In the field of electrical engineering, precise definitions and terminologies are essential to ensure safety, compliance, and efficiency in various electrical systems.
Whether it’s regarding machine rooms, mobile homes, or advanced systems like microgrids, understanding the correct terminology is critical for proper installation, operation, and maintenance.
Machine Room
An enclosed machinery space outside the hoistway, intended for full bodily entry, that contains the electrical driving machine or the hydraulic machine. The room could also contain electrical and/or mechanical equipment used directly in connection with the elevator or dumbwaiter. (620) (CMP—12)
Machine Room and Control Room, Remote (Remote Machine Room and Control Room)
A machine room or control room that is not attached to the outside perimeter or surface of the walls, ceiling, or floor of the hoistway. (620) (CMP—12)
Machinery, Industrial (Industrial Machine)
A power-driven machine (or a group of machines working together in a coordinated manner), not portable by hand while working, that is used to process material by cutting; forming; pressure; electrical, thermal, or optical techniques; lamination; or a combination of these processes. It can include associated equipment used to transfer material or tooling, including fixtures, to assemble/disassemble, to inspect or test, or to package. The associated electrical equipment, including the logic controller(s) and associated software or logic together with the machine actuators and sensors, are considered as part of the industrial machine. (CMP—12)
Machinery Space
A space inside or outside the hoistway, intended to be accessed with or without full bodily entry, that contains the elevator, dumbwaiter, platform lift, or stairway chairlift equipment and could also contain equipment used directly in connection with the elevator, dumbwaiter, platform lift, or stairway chairlift. (620) (CMP—12)
Machinery Space and Control Space, Remote (Remote Machinery Space and Control Space)
A machinery space or control space that is not within the hoistway, machine room, or control room and that is not attached to the outside perimeter or surface of the walls, ceiling, or floor of the hoistway. (620) (CMP—12)
Manufactured Home
A structure, transportable in one or more sections, which in the traveling mode is 2.4 m (8 ft) or more in width or 12.2 m (40 ft) or more in length, or when erected on site is 29.77 m2 (320 ft2) or more, built on a permanent chassis and designed to be used as a dwelling with or without a permanent foundation, whether or not connected to the utilities, and includes plumbing, heating, air conditioning, and electrical systems contained therein. The term includes any structure that meets all the requirements of this paragraph except the size requirements and with respect to which the manufacturer voluntarily files a certification required by the regulatory agency. Calculations used to determine the number of square meters (square feet) in a structure are based on the structure’s exterior dimensions and include all expandable rooms, cabinets, and other projections containing interior space, but do not include bay windows. [501:1.2.12] (CMP—7)
Informational Note No. 1: Unless otherwise indicated, the term mobile home includes manufactured home and excludes park trailers.
Informational Note No. 2: See the applicable building code for the definition of the term permanent foundation.
Informational Note No. 3: See 24 CFR Part 3280, Manufactured Home Construction and Safety Standards, of the Federal Department of Housing and Urban Development, for additional information on the definition.
Manufactured Wiring System
A system containing component parts that are assembled in the process of manufacture and cannot be inspected at the building site without damage or destruction to the assembly and used for the connection of luminaires, utilization equipment, continuous plug-in type busways, and other devices. (604) (CMP—7)
Marina
A facility, generally on the waterfront, that stores and services boats in berths, on moorings, and in dry storage or dry stack storage. [303:3.3.13] (555) (CMP—7)
Maximum Output Power
The maximum power delivered by an amplifier into its rated load as determined under specified test conditions. (640) (CMP—12)
Informational Note: The maximum output power can exceed the manufacturer’s rated output power for the same amplifier.
Maximum Output Power
The maximum 1-minute average power output a wind turbine produces in normal steady-state operation (instantaneous power output can be higher). (694) (CMP—4)
Maximum Voltage
The greatest difference in potential produced between any two conductors of a wind turbine circuit. (694) (CMP—4)
Maximum Water Level
The highest level that water can reach before it spills out. (680) (CMP—17)
Medical Office
A building or part thereof in which the following occur:
- Examinations and minor treatments/procedures performed under the continuous supervision of a medical professional;
- The use of limited to minimal sedation and treatment or procedures that do not render the patient incapable of self-preservation under emergency conditions; and
- No overnight stays for patients or 24-hour operations. [99:3.3.110] (CMP—15)
Membrane Enclosure
A temporary enclosure used for the spraying of workpieces that cannot be moved into a spray booth where open spraying is not practical due to proximity to other operations, finish quality, or concerns such as the collection of overspray. (CMP—14)
Informational Note: See NFPA 33, Standard for Spray Application Using Flammable or Combustible Materials, Chapter 18, for information on the construction and use of membrane enclosures.
Messenger-Supported Wiring
An exposed wiring support system using a messenger wire to support insulated conductors by any one of the following:
- A messenger with rings and saddles for conductor support;
- A messenger with a field-installed lashing material for conductor support;
- Factory-assembled aerial cable;
- Multiplex cables utilizing a bare conductor, factory assembled and twisted with one or more insulated conductors, such as duplex, triplex, or quadruplex type of construction. (CMP—6)
Messenger Wire (Messenger)
A wire that is run along with or integral with a cable or conductor to provide mechanical support for the cable or conductor. (CMP—6)
Metal Shield Connections
Means of connection for flat conductor cables (Type FCC) designed to electrically and mechanically connect a metal shield to another metal shield, to a receptacle housing or self-contained device, or to a transition assembly. (324) (CMP—6)
Microgrid
An electric power system capable of operating in island mode and capable of being interconnected to an electric power production and distribution network or other primary source while operating in interactive mode, which includes the ability to disconnect from and reconnect to a primary source and operate in island mode. (CMP—4)
Informational Note No. 1: See IEEE 1547, IEEE Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interface, IEEE 2030.7, IEEE Standard for the Specification of Microgrid Controllers; IEEE 2030.8, IEEE Standard for the Testing of Microgrid Controllers; and UL1008B, Outline for Source Interconnection, for additional information about microgrids.
Informational Note No. 2: Examples of power sources in microgrids include such items as photovoltaic systems, generators, fuel cell systems, wind electric systems, energy storage systems, electric vehicles that are used as a source of supply, and electrical power conversion from other energy sources.
Microgrid, Health Care (Health Care Microgrid System)
A group of interconnected loads and distributed energy resources within clearly defined boundaries that acts as a single controllable entity with respect to the utility. [99:3.3.75] (517) (CMP—15)
Microgrid Control System (MCS)
A structured control system that manages microgrid operations, functionalities for utility interoperability, islanded operations, and transitions. (CMP—4)
Informational Note: MCS differ from multiple standby generators or uninterruptible power supplies that are evaluated and rated to operate as a single source of backup power upon loss of the primary power source. MCS functions include coordination, transitions, and interoperability between multiple power sources.
Microgrid Interconnect Device (MID)
A device that enables a microgrid system to separate from and reconnect to an interconnected primary power source. (CMP—4)
Mixer
Equipment used to combine and level match a multiplicity of electronic signals, such as from microphones, electronic instruments, and recorded audio. (640) (CMP—12)
Mobile
X-ray equipment mounted on a permanent base with wheels and/or casters for moving while completely assembled. (660) (CMP—12)
Mobile Home
A factory-assembled structure or structures transportable in one or more sections that are built on a permanent chassis and designed to be used as a dwelling without a permanent foundation where connected to the required utilities and that include the plumbing, heating, air-conditioning, and electrical systems contained therein. (CMP—7)
Informational Note: Unless otherwise indicated, the term mobile home includes manufactured home and excludes park trailers.
Mobile Home Lot
A designated portion of a mobile home park designed for the accommodation of one mobile home and its accessory buildings or structures for the exclusive use of its occupants. (550) (CMP—7)
Mobile Home Park
A contiguous parcel of land that is used for the accommodation of mobile homes that are intended to be occupied. (550) (CMP—7)
Module, AC (AC Module)
A complete, environmentally protected unit consisting of solar cells, inverter, and other components, designed to produce ac power. (690) (CMP—4)
Module System, AC (AC Module System)
An assembly of ac modules, wiring methods, materials, and subassemblies that are evaluated, identified, and defined as a system. (690) (CMP—4)
Momentary Rating
A rating based on an operating interval that does not exceed 5 seconds. (660) (CMP—12)
Momentary Rating (Maximum Power)
A rating based on an operating interval that does not exceed 5 seconds. (517) (CMP—15)
Monitor
An electrical or electronic means to observe, record, or detect the operation or condition of the electric power system or apparatus. (750) (CMP—13)
Monopole Circuit
An electrical subset of a PV system that has two conductors in the output circuit, one positive (+) and one negative (—). (690) (CMP—4)
Monorail
Overhead track and hoist system for moving material around the boatyard or moving and launching boats. [303:3.3.16] (555) (CMP—7)
Mooring(s)
Any place where a boat is wet stored or berthed. [303:3.3.17] (555) (CMP—7)
Motion Picture Studio (Television Studio)
A building, group of buildings, other structures, and outdoor areas designed, constructed, permanently altered, designated, or approved for the purpose of motion picture or television production. (530) (CMP—15)
Motor Control Center
An assembly of one or more enclosed sections having a common power bus and principally containing motor control units. (CMP—11)
Motor Fuel Dispensing Facility
That portion of a property where motor fuels are stored and dispensed from fixed equipment into the fuel tanks of motor vehicles or marine craft or into approved containers, including all equipment used in connection therewith. [30A:3.3.11] (CMP—14)
Informational Note: See 511.1 with respect to electrical wiring and equipment for other areas used as lubritoriums, service rooms, repair rooms, offices, salesrooms, compressor rooms, and similar locations.
Multi-Circuit Cable Outlet Enclosure
An enclosure containing one or more multi-circuit plugs, receptacles, or both. (520) (CMP—15)
Multioutlet Assembly
A surface, flush, or freestanding assemblage with a raceway and fittings or other enclosure provided with one or more receptacles, for the purpose of supplying power to utilization equipment. (CMP—18)
Summary Table
| Term | Definition |
|---|---|
| Machine Room | Enclosed space for elevator machinery and related equipment. |
| Remote Machine Room and Control Room | Machine or control room not attached to the hoistway. |
| Industrial Machine | Power-driven machine for material processing, including associated equipment. |
| Machinery Space | Space containing elevator or lift equipment, accessible with or without full entry. |
| Remote Machinery Space and Control Space | Machinery or control space not within the hoistway or attached to its perimeter. |
| Manufactured Home | Transportable dwelling unit with plumbing, heating, and electrical systems. |
| Manufactured Wiring System | Pre-assembled wiring system for luminaires and equipment. |
| Marina | Waterfront facility for boat storage and servicing. |
| Maximum Output Power | Maximum power delivered by an amplifier or wind turbine. |
| Maximum Voltage | Greatest potential difference in a wind turbine circuit. |
| Maximum Water Level | Highest water level before spillage. |
| Medical Office | Building for medical examinations and minor treatments. |
| Membrane Enclosure | Temporary enclosure for spraying workpieces. |
| Messenger-Supported Wiring | Wiring system using a messenger wire for support. |
| Messenger Wire | Wire providing mechanical support for cables or conductors. |
| Metal Shield Connections | Connections for flat conductor cables. |
| Microgrid | Electric power system capable of island and interactive operation. |
| Health Care Microgrid | Microgrid system for health care facilities. |
| Microgrid Control System (MCS) | System managing microgrid operations and transitions. |
| Microgrid Interconnect Device (MID) | Device enabling microgrid separation and reconnection. |
| Mixer | Equipment for combining electronic signals. |
| Mobile | X-ray equipment on a movable base. |
| Mobile Home | Factory-assembled transportable dwelling. |
| Mobile Home Lot | Designated area for a mobile home in a park. |
| Mobile Home Park | Contiguous land for mobile home accommodation. |
| AC Module | Solar cell unit producing ac power. |
| AC Module System | Assembly of ac modules and associated components. |
| Momentary Rating | Rating based on a 5-second operating interval. |
| Monitor | Device for observing or recording electric power system conditions. |
| Monopole Circuit | PV system subset with two conductors. |
| Monorail | Overhead track system for material or boat movement. |
| Mooring(s) | Place for wet storage or berthing of boats. |
| Motion Picture Studio | Facility for motion picture or television production. |
| Motor Control Center | Assembly of motor control units with a common power bus. |
| Motor Fuel Dispensing Facility | Property portion for storing and dispensing motor fuels. |
| Multi-Circuit Cable Outlet Enclosure | Enclosure with multi-circuit plugs or receptacles. |
| Multioutlet Assembly | Assembly with receptacles for supplying power to equipment. |
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