At EC&M, we know NEC issues are very important to our readers. That's why we've dedicated a monthly department to answering your latest Code questions and concerns. When you find yourself stumped by the Code, just e-mail your question to firstname.lastname@example.org for future consideration in Code Quandaries.
Q. I know the standard convention is to run each phase and neutral in each raceway. Does the NEC allow me to run the conductors of each phase in a separate raceway? The reasoning for this would be for ease of alignment to terminate parallel conductors in large equipment.
A. Yes. The NEC allows “isolated phase installations,” as you described, but you must install the conductors in accordance with the requirements of Exception No. 2 to Sec. 300-5(i). It specifies that an “isolated phase installation” is permitted in nonmetallic raceways in close proximity where the installation complies with the requirements of Secs. 300-20 and 310-4.
Sec. 300-20 prohibits induction from heating the metal surrounding the conductor. Sec. 300-20(b) allows for cutting slots in the metal between the individual holes through which the individual conductors pass and for using aluminum locknuts. Sec. 310-4 requires all paralleled conductors in each phase, neutral, or grounded circuit conductor to be the same length, have the same conductor material, area (in circular mils) and insulation type, and be terminated alike.
Commentary in the NEC Handbook states that “isolated phase installations” increase the circuit impedance, which results in a greater conductor voltage drop. In addition, this type of installation should be limited to outdoor, underground installations so as to reduce inductive heating of interior metal as well as reduce electromagnetic interference (EMI) of sensitive electronic equipment.
Q. I have spent the last hour looking at the 1999 NEC to find out if a grounding receptacle must be marked “GFCI Protected” when the circuit's first device is a GFCI receptacle. Does the Code address this? The design is to use one GFCI to protect four other grounding-type receptacles.
A. No. GFCI-protected receptacles are not required to be marked “GFCI Protected.” However, Sec. 210-7(d)(3) specifies that when replacing a receptacle where a grounding means does not exist in the receptacle enclosure, you can replace a nongrounding-type receptacle with a grounding-type receptacle if protected by a ground-fault circuit interrupter. These receptacles must be marked “GFCI Protected” and “No Equipment Ground.”
Q. Specifications require me to run EMT to a refrigeration system's compressor rack. I have been doing electrical work for 16 years, and I have always run flexible raceway to any equipment that vibrates (such as motors and transformers). Is there an actual Code article that tells me I have to use flexible raceway when running a branch circuit to a motor or transformer?
A. No. The NEC does not require a flexible metal raceway for motors or transformers. However, the method of choice for motors and transformers is a flexible raceway — not because the NEC requires it, but because it's the proper engineering choice.
Q. An electrical inspector recently failed one of our cell tower installations because nonconductive optical-fiber cable ran in the same raceway and enclosure with the 120V circuits. Can I run nonconductive optical-fiber cable in the same raceway and/or enclosure with power cable?
A. Yes. Section 770-52(a) states, “Nonconductive optical fiber cables shall be permitted to occupy the same cable tray or raceway with conductors for electric light, power, Class 1, nonpower-limited fire alarm, or medium power network-powered broadband communications circuits operating at 600 volts or less.”
Q. Can we use a 2-pole switch to control 277V lighting and a 120V-exhaust fan in a bathroom by running the circuits through separate poles of the switch?
A. No. The answer comes from the listing of the switch [Sec. 110-3(b)]. Under the guide card information for snap switches on page 103, the 2000 UL White Book says: “Multipole, general-use snap switches have not been investigated for more than a single circuit operation unless marked ‘2-circuit’ or ‘3-circuit.’” The switch cannot be used to switch the two separate branch circuits, unless it is marked.
Q. We were saw cutting for a tenant improvement and hit a series of 1-in. and 2-in. schedule 40 conduits that fed several retail spaces. The conduits in the area we were cutting were buried approximately 2 in. from the top of the slab. Does the NEC specify how deep conduits must be placed when installed in a concrete slab?
Q. Please advise what plenum-rated cables mean for data and voice cables?
A. According to Sec. 725-71(a), 760-71(d), and 800-51(a) and FPN, plenum cable is listed as suitable for use in ducts, plenums, and other spaces used for environmental air, and it has adequate fire-resistant and low smoke-producing characteristics. One method of defining low smoke-producing cable is by establishing an acceptable value of the smoke produced when tested in accordance with the Standard Method of Test for Fire and Smoke Characteristics of Wires and Cables, NFPA 262-1994, to a maximum peak optical density of 0.5 and a maximum average optical density of 0.15. Similarly, you can define fire-resistant cables by establishing a maximum allowable flame travel distance of 5 ft when tested in accordance with the same test.