There is probably no more commonly used article in the National Electrical Code^® than Article 300, Wiring Methods. This article sets the basic rules for premises wiring systems, representing almost every type of conductor, cable, and raceway that moves electrical power from the source (usually a utility company) to the point of use (usually a piece of equipment or an outlet).

What Is Covered

Part I of Article 300 covers all wiring systems operating at 600 volts or less. Part II covers systems over 600 volts, but we’ll leave that for future coverage. Our concern here (and the subject of the vast majority of electrical installations) is wiring 600 volts or less.

Part I does not cover the internal conductors of motors, controllers, and other equipment.

Important Requirements

It is a bit of a stretch to call some NEC^® requirements more important than others, since they are all equally required. Nonetheless, the following are central requirements, in that they apply to a huge number of installations:

·        Single conductors are permitted only where they are part of a standard wiring method. “Standard” here means a wiring method covered specifically by the NEC.

·        All ungrounded conductors, neutral conductors, and equipment grounding conductors of a single circuit must be run in the same raceway, cable, cable tray, trench, or cord. In other words, you are not permitted to run two conductors of a circuit in one cable and another conductor from the same circuit in a separate cable (or raceway). This limitation is necessary to prevent induction heating, which can lead to damage and fire. [300.3(B), 300.3(B)(1), 300.3(B)(4)]

·        Ac or dc circuits of 600 volts or less are allowed to share a raceway, cable, or enclosure. However, the insulation of each must be equal to the highest voltage present. So, a 120-volt circuit is permitted to occupy the same raceway with a 600-volt circuit, as long as every conductor in the raceway is insulated for 600 volts.

·        Circuits over 600 volts and circuits 600 volts or less are not permitted to be run in the same raceway, cable, or enclosure. (Exceptions are made for lighting fixture ballast conductors and control instrument conductors.)

One important change to Article 300 for 2008 is a clarification of wiring in raceways in wet and underground locations. In previous versions of the NEC, there was no direct coverage of conductors inside raceways installed in wet locations, although the 2005 edition [300.5(B)] did cover conductors inside raceways that are installed underground.

The 2008 edition clarifies this whole issue by stating explicitly that the inside of underground raceways are considered wet locations [300.5(B)], and that the inside of raceways in wet locations above ground are also considered wet locations [300.9]. So, all conductors in these locations (that is, inside of these raceways) must comply with 310.8(C). In practice, this generally means that THWN or XHHW conductors must be used. (These are only the most common types of conductors that might be used in these locations. See 310.8 for full coverage.)

Raceways

Metal raceways are required to be properly grounded. The central requirements for grounding are found in Sections 240.4 and 240.5. Section 300.10 expands on these grounding requirements by requiring that all metal raceways and enclosures must be electrically continuous. Further requirements in Article 300 are as follows:

·        No splices are allowed in raceways. There are 8 Exceptions. [300.13(A)]

·        All raceways, cables, and boxes must be securely supported. (300.11)

·        Air flow through a raceway must be prevented (usually by sealing) if the two ends of a run are exposed to noticeably different temperatures. (300.7)

·        Raceways or cable trays that contain electric wiring are not permitted to contain any other type of piping (such as steam, gas, air, drainage). (300.8)

·        All raceways, cables, and boxes must be electrically and mechanically joined together, except as allowed for nonmetallic boxes. (300.10, 300.12)

·        Raceways are not allowed to support other raceways, cables, or nonelectric equipment, unless identified as suitable for the use. Class II cables are allowed to be supported if they are used only for equipment control circuits.

·        Suspended ceiling support wires are permitted to be used to support wiring only in the following circumstances:

                  1. With fire-rated systems: only if they have been tested as part of the                            fire-rated assembly [300.11(A)(1)]

                  2. With non–fire-rated systems: only per the manufacturer’s instructions [300.11(A)(2)]

·        The continuity of a grounded conductor must not be dependent on device connections. On multiwire branch circuits, neutrals aren’t permitted to “feed through” a wiring device, but must be made up with pigtail connections.

·        The above requirement does not apply to conductors that don’t splice or terminate in the box, but rather feed through.

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.

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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