How to correctly install
the wiring methods of Chapter 3 with the help of Article 300
Chapter 3 of the NEC provides detailed requirements for installing wiring and raceways. Among other things, the requirements address the routing, splicing, and securing necessary for a finished installation. Chapter 3 can appear to be overwhelming when you realize it contains 44 Articles. However, all but four of those pertain to a specific type of raceway or wiring product. The first Article in Chapter 3 is Art. 300, which provides the basic rules for wiring methods that are common to the other Articles of Chapter 3. Keep in mind that Article 300 doesn't apply to the internal parts of electrical equipment, nor does it apply to communications systems (except where specifically referenced by Chapter 8).
Use only Chapter 3 wiring methods to install conductors, such as a raceway, cable, or enclosures [300.3].
Install all conductors of a circuit in the same raceway, cable, trench, cord, or cable tray. You'll find four special cases that permit variations of this rule in 300.3(B)(1) through (4).
- Conductors can be run in parallel in accordance with 310.4, and must have all circuit conductors installed within the same raceway, cable tray, trench, or cable.
- You can install equipment grounding conductors (EGC) outside of a raceway or cable assembly for certain existing installations per 250.130(C). You can locate equipment grounding conductors outside of a flexible raceway, if you install the bonding jumper per 250.102(E).
- In nonferrous wiring methods, circuit conductors can be run in different raceways (Phase A in raceway 1, Phase B in raceway 2, etc.), if, in order to reduce or eliminate inductive heating, the raceway is nonmetallic or nonmagnetic, and the installation complies with 300.20(B).
- Neutral conductors can originate in a pullbox that is connected to a column-width panelboard by an auxiliary gutter.
See 725.136(A) for Class 2 and Class 3 circuit conductors. Power and lighting circuits must be separated from those used for control, signal, or communications so the higher voltage conductors do not accidentally energize the control, signal or communications wiring [300.3(C)(1) FPN].
Protecting conductors from physical damage
To route raceways and cables through wooden framing members, make the hole in the center if possible. If a hole isn't at least 1 1/4 in. from the front, install a metal plate per 300.4(A) to protect wiring from nails and the like.
When routing cables through metal framing members, protect them from being cut or damaged by listed bushings [300.4(B)].
To protect cables or raceway-type wiring methods when installed behind panels, such as ceiling tile, they must be supported according to their applicable Articles [300.4(C)].
When cables or raceways run parallel to framing members or furring strips, keep the wiring method at least 1 1/4 in. from the nearest edge of the framing member or furring strip [300.4(D)].
Wiring methods under metal-corrugated sheet roof decking must not be less than 1 1/2 in. from the nearest surface of the roof decking [300.4(E)].
Cables or raceway-type wiring methods installed in grooves and covered by the building finish must have at least 1 1/4 in. of free space or be protected by a 1/16 in. thick steel plate or sleeve [300.4(F)].
Where raceways contain insulated conductors 4 AWG and larger that enter an enclosure, use a fitting, such as a bushing or a box connector, to protect the conductors [300.4.(G)]. This rule applies to threaded IMC and RMC, regardless of the size of the conductors [342.46 and 344.46]. When a raceway terminates in a threaded raceway entry with a smooth, rounded, or flared surface for the conductors, such as a meter hub fitting or a Meyer's hub, which is considered to be sufficient protection for the conductors [300.4(G) Ex].
For underground cables or raceways, ensure a minimum cover per Table 300.5. Cover is measured from the top of the underground cable or raceway to the surface of finished grade [Table 300.5, Note 1].
The interiors of enclosures or raceways installed underground are considered "wet locations." The associated cables and insulated conductors must be listed for use in wet locations per 310.8(C), and any connections or splices made in an underground location must be approved for wet locations [300.5(B)], as shown in Fig. 1.
Direct-buried conductors or cables can be spliced or tapped without a splice box [300.15(G)], if the splice or tap is made following 110.14(B).
When cables are run under a building, they must be installed in a raceway that extends past the outside walls of the building [300.5(C)].
Direct-buried conductors and cables must be protected from damage:
- Where direct-buried cables or conductors emerge from grade, they must be installed in enclosures or raceways. Protection must extend at least 18 in. below grade; protection above ground must extend to a height of at least 8 ft [300.5(D)(1)], as shown in Fig. 2.
- Conductors must be protected at the point where they enter a building [300.5(D)(2)].
- Direct-buried service-lateral conductors not under the exclusive control of the utility and 18 in. or more below grade must have their location identified by a warning ribbon [300.5(D)(3)].
- Where enclosures or raceways for direct-buried conductors or cables are subject to physical damage, the conductors must be installed in rigid metal conduit, intermediate metal conduit, or Schedule 80 PVC conduit [300.5(D)(4)].
- If direct-buried conductors, cables, or raceways are subject to movement by settlement or frost, they must be arranged to prevent damage to conductors or equipment connected to the wiring [300.5(J)].
- Backfill material that won't damage underground conductors, cables, or raceways must be used. Large rocks, paving materials, cinders, large or sharp objects, or corrosive material must not be placed where it could come into contact with the cables or conductors or raceways. Adding fine sand or other suitable material in the ditch can provide protection for buried conductors [300.5(F)].
What about moisture? If moisture could enter a raceway and contact energized live parts, a seal must be installed at one or both ends of the raceway. This is a common problem for equipment located downhill from the supply, or in underground equipment rooms. Section 230.8 provides information for service raceway seals. Section 300.7(A) addresses situations where a raceway travels through different temperature areas and must be sealed. This type of situation does not require an explosion-proof type of seal, products such as duct seal in a junction box should serve the purpose of stopping the circulation of moist air.
Your first line of defense against corrosion is to use materials suitable for the environment [300.6].
Ferrous materials must be protected against corrosion both inside and outside by a coating of listed corrosion-resistant material. For example, when conduit is threaded in the field, threads must be coated with an approved coating that is electrically conductive and corrosion-resistant.
Nonferrous metal raceways such as aluminum rigid metal conduit don't have to meet the provisions of 300.6(A), but have their own requirements in Sec. 300.6(B).
If ferrous parts are protected from corrosion solely by enamel, they must not be used outdoors or in wet locations [300.6(A)(1)].
Boxes or cabinets having a system of organic coatings marked "Raintight," "Rainproof," or "Outdoor Type," can be installed outdoors [300.6(A)(2)].
Ferrous metal equipment can be installed in areas subject to severe corrosive influences, such as in concrete or in direct contact with the earth, if the material is approved for the condition or provided with corrosion protection approved for the condition [300.6(A)(3)].
For example, galvanized steel EMT can be installed in concrete at grade level and in direct contact with the earth only if supplementary corrosion protection is provided (UL White Book, βGuide Information for Electrical Equipment,β www.ul.com/regulators/2008_WhiteBook.pdf). EMT can generally be installed in concrete, including slabs above grade level, without supplementary corrosion protection (Fig. 3).
Aluminum equipment can be installed in areas subject to corrosive influences, if provided with supplementary corrosion protection [300.6(B)].
Nonmetallic equipment can be installed in areas subject to severe corrosive influences, if the materials are identified for the condition and:
- Where exposed to sunlight, the materials are listed or identified as sunlight-resistant [300.6(C)(1)].
- Where subject to exposure to chemical corrosion, materials or coatings are either inherently resistant to chemicals based upon their listing, or are identified for use with the specific chemical [300.6(C)(2)].
If mounting metallic raceway in an indoor wet location, such as a washdown area, each box, fitting, raceway, and cable must have at least 1β4 in. of airspace between it and the wall or supporting surface [300.6(D)].
Raceways exposed to differences in temperature
Along its length, a raceway can be exposed to different temperatures. When the difference is significant, this can result in a problem with condensation inside the raceway. To solve this problem, the NEC requires sealing the raceway with a material approved by the AHJ that will prevent the circulation of warm air to a colder section of the raceway [300.7(A)]. You don't need an explosionproof seal for this purpose.
When raceways are installed in a location where the temperature is subject to changes, the result may be movement of the raceway due to thermal expansion and contraction. Expansion fittings must be installed when the movement is calculated to be ΒΌ in. or more between boxes or fittings on PVC installations [300.7(B) and 352.44].
Table 352.44 provides the expansion characteristics for PVC conduit. For metallic raceways, you can determine the expansion characteristics by multiplying the values from Table 352.44 by 0.20 (ferrous) or 0.40 (aluminum). Table 354.44 provides the expansion characteristics for reinforced thermosetting resin conduit (RTRC) [300.7(B) FPN].
Raceways can't be used for multitasking
Raceways are to be used only for electrical conductors and cables. Nonelectrical components, such as pipes or tubes for steam, water, or gas, must not be installed inside electrical raceways or cable trays [300.8].
Metal raceways, cables, boxes, fittings, cabinets, and enclosures for conductors must be metallically joined [300.10]. This forms a continuous, low-impedance fault current path capable of carrying any fault current likely to be imposed [110.10, 250.4(A)(3), and 250.122] [Fig. 4]. It also eliminates dangerous differences of potential.
Metal raceways and cable assemblies must be mechanically secured to boxes, fittings, cabinets, and other enclosures [300.10]. However, two exceptions apply:
- Short lengths of metal raceways used for the support or protection of cables [300.10 Ex 1].
- Equipment enclosures isolated per 250.96(B) [300.10 Ex 2].
This article has covered many of the essential requirements for the installation of all types of wiring methods. The next installment will include more information about the securing and supporting of wiring methods.