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NEC Article 300 - Wiring Methods 10/30/2003
By Mike Holt for EC&M Magazine

Article 300 benefits extend beyond safety

The primary benefit of following the adequate requirements of Article 300 is the safeguarding of people and property. But, these requirements also reduce conductor failure rates and increase operational efficiency. For power quality problem prevention, Article 300 ranks second only to Article 250.

Article 300 requirements do not apply to signaling and communications systems, except where specifically mentioned. In addition, the requirements of this article do not apply to the internal parts of electric equipment. See 90.7 and Figure 300-1.

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Conductor requirements

You must install individual conductors in a raceway, cable, or enclosure [300.3]. Further, you must put all conductors of a circuit in the same raceway, cable, trench, cord, or cable tray (there are exceptions to this-see Figure 300-2). This requirement minimizes inductive heating of metallic raceways and enclosures, plus it reduces impedance should a ground fault develop.
When separate conductors are in a nonmetallic raceway as permitted in 300.5(I) Ex. 2, you can minimize the inductive heating of the metal enclosure by using aluminum locknuts and by cutting a slot between the individual holes through which the conductors pass (Figure 300-50). Because aluminum is nonmagnetic, it eliminates heating due to hysteresis. However, aluminum conduit, locknuts, and enclosures do carry eddy currents [300.20(B) FPN].

Power conductors of different systems can occupy the same raceway, cable, or enclosure if all conductors have an insulation voltage rating not less than the maximum circuit voltage (Figure 300-5). You must separate control, signal, and communications wiring from power and lighting circuits-so the higher voltage conductors do not accidentally energize them. Exceptions to this requirement allow power conductors to terminate to listed signaling equipment, if the power conductors maintain a minimum of 1/4 in. separation from the low voltage and limited-energy conductors.Where cables and raceways are underground, apply the requirements of 300.5 and Table 300.5. To protect underground cables and conductors from damage, follow the requirements of 300.15.

Conductors in raceways must be continuous between all points of the system. This means you can't place splices in the raceway, except as permitted by 376.56, 378.56, 384.56, 386.56, or 388.56.

In multiwire branch circuits, the removal of a wiring device (e.g., a receptacle) must not interrupt the continuity of the grounded (neutral) conductor. Therefore, you must splice the grounded (neutral) conductors together and provide a pigtail for device terminations (Figure 300-31). The opening of the ungrounded or grounded (neutral) conductor of a 2-wire circuit during the replacement of a device does not cause a safety hazard, so pigtailing of these conductors is not required.

A closer look at raceways

Use raceways, fittings, and supports that are suitable for the environment in which you'll install them (Figure 300-19). Where corrosion protection is necessary (underground and wet locations) and the conduit is threaded in the field, coat the threads with an approved electrically conductive, corrosion-resistant compound.

If condensation from temperature changes might be a problem, fill the raceway with a material approved by the AHJ, to prevent the circulation of warm air to a colder section of the raceway or sleeve. An explosionproof seal is not required for this purpose. See Figure 300-20.

You must provide raceways with expansion fittings where necessary to compensate for thermal expansion and contraction. Table 352.44(A) provides the expansion characteristics for PVC rigid nonmetallic conduit. To determine the expansion characteristics for metal raceways (EMT IMC and RMC), multiply the values from Table 352.44(A) by a multiplier of 0.20.

If you use an expansion fitting with a metal raceway, you must use a bonding jumper to maintain the equipment grounding path [250.98 and 300.10]. Join all metal raceways, cable, boxes, fittings, cabinets, and enclosures to form a continuous low-impedance ground-fault current path. The ground-fault current path must have adequate capacity to carry any fault likely to be imposed on it. See 110.10, 250.4(A)(3), 250.22, and Figure 300-23.

Raceways are designed for the exclusive use of electrical conductors and cables, and cannot contain nonelectrical components (e.g., lines for steam, water, or gas). See Figure 300-22. Raceways and cable sheaths must be mechanically continuous between terminations [300.12]. However, short lengths of metal raceways used for cable protection or support are not required to be electrically continuous, mechanically continuous, or bonded.

Securely fasten in place raceways, cable assemblies, boxes, cabinets, and fittings. You cannot use ceiling support wires or the ceiling grid to support raceways or cables [300.11].

You can support electrical wiring within the cavity of a floor-ceiling or roof-ceiling assembly to independent support wires that are secured at each end [300.11(A)(1)].
If this cavity is fire-rated, the support wires must be distinguishable from the suspended ceiling support wires by color, tagging, or other effective means. If this cavity is non-fire-rated, the support wires do not have to be distinguishable from the suspended ceiling support wires [300.11(A)(2)].
You can support outlet boxes [314.23(D)] and luminaires to the suspended ceiling if you securely fasten them to the ceiling-framing member [410.16(C)].
You cannot use raceways to support other raceways, cables, or equipment (Figure 300-26). However, you can support Class 2 and 3 cables to the raceway that supplies power to the equipment controlled by the circuit. Because Class 2 and 3 cables cannot be installed in the same raceway with the power conductors [725.55(A)], the next best thing is to attach them to the raceway. See 725.58 and Figure 300-27.

You cannot use type AC, NM, or MC cable to support other cables, raceways, or nonelectrical equipment. You can at times use raceways to support threaded boxes, conduit bodies [314.23(E) and (F)], and luminaires [410.16(F)].

If the vertical rise of a raceway exceeds the values of Table 300.19(A), you must support the conductors at the top-or as close to the top as practical (Figure 300-48). The weight of long vertical runs of conductors can cause the conductors to drop out of the raceway, if you don't secure them properly. There have been cases where conductors in a vertical raceway were released from the pulling basket (at the top), and the conductors fell down and out of the raceway injuring the workers!

You must install electrical circuits and equipment in a way that doesn't substantially increase the possible spread of fire or products of combustion. This means sealing openings in fire-rated walls, floors, and ceilings for electrical equipment. Use fire-stop material listed for the specific types of wiring methods and construction structures [300.21].

In general, you must keep wiring and air handling separate (Figure 300-52). You can wire in environmental air space under certain conditions listed in 300.21(C) A space not used for environmental air-handling purposes has no restrictions on wiring methods, so you can use nonplenum cables there (Figure 300-54).

While Article 300 provides a basis for safeguarding people and equipment, it also provides a basis for good system performance. For example, a small engine plant experienced severe power quality problems. Investigation showed the wireways were not electrically continuous. Bonding the wireways eliminated the power quality problems. An extrusion plant was experiencing high scrap rates on a new system installed with signal and power wiring in the same wireways. Separating these dropped the scrap rates to less than one percent.