Tech Talk 1: Volume 1 – Issue 1
January 2002
TECH TALK
Technical Newsletter for Machinery Builders, Electrical Panel Builders,
OEMs, Plant Engineers, and Integrators
New 2002 NEC  Requirement Affects The
Proper Application of Overcurrent Protective
Devices with a Slash Voltage Rating
Proper application of an overcurrent protective device considers
three major ratings of the device:
• Ampere Rating
• Interrupting Rating
• Voltage Rating
The proper application of an overcurrent protective device
according to its voltage rating requires that the voltage rating of the
device be equal to or greater than the system voltage. This practice
is common knowledge in the industry.
The proper application is easy for overcurrent protective devices
(OCPDs) with a straight voltage rating (i.e. 480 V, 240V, 600V,
etc.). However when it comes to devices with a slash voltage rating
(i.e. 480/277, 240/120, 600/347, etc.) it can be difficult.
What is a Slash Voltage Rating?
A device that bears a slash voltage rating has gone through testing
under special conditions. This special testing consists of the lesser
of the two voltages imposed across a single pole of the device
during the interrupting testing for the device. For example, a device
rated and marked 480/277V is only tested with 277 volts across a
single pole not the full 480 volts.
Where can I use slash rated devices?
A slash voltage rated device, (i.e. 480/277V) must only be applied
in a system, where a fault across one pole, will not, under any
circumstances contain a voltage exceeding the lesser of the two
voltages, (i.e. 277V).
Proper application of slash rated devices is addressed by the
National Electrical Code (NEC) in sections:
1999 NEC
2002 NEC
240-85
240.85
430-83(e)
430.83(E)
Changes to the 2002 NEC in Section 240.85 (second paragraph)
limits the application of slash voltage rated devices to solidly
grounded systems only.
Slas h v o ltage rate d dev ices “shall be pe rmitte d to be
applie d in a SOLIDLY GROUNDED circuit whe re
the no minal vo ltage o f any co nducto r to g ro und does
no t excee d the lo we r o f the two v alues ... and the
no minal v o ltage betwee n any two co nducto rs does no t
excee d the hig he r v alue ...”*
* Source: 2002 NEC 240.85

For example, a device rated 480/277V can not be used on:
• 480V corner grounded systems
• 480V resistance grounded systems
• 480V ungrounded systems.
What could happen if slash rated devices are misapplied?
If a slash rated device is applied on a resistance grounded,
ungrounded, or corner-grounded system, full phase to phase
voltage could be imposed across only one pole. This could lead to
a catastrophic failure of the device. This is also a violation of
110.3(B ) of the NEC as well as the product listing and labeling. It
is a misapplication of the intended use of the device and could lead
to a failure in gaining approval or passing a UL inspection if
pertinent.
What other application considerations exist with the use
of slash voltage rated devices?
Ratings for electrical assemblies, and panels, are rated based upon
the characteristics of all the components involved in the assembly.
Since slash voltage rated products are required to be applied on a
specific type system, the use of slash voltage rated devices in
electrical panels or any assembly will place a limitation on the type
of system in which the assembly can be installed (solidly grounded
wye only). This needs to be noted on the label or nameplate.
A Word of Caution: It is very common that equipment designers
may not have the specifics of the electrical system where the
equipment will be installed (such as solidly grounded or resistance
grounded). The limitation to one type of system, solidly grounded
wye, could cause problems late in the design cycle or at the time of
installation if another type of system, i.e. resistance grounded wye,
is encountered. In addition, in today’s world, equipment can be
moved from its original location to another location with a
different type electrical system.
What overcurrent protective devices can be slash rated?
Devices that can be slash rated include but are not limited to:
• Molded Case Circuit Breakers
• Self Protected Type E Combination Starters
• UL 1077 Supplementary Protectors
Summary
Slash rated devices are limited in application to solidly
grounded wye type systems due to the nature of the way that they
are tested, listed, and labeled. Any machine or electrical panel that
uses a slash rated device is limited to installation only in a solidly
grounded wye type system and would require marking that notes
this limitation.
For Further Information on Slash Voltage Ratings Visit
the Cooper Bussmann Website at www.bussmann.com
Features on this Topic Include:
In-depth Technical Paper Available for Download
¥
— Online Interactive Web Training Program
UL1077 S upple me nta ry P rote ctors
 2001 Cooper Bussmann, Inc.
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