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MCC Ratings 47
MCC Ratings
In addition to the various ratings of individual components used
in motor control centers, the overall ratings of the motor control
center must also be considered.
Voltage Rating Motor control centers are rated for 600 volts. This is the
maximum voltage that can be applied to a motor control center.
A motor control center can be connected to a lower voltage.
A motor control center, for example, might be connected to a
three-phase, 480 VAC power supply.
Horizontal Bus The horizontal bus on Siemens motor control centers are made
Current Rating of tin or optional silver plated copper. They are available with
600A, 800A, 1200A, 1600A, and 2000A current ratings. 48
Vertical Bus The vertical bus on the TIASTAR motor control centers are
available with 300A and 600A ratings.
Bus Bracing Motor control centers must be capable of withstanding the
largest potential short-circuit current which can occur in the
selected application. The amount of short-circuit current
available depends on the amount of power available to a facility.
Short-circuit current can be thousands of times higher than
normal current. Bus bars must be braced to withstand this
potential current. Siemens bus bars are braced for 42,000 AIC
(ampere interrupting capacity) with optional bracing available to
100,000 AIC. 49
Temperature Rise The bus bars are the major current carrying component of the
motor control center. Before a motor control center is operated,
bus bars are at the temperature of the surrounding air. This is
known as ambient temperature. Temperature rises in the motor
control center bus bars during operation. The combination of
ambient temperature and allowed temperature rise equals the
maximum temperature of the bus bars.
NEMA and UL both have standards concerning the maximum
temperature rise of bus bars used in motor control centers.
NEMA limits temperature rise to 65°C based on an ambient
temperature of 40°C (104°F), for a maximum operating
temperature of 105°C. UL limits temperature rise to 50°C based
on an ambient temperature of 40°C (104°F), for a maximum
operating temperature of 90°C. Electrical equipment bearing a
UL mark must meet or exceed this standard.
Siemens motor control centers meet or exceed NEMA and
UL standards. Bus bars in Siemens motor control centers are
tested with a maximum temperature rise of 50°C over 40°C
(104°F) ambient. 50
Enclosures
NEMA defines an enclosure as a surrounding case constructed
to provide a degree of protection to personnel against incidental
contact with the enclosed equipment and to provide a degree
of protection to the enclosed equipment against specified
environmental conditions (NEMA Standard 250 - section 2,
definitions).
The following brief descriptions cover enclosures available for
Siemens motor control centers.
Type 1 Enclosure Type 1 enclosures are intended for indoor use primarily to
provide protection against limited amounts of falling dirt and
contact with the enclosed equipment in locations where
unusual service conditions do not exist. 51
Type 1 Gasket Front Type 1 gasketed front, general purpose, indoor enclosure has
the same use as Type 1 except the front of the enclosure is
gasketed. In addition the following parts are gasketed:
Unit separator angles
Right-hand side of front of units
Bottom horizontal cross ties
Lip on top plate
Pilot-device panel
Handle mechanism
Bottom horizontal wireway cover plate
Side holes are plugged
Type 2, Drip-Proof Type 2, drip-proof is an indoor enclosure intended to protect
equipment from falling noncorrosive liquids and dirt. The
enclosure prevents the entrance of dripping liquid at a higher
level than the lowest live part within the enclosure. This
design is a Type 1 gasketed front, or Type 12, with a drip shield
mounted on top of the enclosure.
Type 12 Enclosure Type 12 enclosures are intended for indoor use primarily to
provide a degree of protection against circulating dust, falling
dirt, and dripping noncorrosive liquids. They are not intended
to provide protection against conditions such as internal
condensation.
The Type 12 will provide a greater degree of protection than a
Type 1 gasketed enclosure. The following additional parts are
gasketed:
Hinged side of doors
Top plates
Wireway end-covers
Rear plates
There is no gap between sections, allowing for much greater
dust resistance. In addition, interconnection holes in the side
sheet assemblies are sealed. Bottom plates are included. 52
Type 3R Enclosure Type 3R enclosures are intended for outdoor use primarily to
provide a degree of protection against falling rain and sleet and
protection from contact with the enclosed equipment. They are
not dust, snow, or sleet (ice) proof. They will prevent entrance of
rain at a level higher than the lowest live part. The enclosure has
provisions for locking and drainage.
The enclosure entirely surrounds the motor control center
for outdoor operation. The Type 3R enclosure is designed
to accommodate bottom cable entry and exit only. The 3R
enclosure is not a walk-in type design.
NEMA and IEC The International Electrotechnical Commission (IEC) is another
organization that defines the degree of protection provided by
enclosures. NEMA is primarily associated with equipment used
in North America. IEC is associated with equipment sold in
many countries including the United States.
The IEC designation consists of the letters IP followed by two
numbers. The first number indicates the degree of protection
provided by the enclosure with respect to persons and solid
objects entering the enclosure. The second number indicates
the degree of protection against the ingress of water. The
following chart provides an equivalent conversion between
NEMA and IEC designations.
NEMA
IEC
1 IP10
2 IP11
3R IP14
12 IP52 53
Classification and Types of Wiring
NEMA has established two classification standards (Class I
and Class II) and three types of wiring (A, B, and C) used in the
construction of motor control centers. These are specified by
the customer.
Class I Class I consists of a grouping of combination motor control
units in which each starter and motor operates independently
of the other starters. The factory connects the combination
motor control units to the vertical bus but does not provide
interconnecting wiring between combination motor control
units, different vertical units, or remotely connected devices.
Diagrams of the individual units only are supplied.
Class I, Type A Wiring Type A wiring is only available on Class I motor control
centers. The motor control center manufacturer connects the
combination motor control unit to the vertical bus via the stabs
on the back of the unit. Power is applied to the circuit breaker
from the vertical bus. The circuit breaker is factory wired to
the motor starter. The customer connects the motor leads and
control wiring to the motor starter. There is no interconnecting
wiring between combination motor control units. 54
Class I, Type B Wiring Typically pilot devices, such as indicator lights, pushbuttons, and
selector switches, are used with Class I, Type B wiring. Type B
wiring is divided into two designations: B-d (-d for connection
of load wires directly on starter or contactor terminals) and B-t
(-t for connection of load wires to unit mounted load terminal
blocks).
When Type B-d wiring is specified, terminal blocks are furnished
near the wireway for control circuit connections. Motor leads
are connected directly to the overload relay terminals.
When Type B-t wiring is specified, terminal blocks are furnished
near the wireway for control circuit connections and for motor
starter leads. Type B-t wiring can be used on starters up to