Article 100 NEC Definitions (Letter F)

From flameproof enclosures to fuse types and energy storage systems, each term is carefully defined according to NEC standards.

These definitions provide clarity on equipment functionality, installation requirements, and operational principles, helping engineers, electricians, and inspectors apply best practices in the field.

Facility, On-Site Power Production

(On-Site Power Production Facility)
The normal supply of electric power for the site that is expected to be constantly producing power. (695) (CMP—13)

Fastened-in-Place

Mounting means of equipment in which the fastening means are specifically designed to permit removal without the use of a tool. (625) (CMP—12)

Fault-Managed Power (FMP)

A powering system that monitors for faults and controls current delivered to ensure fault energy is limited. (726) (CMP—3)

Informational Notes:

1. The monitoring and control systems differentiate fault-managed power from electric light and power circuits; therefore, alternative requirements to those of Chapters 1 through 4 are given regarding minimum wire sizes, ampacity adjustment and correction factors, overcurrent protection, insulation requirements, and wiring methods and materials.
2. A fault-managed power circuit is also commonly referred to as a Class 4 circuit.

Fault Current

The current delivered at a point on the system during a short-circuit condition. (CMP—10)

Fault Current, Available (Available Fault Current)

The largest amount of current capable of being delivered at a point on the system during a short-circuit condition. (CMP—10)

Informational Note: A short-circuit can occur during abnormal conditions such as a fault between circuit conductors or a ground fault. See Informational Note Figure 100.1.

Fault Protection Device

An electronic device that is intended for the protection of personnel and functions under fault conditions, such as network-powered broadband communications cable short or open circuit, to limit the current or voltage, or both, for a low-power network-powered broadband communications circuit and provide acceptable protection from electric shock. (830) (CMP—16)

Feeder

All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device. (CMP—10)

Feeder Assembly

The overhead or under-chassis feeder conductors, including the equipment grounding conductor, together with the necessary fittings and equipment; or the power-supply cord assembly for a mobile home, recreational vehicle, or park trailer, identified for the delivery of energy from the source of electrical supply to the panelboard within the mobile home, recreational vehicle, or park trailer. (CMP—7)

Festoon Lighting

A string of outdoor lights that is suspended between two points. (CMP—18)

Fibers/Flyings, Combustible (Combustible Fibers/Flyings)

Fibers/flyings, where any dimension is greater than 500 µm in nominal size, which can form an explosible mixture when suspended in air at standard atmospheric pressure and temperature. [499:3.3.4.1] (CMP—14)

Informational Notes:

1. Extracted from NFPA 499—2021, which provides guidance on hazardous (classified) locations.
2. Section 500.5(D) defines a Class III location. Examples include metal flake and particle board core material.
3. Standards for explosibility testing include ASTM E1226, ISO 6184-1, and ISO/IEC/UL 80079-20-2.

Fibers/Flyings, Ignitible (Ignitible Fibers/Flyings)

Fibers/flyings where any dimension is greater than 500 µm in nominal size, which are not likely to be in suspension in quantities to produce an explosible mixture but could produce an ignitible layer fire hazard. [499:3.3.4.2] (CMP—14)

Informational Notes:

1. Extracted from NFPA 499—2021.
2. Section 500.5 prescribes a Class III location where ignitible fibers/flyings are present but not suspended in sufficient quantity to cause an explosion hazard.

Field Evaluation Body (FEB)

An organization or part of an organization that performs field evaluations of electrical or other equipment. [790:3.3.4] (CMP—1)

Informational Note: NFPA 790—2021 provides guidelines for the qualification and competency of bodies performing field evaluations of electrical products.

Field Labeled (as applied to evaluated products)

Equipment or materials to which a label, symbol, or other identifying mark of an FEB has been attached, indicating compliance with requirements described in an accompanying field evaluation report. [790:3.3.6] (CMP—1)

Fire Alarm Circuit

The portion of the wiring system between the load side of the overcurrent device or the power-limited supply and the connected equipment of all circuits powered and controlled by the fire alarm system. Classified as either non-power-limited or power-limited. (CMP—3)

Fire Alarm Circuit, Non-Power-Limited (NPLFA)

A fire alarm circuit powered by a source that is not power limited. (CMP—3)

Informational Note: See 760.41 and 760.43 for requirements for non-power-limited fire alarm circuits.

Fire Alarm Circuit, Power-Limited (PLFA)

A fire alarm circuit powered by a power-limited source. (CMP—3)

Informational Note: See 760.121 for requirements on power-limited fire alarm circuits.

Fitting

An accessory such as a locknut, bushing, or other part of a wiring system that is intended primarily to perform a mechanical rather than an electrical function. (CMP—1)

Fixed (as applied to equipment)

Equipment that is fastened or otherwise secured at a specific location. (680) (CMP—17)

Fixed-in-Place

Mounting means of equipment using fasteners that require a tool for removal. (625) (CMP—12)

Flameproof «d»

A type of protection where the enclosure withstands an internal explosion of a flammable mixture that has entered the interior, without suffering damage or causing ignition through joints or structural openings. The external explosive gas atmosphere consists of one or more gases or vapors for which it is designed. (CMP—14)
Informational Note: See ANSI/UL 60079-1, Explosive Atmospheres — Part 1: Equipment Protection by Flameproof Enclosures «d», for additional information.

Flammable Anesthetics

Gases or vapors such as fluroxene, cyclopropane, divinyl ether, ethyl chloride, ethyl ether, and ethylene, which could form flammable or explosive mixtures with air, oxygen, or reducing gases such as nitrous oxide. (517) (CMP—15)

Flexible Bus Systems

An assembly of flexible insulated bus with a system of associated fittings used to secure, support, and terminate the bus. (CMP—8)
Informational Note: These systems are engineered for specific site locations and are ordinarily assembled at the installation site from manufacturer-specified components.

Flexible Insulated Bus

A flexible rectangular conductor with an overall insulation. (CMP—8)

Flywheel ESS (FESS)

A mechanical energy storage system (ESS) composed of a spinning mass, referred to as a rotor, and an energy conversion mechanism such as a motor-generator that converts mechanical energy into electrical energy. (706) (CMP—13)
Informational Note: Two primary types of rotor construction exist: solid metal mass design and composite fiber design.

Footlight

A border light installed on or in the stage. (520) (CMP—15)

Forming Shell

A structure designed to support a wet-niche luminaire assembly and intended for mounting in a pool or fountain structure. (680) (CMP—17)

Fountain

An ornamental or recreational water feature from which one or more jets or streams of water are discharged into the air, including splash pads, ornamental pools, display pools, and reflection pools. Drinking water fountains and water coolers are excluded from this definition. (680) (CMP—17)

Frame

The chassis rail and any welded additions thereto with a metal thickness of at least 1.35 mm (0.053 in.). (551) (CMP—7)

Free Air (as applied to conductors)

An open or ventilated environment allowing for heat dissipation and airflow around an installed conductor. (CMP—6)

Fuel Cell

An electrochemical system that consumes fuel to produce an electric current. The main chemical reaction does not involve combustion, though combustion sources may be present within the system, such as reformers or fuel processors. (CMP—4)

Fuel Cell System

A complete set of equipment used to convert chemical fuel into usable electricity, typically consisting of a reformer, stack, power inverter, and auxiliary equipment. (CMP—4)

Fuse

An overcurrent protective device with a fusible part that opens the circuit when overheated by an overcurrent. (CMP—10)
Informational Note: A fuse comprises all necessary parts to perform its function, though it may or may not include all components for circuit connection.

Types of Fuses:

• Electronically Actuated Fuse: An overcurrent protective device with a control module for current sensing, electronic time-current characteristics, and tripping energy, along with an interrupting module that stops the current. May or may not operate in a current-limiting fashion. (CMP—10)
• Expulsion Fuse: A vented fuse where the arc is extinguished by gases produced from the arc and fuseholder lining, either alone or with a spring. (CMP—10)
• Nonvented Power Fuse: A fuse designed to prevent the escape of arc gases, liquids, or solid particles during circuit interruption. (CMP—10)
• Power Fuse: A fuse that can be vented, nonvented, or controlled vented, using solid material, granular material, or liquid to extinguish the arc. (CMP—10)
• Vented Power Fuse: A fuse with provisions for the escape of arc gases, liquids, or solid particles during circuit interruption. (CMP—10)

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Summary Table

Term	Definition
Facility, On-Site Power Production	Normal power supply that is continuously producing electricity.
Fastened-in-Place	Equipment that can be removed without tools.
Fault-Managed Power	System that monitors and limits fault energy.
Fault Current	Current delivered during a short-circuit condition.
Available Fault Current	Maximum current during a short-circuit.
Fault Protection Device	Device limiting voltage/current in low-power circuits.
Feeder	Conductors between power source and branch-circuit device.
Feeder Assembly	Conductors supplying energy to mobile electrical systems.
Festoon Lighting	Outdoor string lights suspended between two points.
Combustible Fibers/Flyings	Particles capable of forming an explosible mixture.
Ignitible Fibers/Flyings	Particles that pose a fire hazard but not an explosion risk.
Field Evaluation Body	Organization evaluating electrical equipment compliance.
Field Labeled	Mark indicating compliance with field evaluation standards.
Fire Alarm Circuit	Wiring system controlling fire alarm circuits.
NPLFA Circuit	Fire alarm circuit with non-power-limited source.
PLFA Circuit	Fire alarm circuit with power-limited source.
Fitting	Mechanical accessory in an electrical system.
Fixed Equipment	Equipment secured in a specific location.
Fixed-in-Place	Equipment requiring a tool for removal.

Term	Definition
Flameproof «d»	Enclosure withstands internal explosion without damage or external ignition.
Flammable Anesthetics	Gases or vapors that form flammable/explosive mixtures with air or oxygen.
Flexible Bus Systems	Insulated flexible bus system with associated fittings.
Flexible Insulated Bus	Flexible rectangular conductor with insulation.
Flywheel ESS (FESS)	Mechanical ESS with a rotor and motor-generator for energy conversion.
Footlight	Border light installed on or in a stage.
Forming Shell	Structure supporting wet-niche luminaire in pools or fountains.
Fountain	Ornamental water feature with jets/streams of water.
Frame	Chassis rail with a minimum metal thickness of 1.35 mm (0.053 in.).
Free Air (Conductors)	Open/ventilated environment allowing heat dissipation around conductors.
Fuel Cell	Electrochemical system generating electricity from fuel without combustion.
Fuel Cell System	Complete fuel-to-electricity conversion system.
Fuse	Overcurrent protective device with a fusible circuit-opening element.
Electronically Actuated Fuse	Fuse with electronic control for current sensing and trip initiation.
Expulsion Fuse	Vented fuse that uses arc-produced gases for extinction.
Nonvented Power Fuse	Fuse preventing escape of arc gases, liquids, or solids.
Power Fuse	Fuse extinguishing arc using solid, granular, or liquid material.
Vented Power Fuse	Fuse allowing escape of arc gases, liquids, or solids during operation.