Understanding NEMA Wiring Classes
NEMA/EEMAC Wiring Class
NEMA (National Electrical Manufacturers Association) is an American trade association and the largest one in the US related to electrical equipment manufacturers. It is supported by over 450 electrical equipment manufacturers from around the world and is involved in industry standardization. Over the years, NEMA has published over 600 standards related to the manufacture, installation, use, maintenance, and safety of electrical equipment in electrical and control industries. NEMA standards are the de-facto standards in most of the world.
Similarly, the EEMAC (Electrical Equipment Manufacturers Association of Canada) is the Canadian counterpart of the US NEMA. Even though EEMAC is still referenced in various specifications, these standards are no longer actively maintained.
NEMA/EEMAC Wiring Class standards relate to the proper and safe wiring of motor control centers. A motor control center (MCC), in the context of NEMA ICS-3-2000 and ICS-18-2001, is a group of individual motor starters, over-current protection devices, Variable Frequency Drives (VFDs), Variable Speed Drives (VSDs), starting and inrush current control circuitry, transformers, and logic controllers in a NEMA enclosure.
The relevant NEMA Standards are ICS-18-2001 (Motor Control Centers) and ICS-3-2000 (Industrial Control and Systems: Medium Voltage Controllers Rated 2001 to 7200 Volts AC). These Standards are derived from an obsolete NEMA Standard ICS-3-1993 named “Factory Built Assemblies”. Part 1 (Motor Control Centers Rated Not More Than 600 Volts AC) of Standard ICS-3-1993 has been revised as ICS-18-2001. The other 2 Parts of ICS-3-1993 relate to higher voltage motor control centers, and have been renamed as Parts 1 and 2 of ICS-3-2000.
The Standards define two classifications of the motor control center assembly and three classifications according to their wiring. The assembly classes are further bifurcated according to whether they come with manufacturer-standard drawings or with installer-customized drawings. When specifying a motor control center, a Class (I or II) of MCC, can be combined with a Type (A, B, or C) of wiring.
NEMA Motor Control Center Classes
A motor control center (MCC) can be classified as either Class I or Class II; depending on whether it is supplied as a standalone unit or as a customized unit according to purchaser specifications.
NEMA Class I (Independent Units with manufacturer-standard drawings):
Class I motor control centers shall consist of a combination of motor-control units, power connections, and other electrical devices arranged in a suitable assembly. The drawings furnished by the manufacturer shall include manufacturer’s standard diagrams for each individual unit consisting of drawings that Identify electrical devices, indicate terminal numbering designations, and indicate electrical connections and overall dimensions of the motor control center, locations of outgoing and incoming line terminals, identification and location of units in the MCC, available conduit entrance areas, mounting dimensions.
NEMA Class II (Interconnected Units with manufacturer-standard drawings):
Class II motor control centers are the same as Class I motor control centers except with the addition of electrical interlocking and wiring diagrams furnished by the manufacturer between units as described in the overall plant control system diagrams.
NEMA Class IS (Independent Units with Custom Drawing Requirements):
Class IS MCCs are the same as Class I MCCs except that customized drawings shall be provided instead of standard drawings according to user specifications.
NEMA Class IIS (Interconnected Units with Custom Drawing Requirements):
Class IIS MCCs are the same as Class II MCCs except that customized drawings shall be provided instead of standard drawings according to user specifications.
NEMA Wiring Types
According to NEMA standard ICS 18-2001 (R2007), wiring shall be classified according to three Types:
NEMA Type A Wiring:
Both control and power cables from outside the MCC are directly terminated to the device terminals inside a feeder or starter unit. Terminal blocks are not provided for either control or power cables. Only Class I MCCs may have this type of wiring.
NEMA Type B wiring
MCC Specifications with Listed EEMAC Wiring Type
Combination starters in a MCC are equipped with terminal blocks for control wiring termination. Field control wiring connections are made at each marked terminal block. Type B wiring is subdivided into two categories:
The power cables are directly connected to a relay’s terminals. The terminal block should be placed adjacent to vertical wire-ways in a readily accessible location.
Power cables are connected to a separate power terminal block, which should be located immediately adjacent to the unit.
NEMA Type C Wiring:
The field control wiring is connected by the user to a master terminal block mounted in the horizontal wire-way at top or bottom of a vertical section. The control wiring from the combination starter termination block to a master terminal block is done at the factory.