Electrical System Design-Wiring Materials: Raceways and Boxes

What is rigid metal conduit (RMC?)

Rigid metal conduit (RMC) is a threadable raceway of circular cross section designed for the physical protection and routing of conductors and cables. It is the universal raceway. This definition applies all throughout the Code.

Where is RMC permitted?

RMC is permitted under all atmospheric conditions and in all types of occupancies per 344.10(A).

Can we use dissimilar metals in RMC?

When installing RMC, dissimilar metals that could cause galvanic action should be avoided.

When can we use aluminum fittings and enclosures with RMC?

Aluminum fittings and enclosures are permitted to be used with RMC and vice versa where they are not subject to severe corrosive influences.

When can we use stainless steel RMC?

Stainless steel RMC is also permitted, provided it is installed using only stainless-steel fittings and approved accessories such as outlet boxes and enclosures.

What is a nipple in the context of electrical work?

A nipple is a short piece of conduit or tubing that does not exceed 24″ in length.

How do nipples work?

. Nipples are used to join enclosures that are mounted close together. The wiring passes through the nipple from one enclosure to the other.

Where do we find the requirements for conductor fill for nipples?

Chapter 9, Notes to Tables

What’s rigid polyvinyl chloride conduit?

Rigid polyvinyl chloride (PVC) conduit is a conduit made of materials other than metal designed for the installation of electrical conductors and cables.

Are rigid PVC conduit materials recognized for direct burial in the earth?

These materials are recognized as having suitable physical characteristics for direct burial in the earth.

What materials are used in the practice of rigid PVC conduit?

These materials are recognized as having suitable physical characteristics for direct burial in the earth. Some of the materials used in the manufacture of these products include fiber, asbestos cement, soapstone, rigid polyvinyl chloride, fiberglass epoxy, high-density polyethylene (for underground use), and rigid polyvinyl chloride (for aboveground use).

What is the maximum sum of the degrees of angles of bends in a total run of PVC?

Per 352.26, a total run of PVC shall not exceed 360° of bends (including kicks and offsets).

What is a bend?

A bend is any change in the direction of a raceway.

How many 90 degree bends are permitted between pull points?

There shall not be more than the equivalent of four 90° bends between pull points.

What’s a kick?

A single bend in a raceway

What are kicks used for?

Kicks are often used for a raceway entrance and exit in an enclosure.

What’s an offset?

An offset is a double bend in a raceway, each containing the same number of degrees.

What are offsets used for?

Offsets are commonly used when raceways are routed around obstacles, other raceways, etc.

What are possible angles for bends?

Bends can be any angle necessary to meet job requirements.

What is electrical metallic tubing (EMT?)

Electrical metallic tubing (EMT) is a light-weight tubular steel raceway that does not have threads on the ends.

What is the lightest metallic raceway manufactured?

EMT is the lightest metallic raceway manufactured.

How are lengths of EMT joined?

The lengths are joined by using fittings with set screws or compression fittings.

Is stainless steel EMT allowed?

Stainless steel EMT is also permitted and must be installed using only stainless-steel fittings and approved accessories such as outlet boxes and enclosures.

What is flexible metal conduit?

Flexible metal conduit (FMC) is a raceway of metal strips formed into a circular cross-sectional raceway.

Where is FMC permitted?

It is permitted in exposed and concealed locations.

What’s another name for FMC?

Greenfield

Where is FMC not allowed?

Wet locations, hoistways other than per 620.21(A)(1), storage battery rooms, hazardous locations except as permitted elsewhere in the Code, where exposed to materials like gasoline and oil products that could deteriorate the conductors, underground or in poured concrete, and where subject to physical damage.

What kind of cross section do liquidtight flexible metal conduit and liquidtight flexible nonmetallic conduit have?

Circular cross section

What is liquidtight flexible metal conduit (LFMC)?

Liquidtight flexible metal conduit (LFMC) is a raceway of circular cross section with an outer liquidtight, nonmetallic, and sunlight-resistant jacket over an inner flexible metal core with associated couplings, connectors, and fittings for the installation of electric conductors.

What is the most common name for LFMC?

Sealtite

What couplings, connectors, and fittings shall we use when running LFMC?

Associated couplings, connectors, and fittings shall be used when running this raceway.

What do we need to do with cut edges of conduit during installation of LFMC?

We need to trim them inside and out to ensure removal of rough edges. 

What sizes of LFMC are permitted?

Sizes smaller than 1/2″ are prohibited, except for size 3/8″ where used for special applications, such as flexible connections to motors, as part of an approved assembly or for fixture whips not over 6′.

What is a strut-type channel raceway?

A strut-type channel raceway is a surface raceway formed of moisture-resistant and corrosion-resistant metal.

What are permissible materials for strut-type channel raceways?

These listed channel raceways may be galvanized, stainless, enameled, PVC coated, or aluminum.

How should strut-type channel raceways be mounted?

These raceways are intended to be mounted to the surface of or suspended from a structure.

What shall the covers of strut-type channel raceways be?

They shall be either metallic or nonmetallic.

What is a surface raceway?

A surface raceway is an enclosed channel for conductors that is attached to a surface.

How are surface raceways sized?

Surface raceways are available in several sizes. Each size has a complete line of boxes and accessories needed to install that size surface raceway as a system.

What are the advantages of surface raceways?

They’re convenient when adding to/extending an existing system in an installation where fishing cables is difficult.

Where should we install surface raceways?

Surface raceways are for installations in dry locations.

What is a multioutlet raceway?

A multioutlet assembly is a surface, flush, or freestanding raceway that contains conductors and receptacles.

When can we field install additional conductors in multioutlet assemblies?

There is no provision for field installation of additional conductors except where the product is marked to indicate the number, type, and size of additional conductors which may be field installed.

What is the difference between multioutlet assemblies and surface raceways?

Multioutlet assemblies contain conductors or receptacles that are assembled in the field or at the factory.

How are multi-outlet assemblies used?

Multioutlet assemblies are popular for work benches, laboratories, countertop areas, and for upgrading older systems to meet today’s requirements.

What is armored cable (AC)?

Armored cable (AC) is a factory assembly that contains the conductors within a jacket made of a spiral wrap of steel.

Where does the bare bonding strip in AC go?

. This bare bonding strip is in intimate contact with the outer metal jacket for its entire length to ensure a low-impedance path on the outer armor in the event of a ground fault.

Where can we use AC cable?

AC cable may be used only in dry locations.

What is metal-clad (MC) cable?

Metal-clad (MC) cable is a factory assembly of one or more conductors with or without fiber-optic members enclosed in a metallic armor of interlocking metal tape or a smooth or corrugated metallic sheath.

Does MC cable have an internal bonding strip?

No

How are the conductors in type MC cable enclosed?

The conductors are enclosed in a metal sheath with one of the following configurations per 330.2: smooth sheath, corrugated sheath, or interlocked sheath.

What is nonmetallic-sheathed (NM) cable?

Nonmetallic-sheathed (NM) cable is a factory assembly of two or more insulated conductors having an outer sheath of moisture-resistant, flame-retardant, nonmetallic material.

How is NM used?

NM is an economical wiring method used in residential units and small commercial locations.

What’s another name for NM cable in the field?

Romex

What’s service-entrance (SE) cable?

Service-entrance (SE) cable is a single or multiconductor assembly with or without an overall covering.

What types of service-entrance cable are recognized by the NEC?

Type SE and Type USE

What’s a cable tray system (CTS)?

A cable tray system (CTS) is an assembly of sections and associated fittings that form a rigid structural system used to support cables and raceways.

What configurations are cable tray systems available in?

These systems are available in different configurations including ladder, trough, ventilated trough, solid bottom, and other similar structures.

How are cable trays for multiconductor cables sized?

We size these with Table 392.22(A).

Where can we install multiconductor cables rated 2000V or less?

Multiconductor cables rated 2000 V or less are permitted to be installed in ladder, ventilated trough, solid bottom, or ventilated channel cable trays.

What is a wireway?

A wireway is a metallic or nonmetallic trough with a hinged or removable cover designed to house and protect conductors and cables.

How shall we install wireways?

Wireways shall be installed as a complete system before the conductors are installed.

How do we size wireways?

They’re sized by Chapter 9, Table 5.

What’s a busway?

A busway is a raceway consisting of a grounded metal enclosure that contains factory-assembled aluminum or copper busbars that are supported on insulators.

Where can we install busways?

Busways are available for either indoor or outdoor use. If used in an outdoor location, the busway shall be listed for outdoor use.

What are the max ampacities of busways?

Their ampacity can be as high as 6500A for copper bus and 5000A for aluminum bus.

What are the three basic types of busway?

The three basic types of busways are busways that provide an opening for plugging in a disconnect with an overcurrent device, busways that will not accept a plug-in disconnect, and trolley busways.

What does Section 368.17(A) require?

Section 368.17(A) requires that, in general, busways be protected against overcurrent in accordance with the current rating of the busway.

What does Section 368.17(B) require?

Section 368.17(B) requires overcurrent protection at the point where busways are reduced in ampacity.

What does the exception to Section 368.17(B) say?

Section 368.17(B), Exception, permits only industrial establishments to reduce the ampacity of busways without providing overcurrent protection, provided the following three conditions are met:

  (1)  The length of the reduced busway does not exceed 50′.

  (2)  The reduced busway has an ampacity of at least 1/3 of the rating or setting of the overcurrent device ahead of where   the reduction takes place.

  (3)  The reduced busway is free from contact with combustible material.

What’s an auxiliary gutter?

An auxiliary gutter is a sheet-metal or flame-retardant nonmetallic enclosure with a hinged or removable cover that is used to supplement wiring space for conductors or busbars. These gutters supplement wiring space at meters, panelboards, switchboards, and similar types of equipment.

How are auxiliary gutters and wireways different?

An auxiliary gutter and associated fittings are identical to wireways and associated fittings. The only difference between them is their intended use.

What is a box?

A box is a metallic or nonmetallic electrical enclosure used for equipment, devices, and pulling or terminating conductors.

What is an outlet box?

An outlet box is any point in a wiring system where current is taken to supply utilization equipment.

What is a device box?

A device box is a box that houses an electrical device. For example, a device box may contain a switch or receptacle.

What do pull boxes and junction boxes do?

They provide access to the raceways entering and leaving the boxes.

What is a pull box?

A pull box is a box used as a point to pull or feed electrical conductors into the raceway system.

What’s a junction box?

A junction box is a box in which splices, taps, or terminations are made.

What do we have to do if a conduit run has a total of more than 360 degrees of bends and offsets?

If a conduit run has a total of more than 360° of bends and offsets, a pull box must be installed.

What must vertical runs provide?

Vertical runs must provide at least one support method for each conductor at (or close to) the top of the vertical raceway.

How do we size pull and junction boxes?

Pull and junction boxes are sized for straight or angle pulls. The minimum-size pull and junction boxes to be used with raceways or cables containing conductors 4 AWG or larger shall be determined based on the type of pull. The length of boxes containing straight pulls shall not be less than eight times the trade diameter of the largest raceway per 314.28(A)(1).

What is a conduit body?

A conduit body is a conduit fitting that provides access to the raceway system through a removable cover at a junction or termination point.

How are conduit bodies named?

Conduit bodies are named based upon their similarity to letters of the alphabet.

How do we calculate box size?

Boxes shall be large enough to provide enough free space for all the enclosed conductors. This free space is needed so that any heat given off by the conductors in the form of I²R losses can be dissipated. Box size is calculated by the number and size of conductors contained within the box.

When can we use the framing members of suspended ceiling systems as support for boxes?

Framing members of suspended ceiling systems are permitted as support for boxes if the framing member is adequately supported and securely fastened to the building structure and the enclosure is limited in size to not more than 100 cu in.

How shall we fasten boxes to the framing member?

Boxes shall be fastened to the framing member by a mechanical means such as bolts, screws, or clips identified for the use. See Figure 8-27. Section 314.23(D)(2) requires that enclosures be secured using identified means to provide a tight support wire that is fastened at each end and that the installation comply with 300.11(A).

How shall we support boxes that aren’t more than 100 cubic inches and have threaded entries or hubs?

Boxes that are not over 100 cu in. and have threaded entries or hubs may be supported where two or more conduits are threaded wrenchtight into the enclosure or identified hubs. These conduits shall be supported within 3′ of the box on two or more sides to provide a rigid and secure installation.

What does the exception to 314.23(E) permit?

Section 314.23(E), Exception, permits RMC, IMC, EMT, PVC, and RTRC conduit to support conduit bodies, provided the conduit bodies are not larger than the largest trade size of the conduit or tubing.