Using Two Cables For A Three-Way Switch, Mixing Plastic And Metal, And More

If you have a problem related to the National Electrical Code (NEC), are experiencing difficulty in understanding a Code requirement, or are wondering why or if such a requirement exists, ask Charlie, and he will let the Code decide. Questions can be sent to Answers are based on the 2011 NEC.

Thank you for commenting. Yes, NEC 300.3(B) requires all conductors of the same circuit to be contained in the same cable—unless otherwise permitted in accordance with 300.3(B)(1) through (4). NEC 300.3(B)(3) permits conductors in wiring methods with a nonmetallic (NM) sheath to be run in different cables. NEC 300.3(B) requires all circuit conductors of an individual circuit to be grouped to reduce inductive heating. This is not a problem with nonferrous wiring methods.

According to NEC 314.3, NM boxes can only be used with NM raceway and NM cable wiring methods, unless the requirements of exceptions No. 1 and No. 2 are followed. Basically, the Code requires a means of ensuring the electrical continuity of the metal raceways and a provision for attaching an equipment bonding jumper inside the box. Compliance can be accomplished by running an equipment grounding conductor with the circuit conductors and providing a grounding terminal or clip in the box.

NEC 334.12(B)(4) does not permit Type NM cable to be used in wet or damp locations. NEC 300.5(B) requires the interiors of raceways installed underground to be considered a wet location.

NEC 430.6(A)(1) directs you to Table 430.250 where the full-load current of a 10-hp, three-phase, 208V motor is shown as 30.8 amperes (A). The maximum rating of the motor branch-circuit overcurrent device (inverse-time breaker) is 30.8 × 2.5 = 61.6A as determined by 430.52. A 60A overcurrent device probably protects your motor branch circuit, and if you follow the requirements of 430.72(B)(2) Column C, you will find that, where the conductors extend beyond the motor-control enclosure, the motor branch-circuit protective device will adequately protect the rating of the 12 AWG copper conductors. Therefore, supplementary overcurrent protection will not be required.

If the calculated load exceeds 50A, a permanent feeder must be installed in accordance with 550.10(I) where it permits using 310.15(B)(7) to size the feeder conductors to a mobile home.

NEC 334.15(C) specifically addresses installing NM cable run exposed in crawl spaces. It shall be permissible to secure cables not smaller than two 6 AWG or three 8 AWG conductors directly to the lower edges of the joists. Smaller cables must be run through bored holes in joists or on running boards. However, some crawl spaces are typically open to damp or wet conditions, and 334.12(4) does not permit the installation of Type NM cable in wet or damp locations.

In accordance with 250.50, all grounding electrodes as described in 250.52(A)(1) through (A)(6) that are present at the building must be bonded together to form the grounding electrode system. Where none of these grounding electrodes exist, one or more of the grounding electrodes shown in 250.52(A)(4) through (A)(7) must be installed and used. For the building in question, none of the electrodes described in 250.52(A)(1) through (A)(6) are present except probably a concrete-encased electrode as described in 250.52(A)(3). Therefore, you must use the concrete-encased electrode as your grounding electrode and connect a grounding electrode conductor from the concrete-encased electrode to the grounded service conductor at the service disconnecting means [250.24(A)(1)]. The grounding electrode conductor to the concrete-encased electrode is not required to be larger than 4 AWG copper wire [250.66(B)]. With respect to the plastic piping to the metal well casing, bonding the well casing to the pump circuit equipment grounding (bonding) conductor is required by 250.112(M). The pump circuit equipment grounding (bonding) conductor is required by 250.112(L). NEC 250.104(A)(1) requires the interior metal water piping system to be bonded to the grounded service conductor.

A simple reactance ballast is a two-wire ballast usually found in low-wattage fluorescent fixtures.

An ungrounded conductor can reach any voltage within that system with respect to earth. When one conductor is grounded, a fixed potential to ground is established. For example, if you ground the common conductor of a single-phase, three-wire, 120/240V transformer secondary, the highest voltage to ground that can be reached by any conductor in that system is 50 percent of the transformer secondary potential (or 120V) because the grounded conductor is tapped at the midpoint of the 240V secondary winding.

You are essentially correct in your sizing requirements. The only concern I have is with you calling one electrode a primary grounding source and another electrode a secondary grounding source. All of the electrodes shown in 250.52(A)(1) through (A)(6), if available on the premises, must be bonded together to form the grounding electrode system. The size of the grounding electrode conductor must not be less than given in Table 250.66, except as permitted in 250.66(A) through (C). NEC 250.66(B) relates to connections to a concrete-encased electrode. The portion of the conductor that is the sole connection to the concrete-encased grounding electrode is not required to be larger than 4 AWG copper wire. It can be smaller if permitted by Table 250.66 but is not required to be larger. NEC 250.64(F) requires that the grounding electrode conductor must be sized for the largest grounding electrode conductor required among all the electrodes connected to it. The sole connection means that, if you run a grounding electrode conductor from the neutral bar to the concrete-cased electrode and bond from the concrete-encased electrode to an underground metal water pipe, the size of the grounding electrode conductor must be sized for the underground metal water pipe. The grounding electrode conductor is the conductor used to connect the grounding electrode(s) to the grounded (neutral) conductor at the service.

About the Author

Charlie Trout

Code Contributor

Charlie Trout is most known for his work with the National Electrical Code (NEC). He helped write the NEC Since 1990; he was a member of NECA’s National Codes & Standards Committee and chairman of the National Fire Protection Association (NFPA)’s...

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