The Importance of Article 300 to Wiring Systems

Let’s reiterate the importance of NEC^® Article 300. Section 300.1(A) begins this article by stating: “This article covers wiring methods for all wiring installations unless modified by other articles.” Then 300.1(B) goes on to explain that this article does not apply to the wiring inside of motors, controllers, and similar equipment. However, almost every other type of wiring you ever install in or on a building will require you to know and understand Article 300. It’s that important.

Protection of Conductors

All conductors must be protected from physical damage. Sections 300.4(A) and 300.4(B) mandate specific protection methods, including the following:

·    When cables or raceways are installed through wood structural members, the edge of the bored holes must be at least 1¼ inch from the edge of the wood framing member.

·    Where the clearance specified above is not possible, a 1/16-inch steel plate must be installed to cover the area of the wiring.

·    Notches can be cut in wood, if the strength of the structure won’t be jeopardized. Wiring can be laid in the notches, and a 1/16-inch steel plate must be installed to cover the area.

·    The steel plate mentioned above for covering notches is not required where the wiring method is rigid metal conduit, intermediate metal conduit, electrical metallic tubing, or rigid nonmetallic conduit.

·    Where nonmetallic-sheathed cables go through metal framing members (usually metal studs), a listed grommet or bushing must protect the hole that the cable passes through.

·    In locations where nails or screws are likely to damage nonmetallic-sheathed cables or electrical nonmetallic tubing, a 1/16-inch steel plate must be installed to cover the area.

Underground Wiring

Directly buried cables or conduits must meet the depth requirements given in Table 300.5

of the NEC. You’ll notice that warning tape is required for service conductors, but not for branch circuit and feeder circuits. Nonetheless, placing such a tape in a trench above the conductors would be both sensible and a cheap, easy way to prevent accidents.

Other requirements are as follows:

·    Cables installed underground beneath a building must be in a raceway that extends the full length of the building. [300.5(C)]

·    Conductors or cables directly buried must be protected from below where they emerge from the ground to a point 8 feet above the grade level by an enclosure or raceway. [300.5(D)(1)]

·    Conductors that enter a building must be protected up to the point of entrance. [300.5(D)(2)]

·    Directly buried conductors are allowed to be spliced without a junction box. However, the splicing method must be approved for the intended use. [300.5(E)]

·    Care must be taken so that backfill won’t damage conductors. When necessary, a layer of sand or gravel can be placed over the conductors to protect them, , or some other means of protection used.[300.5(F)]

·    Conduits or raceways that could allow moisture to contact live parts must be sealed. [300.5(G)]

·    Cables that terminate underground with directly buried wiring protruding from them must be terminated with a bushing or an equivalent sealing compound.

·    All conductors of the same circuit must be installed in the same raceway, or, for open conductors or cables, close to each other in the trench. [300.5(I)]

·    Parallel conductors are allowed, but each raceway must contain all the conductors of the same circuit. [300.5(I), Exception 1]

·    All metal raceways, cables, and fittings must be suitable for the area in which they are installed. If corrosive conditions exist, raceways must be covered with a corrosion-resistant coating or other suitable protection.

·    In wet locations and in locations where the walls are frequently washed (such as dairies or laundries), all raceways, cables, enclosures, and the like must be installed with an air space of at least 1/4 inch between them and the surface on which they are mounted. This requirement also applies when raceways, cables, or enclosures are mounted on absorbent materials such as damp paper or wood. [300.6(D)]

Boxes

We can summarize the requirements in Article 300 for boxes as follows:

·    Boxes or fittings are required at every splice or pull point in raceways. Wireways, gutters, and cable trays are excepted.

·    Boxes or fittings are required at every splice or pull point for AC, MC, MI, NM, or other cables (with exceptions). [300.15(A) through (M)]

·    Fittings and connectorsmust be used only with the system for which they were designed. For example, no flexible metal conduit is allowed to be used on conduit.

·    A box or fitting with a separately bushed hole for each conductor must be used where a change is made between raceway or cable systems and open or knob-and-tube wiring methods.[300.16(A), 300.16(B)]

·        Where a raceway terminates behind a switchboard, a bushing on the end of the raceway is permitted to be used instead of the box with bushed holes mentioned above. [300.16(B)]

Conductors
Article 300 contains a number of rules for conductors, including the following:

·    The number and size of conductors in raceways must be small enough to allow for heat dissipation, as well as ease of installation and removal.

·    Raceways must be completely installed before conductors are pulled into the raceway system. [300.18(A)]

·    At least 6 inches of free conductor must be left at every box, fitting, or other splice point (300.14). Conductors that are not spliced in a box (which are said to “feed through”) are excepted from this requirement. (It is a good idea to leave plenty of even feed-through free conductors available; because you never know what kinds of modifications may be required in the future.)

·    Conductors that are installed in vertical raceways must be supported. The support requirements are given in Table 300.19(A).

·    Conductors of AC systems must be grouped together in circuits to avoid induction heating. Where single conductors are used, inductive effects must be avoided by cutting notches in the metal between the holes the conductors pass through or running the conductors through a box or enclosure wall made of insulating material. (300.20)

·    Openings around electrical penetrations in fire-rated walls, ceilings, and floors,. must be properly sealed against the spread of fire. (300.21)

·    No types of wiring are allowed in ducts that transport dust, loose stock, or flammable vapors. [300.22(A)]

Plenums and Environmental Air Spaces

Wiring methods for spaces that handle environmental air (that is, the air we breathe) are especially important concerns. Not only can smoke travel quickly through environmental air spaces, but some burning materials also give off toxic fumes. So, all wiring methods in air spaces must be carefully chosen. Section 300.21 and especially Section 300.22 address the requirements for plenums.

Key Definitions from Article 100

Grounding Conductor. A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.

Grounding Conductor, Equipment (EGC). The conductive path installed to connect normally non–current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both.

Equipment. A general term, including material, fittings, devices, appliances, luminaires, apparatus, machinery, and the like used as a part of, or in connection with, an electrical installation.

Plenum. A compartment or chamber to which one or more air ducts are connected and that forms part of the air distribution system.

Premises Wiring (System). Interior and exterior wiring, including power, lighting, control, and signal circuit wiring together with all their associated hardware, fittings, and wiring devices, both permanently and temporarily installed. This includes (a) wiring from the service point or power source to the outlets or (b) wiring from and including the power source to the outlets where there is no service point. Such wiring does not include wiring internal to appliances, luminaries, motors, controllers, motor control centers, and similar equipment.

Raceway. An enclosed channel of metal or nonmetallic materials designed expressly for holding wires, cables, or busbars, with additional functions as permitted in this Code. Raceways include, but are not limited to, rigid metal conduit, rigid nonmetallic conduit, intermediate metal conduit, liquidtight flexible conduit, flexible metallic tubing, flexible metal conduit, electrical nonmetallic tubing, electrical metallic tubing, underfloor raceways, cellular concrete floor raceways, cellular metal floor raceways, surface raceways, wireways, and busways.

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