Article 310—Conductors for General Wiring;
Article 410—Lighting Fixtures, Lampholders, Lamps, and Receptacles;
Article 501—Class I Locations; and
Article 680—Swimming Pools, Fountains, and Similar Installations
Grounding electrode connections
Q: Is it permissible to connect the grounding electrode conductor from two 8-foot ground rods to the metal water pipe or does this grounding electrode conductor have to be run to the service disconnect switch? The ground rods are used to supplement the buried metal water pipe.
A: Where there is at least 10 feet of buried metal water pipe, it must be used as a grounding electrode. The grounding electrode conductor must be connected to the metal water pipe within 5 feet of the building entrance.
The grounding electrode conductor connected to the ground rods does not have to be larger than No. 6 copper and may be terminated on the water pipe within 5 feet of where it enters the building. This information may be found in Section 250-50.
Bonding around swimming pools
Q: Because of the close proximity of property lines to the edges of a swimming pool, a 3/4-inch rigid metal conduit supplying power to a boat dock must be run within 5 feet of the pool. Does this conduit have to be bonded to the swimming pool reinforcing steel even though it is effectively grounded?
A: The question does not indicate whether the rigid metal conduit is above or below ground. If it is underground, Section 680-10 requires a minimum burial depth of 6 inches.
This Section has no requirement for bonding the metal raceway to the pool reinforcing steel. If the conduit is above ground and less than 12 feet above the maximum water level of the pool, it must be bonded to the pool bonding grid with a No. 8 or larger copper bonding jumper.
Sizing the neutral conductor
Q: How should the neutral be sized where supplying single-phase, 208-volt branch circuits and feeders from three-phase, 208-volt wye system? Specifically, can the neutral on a three-wire, 208-volt, 200-ampere feeder be reduced where all loads are connected for 120-volt operation? Under what conditions is a grounded circuit conductor (neutral) counted as a current carrying conductor? When is the grounded circuit conductor not counted as a current carrying conductor?
A: Even though the 120-volt loads connected to the three-wire, 208-volt feeder are balanced, or nearly equally balanced, between the ungrounded conductors and grounded circuit conductor, the grounded circuit conductor (neutral) cannot be reduced in size because it carries approximately the same amount of current as the ungrounded feeder conductors. This is because the grounded circuit conductor is only 120 electrical degrees out of phase with the ungrounded feeder conductors. For zero current flow on the grounded circuit conductor (neutral), the two ungrounded circuit conductors of a three-wire feeder must be 180 electrical degrees apart, both ungrounded conductors must be carrying the same load in amperes, and the power factor must be the same on both ungrounded conductors.
The National Electrical Code (NEC) has this to say about neutral conductors that are, or are not, current-carrying conductors. “In a 3-wire circuit consisting of two phase wires and the neutral of a four-wire, three-phase wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of Section 310-15(b)(2)(a).” This sentence is in Section 310-15(b)(4)(b) and requires the use of derating factors where four or more circuit conductors are in a raceway or cable assembly.
The answer to the question of whether the neutral conductor size can be reduced on a three-wire, 208-volt, 200-ampere feeder that supplies only 120-volt loads is “no” for the reasons previously mentioned.
The neutral or grounded phase conductor is a current-carrying conductor on all two-wire, single-phase circuits where the grounded conductor is present. It is a current-carrying conductor on three-wire, wye-connected circuits as described earlier. The neutral is also considered to be a current-carrying conductor on four-wire, three-phase, wye systems which have a major portion of the load that is nonlinear. Since some harmonic currents are additive in the neutral, the circuit-grounded conductor must be counted as a current-carrying conductor.
The circuit grounded conductor (neutral) is not assumed to be a current-carrying conductor where connected to three-wire, single-phase circuits, and where supplying phase-to-neutral loads without harmonics on three-phase, four-wire wye connected systems.
Wiring fluorescent fixtures
Q: I am wiring rows of surface-mounted fluorescent fixtures end-to-end in a department store. In each row I am wiring one lighting fixture for night lighting. The electrical inspector says that I cannot run the night light circuit through the other fixtures because the fixtures are not listed for use as a raceway. Is he right?
A: Section 410-31 of the NEC permits the type of installation mentioned in the question. This section has the title: Fixtures as Raceways, and reads like this: “Fixtures shall not be used as a raceway for circuit conductors.” However, there are three exceptions. The first exception allows branch circuit conductors to pass through lighting fixtures where they are listed for use as a raceway. The second exception limits the number of branch circuit conductors that can be carried through lighting fixtures that are assembled end-to-end to form a continuous row, or that are connected together by recognized wiring methods. Under this exception, a two-wire or multiwire branch circuit can be run through the fixtures, and the fixtures do not have to be listed as a raceway.
Exception No. 3 permits the two-wire circuit for night lighting to be installed along with the branch circuit conductors that supply the fixtures. Since this exception is related to Exception No. 2, the extra two-wire circuit is permitted to be run from fixture-to-fixture without requiring the fixtures to be marked, “Suitable for use as a raceway.”
Q: Is it necessary to have a seal fitting in a rigid conduit that passes through a Class I division 1 area?
A: Not always. Generally, a sealing fitting is required wherever a rigid metal conduit enters or leaves a Class I, Division 1 area. Section 501-5(a)(4) contains this statement: “In each conduit run leaving a Class I, Division 1 location. The sealing fitting shall be permitted on either side of the boundary of such location within 10 feet (3.05m) of the boundary, and shall be designed and installed so to minimize the amount of gas or vapor within the Division 1 portion of the conduit from being communicated to the conduit beyond the seal. Except for approved explosionproof reducers at the conduit seal, there shall be no union, coupling, box, or fitting between the conduit seal and the point at which the conduit leaves the Division 1 location.”
There is an exception to this rule. A sealing fitting is not required where a metal conduit that contains no unions, couplings, boxes, or fittings passes through a Class I, Division 1 area and extends for at least 12 inches beyond each boundary and terminates in unclassified locations. Because gases or vapors cannot enter the conduit system under these conditions, seals are not required.
Paralleling type MC cables
Q: We propose to install two parallel runs of Type MC Cable for a 600-ampere, 208Y/120 volt, four-wire feeder. The only cable in stock at a local electrical supply company is four-conductor, 500 kcmil aluminum conductors with a No. 1 aluminum equipment grounding conductor. The conductor insulation is Type XHHW-2. Is it permissible to run a No. 2/0 bare aluminum equipment grounding conductor along with the Type MC Cables to satisfy the minimum-size equipment grounding conductor requirement in Section 250-122 and Table 250-122?
A: No. You cannot run an equipment grounding conductor along with the MC Cables to satisfy the requirements in Section 250-122. In fact, part (f) of Section 250-122 says that where conductors are run in parallel in multiple raceways or cables, the equipment grounding conductors must be run in parallel in each raceway or cable. And part (f)(1) requires that each parallel equipment grounding conductor be sized on the basis of the ampere rating of the overcurrent device protecting the circuit conductors in the cable. Table 250-122 is used for this purpose. According to the Table, a No. 2/0 aluminum equipment grounding conductor is required in each cable where the overcurrent device is 600 amperes.
At least one Type MC Cable manufacturer produces a cable with an oversize equipment grounding conductor. The prime purpose for manufacturing these cables is for paralleling. This manufacturer lists six combinations of circuit conductor sizes and equipment grounding conductor sizes that allow paralleling of two cables for a 400-ampere feeder all the way up to 11 cables in parallel for a 4,000-ampere feeder.
Since cables with oversize equipment grounding conductors are catalog items, the cost should be reasonable, and lead time for shipping and receiving should be short.
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at (504) 254-2132.