Article 300: Wiring Methods

Table of contents:

Article 300 Wiring Methods

I. General Requirements



This article applies to all wiring installations except as noted in other articles. It does not apply to integral parts of equipment (e.g., motors, motor control centers, etc.) Metric designators and trade sizes for conduit, tubing, etc. are shown in Table 300.1(C).

Table 300.1(C). Metric Designator and Trade Sizes

Metric Designator

Trade Size























Note: The metric designators and trade sizes are for identification purposes only and are not actual dimensions.



This chapter applies to voltages up to and including 600 V when not otherwise noted and over 600 V only when specifically mentioned in the National Electrical Code®.



(A) Single Conductors. The single conductors that are specified in Table 310.13 can be installed only as part of a recognized wiring method described in Chapter 3. Single overhead conductors are permitted if installed in accordance with 225.6.

(B) Conductors of the Same Circuit. All conductors of the same circuit including the grounded conductor and equipment grounding conductors must be installed in the same raceway, cable, etc. There are exceptions.

(C) Conductors of Different Systems

(1) 600 Volts, Nominal, or Less. Conductors of 600 V or less can occupy the same wiring enclosure if all conductors have insulation rated for the maximum voltage used. Refer to Section 690.4(B) for solar photovoltaic systems and to Section 725.55(A) for Class 2 and Class 3 conductors.

(2) Over 600 Volts, Nominal. Conductors of over 600 V cannot be in the same wiring enclosure with those rated 600 V or less except for secondary wiring to electric discharge lamps, primary leads of electric discharge lamp ballasts, and excitation, control, relay, ammeter conductors, motor switch gear and control assemblies, and in manholes under certain conditions.

Reference should be made to this section in the NEC® for a complete explanation.


Protection against Physical Damage


Underground Installations

(A) Minimum Cover Requirements. The cover requirements for direct buried cable or conduit are shown in Table 300.5. The reader should refer to it.

(B) Listing

(C) Underground Cables under Buildings

(D) Protection from Damage

(E) Splices and Taps

(F) Backfill

(G) Raceway Seals

(H) Bushing

(I) Conductors of the Same Circuit

(J) Ground Movement

(K) Directional Boring


Protection against Corrosion

Generally, all material used must be suitable for use in the installed environment.

(A) Ferrous Metal Equipment. Ferrous material must be protected both inside and out by a coating of corrosion-resistant material. If corrosion protection is required and threading takes place in the field, then the threads must be coated with an identified electrical conductive, corrosion-resistant compound. Stainless steel need not have a protective coating. If protected only by enamel ferrous metal equipment shall not be used outdoors or in wet locations as noted in 300.6(D). Boxes or cabinets with an approved organic coating and marked "Raintight," "Rainproof," or "Outdoor Type" can be used outdoors. If the material is approved for use in concrete, in direct contact with the earth, or subject to severe corrosion or has approved corrosion protection, then it can be used there.

(B) Non-Ferrous Metal Equipment. Non-ferrous material must have supplementary corrosion protection if embedded in or encased in concrete or in direct contact with the earth.

(C) Nonmetallic Equipment. Nonmetallic equipment must be made of material approved for the conditions in which it is installed. If exposed to sunlight then the material must be listed as sunlight resistant or be identified as sunlight resistant. If exposed to chemicals or their vapors, the material must be inherently resistant to the chemicals or identified for the specific chemical reagent.

(D) Indoor Wet Locations. The cable must be installed with at least 6 mm (¼ in.) of air space from the wall or supporting surface when installed exposed. There is an exception.


Raceways Exposed to Different Temperatures


Installations of Conductors with Other Systems


Electrical Continuity of Metal Raceways and Enclosures

There must be electrical continuity between all metal race-ways, cable armor, and metal enclosures. There are two exceptions.


Securing and Supporting


Mechanical ContinuityRaceways and Cables


Mechanical and Electrical ContinuityConductors

Splices or taps are not permitted within a raceway. Exceptions to this are noted in Sections 300.15; 368.8(A); 376.56; 378.56; 384.56; 386.56; 388.56; or 390.6. In a multiwire branch circuit the continuity cannot depend on device connections that can be removed.


Length of Free Conductors at Outlets, Junctions, and Switch Points

At least 150 mm (6 in.) of conductors must be left at every outlet, junction, or switch point for splices or connections. When the opening to the point of connection is less than 200 mm (8 in.), the conductors must extend at least 75 mm (3 in.).


Boxes, Conduit Bodies or FittingsWhere Required

For concealed knob and tube wiring a box must be installed at each switch point and outlet. Use fittings and connectors only for the purpose that they are listed and designed for. If the wiring method is conduit, tubing, Type AC cable, Type MC cable, Type MI cable, non-metallic sheathed cable or other cables, then a conduit body or box must be installed at each conductor splice point, switch point, outlet point, termination point, pull point and junction point. This requirement for the use of a conduit body or box is in force unless not required by (A) through (M) following.

(A) Wiring Methods With Interior Access. If the outlet method has a removable cover that is accessible after installation.

(B) Equipment. A wiring compartment or junction box that is part of approved equipment is allowed instead of a box.

(C) Protection. If the cables enter or leave conduit or tubing that is used for support or protection against physical damage. There are requirements for the use of fittings.

(D) Type MI Cable. If accessible fittings are used for straight through splices.

(E) Integral Enclosure. A wiring device with an integral enclosure that has brackets that fasten it to wall or ceilings for use with nonmetallic sheathed cable is permitted.

(F) Fitting. A fitting can be used instead of a conduit body or box as long as there are splices or terminations in the fitting. The fitting has to be accessible after the installation.

(G) Direct-Buried Conductors. As permitted in 300.5(E).

(H) Insulated Devices. As permitted in 334.40(B) where the insulated device is supplied by nonmetallic-sheathed cable.

(I) Enclosures. Where a splice, switch terminal, or pull point is in a cabinet or cut out box in an enclosure for a switch or overcurrent device as noted in 312.8 in a motor controller as noted in 430.10(A) or in a motor center.

(J) Luminaires (Fixtures). If the fixture is used as a raceway as permitted in 410.31 and 410.32.

(K) Embedded. Where conductors are embedded as permitted in 424.40, 424.41(D), 426.22(B), 426.24(A), and 427.19(A) they are not required for splices.

(L) Manholes and Handhole Enclosures. If only qualified persons have access in manholes except when connecting to electrical equipment and compliance with Part V of Article 110 for manholes and 314.30 for handhole enclosures.

(M) Closed Loop. If a device is identified and listed for installation without a box and is used with a closed loop power distribution system.


Raceway or Cable to Open or Concealed Wiring


Number and Size of Conductors in Raceway

In general, the number and size of conductors in a raceway must allow the dissipation of heat, easy installation, and withdrawal.


Raceway Installations

Raceways other than exposed raceways or busways that have hinged or removable covers must be installed complete between outlet, junction, or splicing points before the conductors are installed. Three exceptions are: (1) for prewired assemblies, (2) when necessary for the installation of utilization equipment, and (3) short sections used for protection from physical damage. There is a restriction on the use of welding.


Supporting Conductors in Vertical Raceways

Conductors must be supported at the top of vertical risers with additional support as noted in Table 300.19(A) of the NEC® when the rise is greater than the values in that table.


Induced Currents in Metal Enclosures or Metal Raceways

The effects of induced currents in metal enclosures must be minimized. This is done by grouping all phases, neutral, and grounding conductors in one raceway. If individual conductors are used, a slot can be cut between the holes made in the metallic enclosure through which the individual conductor passes. There are some exceptions.


Spread of Fire or Products of Combustion


Wiring in Ducts, Plenums, and Other Air-Handling Spaces

(A) Ducts for Dust, Loose Stock, or Vapor Removal. Wiring of any type is prohibited in these types of installations. This includes ventilation of commercial-type cooking equipment.

(B) Ducts or Plenums Used for Environmental Air. The following can be used when the duct or plenum was specifically made for this purpose: Type MI, Type MC cable with the specified sheath, EMT flexible metallic tubing, intermediate metal conduit, or rigid metal conduit. When necessary to connect physically adjustable items, flexible metal conduit can be used, but only in lengths not longer than 1.2 m (4 ft.). Only devices that act on or sense the contained air, such as thermostats, are permitted. If lighting fixtures are installed for maintenance or repair, they must be of the enclosed gasketed type. This paragraph applies primarily to fabricated ducts and plenums.

(C) Other Space Used for Environmental Air. This paragraph applies to spaces other than those in Sections 300.22(A) and (B) for environmental air handling. Habitable rooms or areas of buildings which are not primarily used for air handling are not included. Only totally enclosed nonventilated insulated busways that do not have a provision for plug-in connections and Type MI cable, Type MC cable without an overall nonmetallic covering, as well as Type AC cable or other factory-assembled multiconductor control or power cable made to be installed in these areas can be installed in this space. Other types of cables and conductors can be used but only when installed in EMT, flexible metallic tubing, intermediate metal conduit, rigid and flexible metal conduit, where accessible surface metal raceway or metal wireway with metal covers or solid bottom metal cable tray with solid metal covers. Also permitted is electric equipment with a metal enclosure or a listed nonmetallic enclosure with low smoke- and fire-resistive qualities suitable for the ambient temperature unless prohibited in another place in the Code.

(D) Information Technology Equipment. Refer to Article 645.


Panels Designed to Allow Access


II. Requirements for Over 600 Volts, Nominal


Covers Required


Conductors of Different Systems

See Section 300.3(C)(2).


Conductor Bending Radius


Protection against Induction Heating


Aboveground Wiring Methods


Braid-Covered Insulated Conductors-Exposed Installation


Insulation Shielding


Moisture or Mechanical Protection for Metal Sheathed Cables


Underground Installations

Article 90 Introduction


Wiring and Protection

Wiring Methods and Materials

Equipment for General Use

Special Occupancies

Special Equipment

Special Conditions


Annex C. Conduit and Tubing Fill Tables for Conductors and Fixture Wires of the Same Size

Annex D. Examples

Annex E. Types of Construction

Pocket Guide to the National Electrical Code 2005
Pocket Guide to the National Electrical Code(R), 2005 Edition (8th Edition)
ISBN: 0131480014
EAN: 2147483647
Year: 2004
Pages: 120 © 2008-2020.
If you may any questions please contact us: