059 must be verified to have required spacing.
PDB in 480V UL 1995 HVAC Equipment must have spacing
of
3
⁄
8
˝ through air and
1
⁄
2
˝ over surface. PDBs listed to UL 1953
meet this spacing as well as most terminal blocks
recognized to UL 1059.
PDB in UL508A, 480V Branch Circuit must have spacing of
3
⁄
8
˝
through air and
1
⁄
2
˝ over surface. PDBs listed to UL 1953 meet
these spacing as well as most terminal blocks
recognized to UL 1059.
3
© 2011 Cooper Bussman, Inc.
Spacing Requirements for Power Distribution
and Terminal Blocks
PDBs installed in wireways must be listed to UL1953 and
provided with a cover per NEC
®
376.56(B). Terminal blocks
recognized to UL1059 are not permitted because they are
recognized, not listed.
Note: Marking the equipment with its short-circuit current
rating (SCCR) is another important consideration in the proper
application of PDBs. Cooper Bussmann has a new line of
PDBs with high short-circuit current ratings which makes it
easier to achieve a high short-circuit current rating for the
entire assembly, see the new Cooper Bussmann PDBs with
High SCCR Datasheet (Reorder #3134). For more
information on the topic of SCCRs for industrial control
panels, see the new Cooper Bussmann Simplified Guide to
Understanding Short-Circuit Current Rating (Reorder #7009).
For more information or questions, please contact Cooper
Bussmann Application Engineering at (636) 527-1270 or
fusetech@CooperBussmann.com.
Also visit us on the web at
cooperbussmann.com.
UL 1953 LISTED HIGH SCCR POWER DISTRIBUTION BLOCK APPLICATION CHART @ 600V
Cooper
Bussmann Part
Number
PDB204-3
PDB220-3
PDB220-3
PDB280-3
PDB321-1
PDB321-1
PDB321-2
PDB321-2
PDB321-3
PDB321-3
PDB323-3
PDB323-3
PDB370-3
PDB370-3
PDB371-3
PDB371-3
Description
Line 2/0-8 Cu, Load 2/0-8 Cu, 3 Pole
Line 2/0-8 Cu, Load (4) 4-12 Cu, 3 Pole
Line 2/0-8 Cu, Load (4) 4-14 Cu, 3 Pole
Line 2/0-8 Cu, Load 1/4 - 20 X 3/4 STUD, 3 Pole
Line 2/0-8 Cu, Load (6) 4-12 Cu, 1 Pole
Line 2/0-8 Cu, Load (6) 4-14 Cu, 1 Pole
Line 2/0-8 Cu, Load (6) 4-12 Cu, 2 Pole
Line 2/0-8 Cu, Load (6) 4-14 Cu, 2 Pole
Line 2/0-8 Cu, Load (6) 4-12 Cu, 3 Pole
Line 2/0-8 Cu, Load (6) 4-14 Cu, 3 Pole
Line 350-4 Cu, Load (6) 4-8 Cu, 3 Pole
Line 350-4 Cu, Load (6) 4-12 Cu, 3 Pole
Line 350-4 Cu, Load (12) 4-8 Cu, 3 Pole
Line 350-4 Cu, Load (12) 4-14 Cu, 3 Pole
Line 350-4 Cu, Load (6) 2-6 Cu and (3) 1/0-6 Cu, 3 Pole
Line 350-4 Cu, Load (9) 2-12 Cu, 3 Pole
Max OCPD
Class J, T, CC or G
Class J, T, CC or G
Class J, T, CC or G
Class J, T, CC or G
Class J, T, CC or G
Class J, T, CC or G
Class J, T, CC or G
Class J, T, CC or G
Class J, T, CC or G
Class J, T, CC or G
Class J, T, CC or G
Class J, T, CC or G
Class J, T, CC or G
Class J, T, CC or G
Class J, T, CC or G
Class J, T, CC or G
SCCR (kA)
200
200
100
200
200
100
200
100
200
100
200
100
200
100
200
100
UL 508A
Branch
Circuit
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
UL 508A
Feeder
Circuit
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
UL 1995
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
YES
NEC
®
376.56(B)
YES*
YES*
YES*
YES*
YES*
YES*
YES*
YES*
YES*
YES*
YES*
YES*
YES*
YES*
YES*
YES*
200A
200A
175A
200A
400A
175A
400A
175A
400A
175A
400A
175A
400A
175A
400A
175A
*Optional covers required. For PDB204-1, -3, PDB220-1, -3, and PDB280-1, -3, Use CPB162-1 (One cover required per pole). For PDB321-1, -2, -3, Use CPB-1, -2, -3. For
PDB323-1, -3, PDB370-1, -3, PDB371-1, -3, use CPDB-1 (One cover required per pole).
© 2011 Cooper Bussman, Inc.
4
Spacing Requirements for Power Distribution
and Terminal Blocks
UL 1059 Recognized High SCCR Power Terminal Block Application Chart @ 600V
Cooper
Bussmann
Part Number Description
14002-2UL
Line 2-14Cu, Load 2-8 Cu, 2-Pole
14002-2UL
Line 2-14Cu, Load 2-14 Cu, 2-Pole
14002-3UL
Line 2-14Cu, Load 2-8 Cu, 3-Pole
14002-3UL
Line 2-14Cu, Load 2-14 Cu, 3-Pole
14002-4UL
Line 2-14Cu, Load 2-8 Cu, 4-Pole
14002-4UL
Line 2-14Cu, Load 2-14 Cu, 4-Pole
16204-1
Line 2/0-8 Cu, Load 2/0-8 Cu, 1-Pole
16204-2
Line 2/0-8 Cu, Load 2/0-8 Cu, 2-Pole
16204-3
Line 2/0-8 Cu, Load 2/0-8 Cu, 3-Pole
16220-1
Line 2/0-8 Cu, Load (4) 4-12 Cu, 1-Pole
16220-1
Line 2/0-8 Cu, Load (4) 4-14 Cu, 1-Pole
16220-2
Line 2/0-8 Cu, Load (4) 4-12 Cu, 2-Pole
16220-2
Line 2/0-8 Cu, Load (4) 4-14 Cu, 2-Pole
16220-3
Line 2/0-8 Cu, Load (4) 4-12 Cu, 3-Pole
16220-3
Line 2/0-8 Cu, Load (4) 4-14 Cu, 3-Pole
16280-1
Line 2/0-8 Cu, Load 1/4-20 X 3/4 STUD, 1-Pole
16280-2
Line 2/0-8 Cu, Load 1/4-20 X 3/4 STUD, 2-Pole
16280-3
Line 2/0-8 Cu, Load 1/4-20 X 3/4 STUD, 3-Pole
16321-1
Line 2/0-8 Cu, Load (6) 4-12 Cu, 1-Pole
16321-1
Line 2/0-8 Cu, Load (6) 4-14 Cu, 1-Pole
16321-2
Line 2/0-8 Cu, Load (6) 4-12 Cu, 2-Pole
16321-2
Line 2/0-8 Cu, Load (6) 4-14 Cu, 2-Pole
16321-3
Line 2/0-8 Cu, Load (6) 4-12 Cu, 3-Pole
16321-3
Line 2/0-8 Cu, Load (6) 4-14 Cu, 3-Pole
16323-1
Line 350-4 Cu, Load (6) 4-8 Cu, 1-Pole
16323-1
Line 350-4 Cu, Load (6) 4-12 Cu, 1-Pole
16323-2
Line 350-4 Cu, Load (6) 4-8 Cu, 2-Pole
16323-2
Line 350-4 Cu, Load (6) 4-12 Cu, 2-Pole
16323-3
Line 350-4 Cu, Load (6) 4-8 Cu, 3-Pole
16323-3
Line 350-4 Cu, Load (6) 4-12 Cu, 3-Pole
16370-1
Line 350-4 Cu, Load (12) 4-8 Cu, 1-Pole
16370-1
Line 350-4 Cu, Load (12) 4-14 Cu, 1-Pole
16370-2
Line 350-4 Cu, Load (12) 4-8 Cu, 2-Pole
16370-2
Line 350-4 Cu, Load (12) 4-14 Cu, 2-Pole
16370-3
Line 350-4 Cu, Load (12) 4-8 Cu, 3-Pole
16370-3
Line 350-4 Cu, Load (12) 4-14 Cu, 3-Pole
16371-1
Line 350-4 Cu, Load (6) 2-6 and (3) 1/10-6 Cu, 1-Pole
16371-1
Line 350-4 Cu, Load (9) 2-12 Cu, 1-Pole
16371-2
Line 350-4 Cu, Load (6) 2-6 and (3) 1/10-6 Cu, 2-Pole
16371-2
Line 350-4 Cu, Load (9) 2-12 Cu, 2-Pole
16371-3
Line 350-4 Cu, Load (6) 2-6 and (3) 1/10-6 Cu, 3-Pole
16371-3
Line 350-4 Cu, Load (9) 2-12 Cu, 3-Pole
*May be possible to use if spaced as required per UL 508A Standard
UL Recognized to Use Group C
UL 508A UL 508A
Branch Feeder
SCCR (kA) Circuit Circuit
200
YES
NO
100
YES
NO
200
YES
NO
100
YES
NO
200
YES
NO
100
YES
NO
200
YES
NO*
200
YES
NO
200
YES
NO
200
YES
NO*
100
YES
NO*
200
YES
NO
100
YES
NO
200
YES
NO
100
YES
NO
200
YES
NO*
200
YES
NO
200
YES
NO
200
YES
YES
100
YES
YES
200
YES
YES
100
YES
YES
200
YES
YES
100
YES
YES
200
YES
NO*
100
YES
NO*
200
YES
NO
100
YES
NO
200
YES
NO
100
YES
NO
200
YES
NO*
100
YES
NO*
200
YES
NO
100
YES
NO
200
YES
NO
100
YES
NO
200
YES
NO*
100
YES
NO*
200
YES
NO
100
YES
NO
200
YES
NO
100
YES
NO
UL
NEC
®
1995 376.56(B)
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
Max OCPD
200A Class J, T, CC or G
175A Class J, T, CC or G
200A Class J, T, CC or G
175A Class J, T, CC or G
200A Class J, T, CC or G
175A Class J, T, CC or G
200A Class J, T, CC or G
200A Class J, T, CC or G
200A Class J, T, CC or G
200A Class J, T, CC or G
175A Class J, T, CC or G
200A Class J, T, CC or G
175A Class J, T, CC or G
200A Class J, T, CC or G
175A Class J, T, CC or G
200A Class J, T, CC or G
200A Class J, T, CC or G
200A Class J, T, CC or G
400A Class J, T, CC or G
175A Class J, T, CC or G
400A Class J, T, CC or G
175A Class J, T, CC or G
400A Class J, T, CC or G
175A Class J, T, CC or G
400A Class J, T, CC or G
175A Class J, T, CC or G
400A Class J, T, CC or G
175A Class J, T, CC or G
400A Class J, T, CC or G
175A Class J, T, CC or G
400A Class J, T, CC or G
175A Class J, T, CC or G
400A Class J, T, CC or G
175A Class J, T, CC or G
400A Class J, T, CC or G
175A Class J, T, CC or G
400A Class J, T, CC or G
175A Class J, T, CC or G
400A Class J, T, CC or G
175A Class J, T, CC or G
400A Class J, T, CC or G
175A Class J, T, CC or G
© 2011 Cooper Bussman, Inc.
5
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