Grounding-Electrode Conductor Sizes, Disconnects and More

Article 210—Branch Circuits; Article 250—Grounding and Bonding; Article 320—Armored Cable: Type AC; Article 334—Nonmetallic-Sheathed Cable: Types NM, NMC and NMS; Article 430—Motors, Motor Circuits and Controllers; Article 440—Air-Conditioning and Refrigeration Equipment; Article 702—Optional Standby Systems

The 2006 edition of the Guide Information for Electrical Equipment (White Book) published by Underwriters Laboratories Inc. is also mentioned.

Grounding-electrode conductor size
Q: Where the sizes of service-entrance conductors are increased to reduce voltage drop, does the
National Electrical Code (NEC) require an increase in the size of the grounding-electrode conductor (for example, a service that consists of 250 Kcmil copper conductors protected by a 150-ampere circuit breaker)?

A: The answer depends on the type of grounding electrode that is used. If there are no grounding electrodes available and two ground rods are used because the resistance of one exceeds 25 ohms, the grounding-electrode conductor does not have to be larger than 6 AWG copper wire

A 4 AWG copper conductor meets the requirement for the grounding-electrode conductor where 20 feet or more of ½-inch or larger reinforcing steel embedded in concrete that is in contact with the earth is the grounding electrode. A 2 AWG copper conductor satisfies the requirement for a ground ring where the ground ring consists of 20 feet or more of 2 AWG copper conductor buried in the earth.

Where there is 10 feet or more of buried metal water pipe, the grounding-electrode conductor cannot be smaller than 2 AWG copper. Table 250.66 is used to size the grounding-electrode conductor where a metal water pipe is the grounding electrode. In addition, the metal water pipe must be supplemented by one or more of the following: the metal frame of a building, a concrete-encased electrode, ground ring, rod, pipe or plate electrodes.

Table 250.66 must be used to size the grounding-electrode conductor that is run to the metal water pipe electrode. The title for the table is “Grounding Electrode Conductor for Alternating-Current Systems,” and the title for the left-hand column is “Size of Largest Ungrounded Service-Entrance Conductor or Equivalent Area for Parallel Conductors (AWG/Kcmil).” Notice the table is based on the size of the service-entrance conductors, not on the overcurrent protection at the termination of the service-entrance conductors.

For the example, the minimum size grounding-electrode conductor is 2 AWG copper or 1/0 aluminum where buried metal water pipe is the grounding electrode. Although Table 250.122 permits a 6 AWG copper or 4 aluminum conductor for equipment grounding for a 150-ampere feeder, this table cannot be used to size the grounding-electrode conductor for a service.

Optional standby generator switch
Q: Is it acceptable to use a multipole, double-throw switch to transfer loads from the normal power source to an optional standby generator?

A: Article 702—Optional Standby Systems permits manual transfer of optional loads from the normal power source to an on-site power source, and 702.6 has requirements for transfer switches. Part of 702.6 reads, “Transfer equipment shall be suitable for the intended use and designed and installed so as to prevent the inadvertent interconnection of normal and alternate sources of supply in any operation of the transfer equipment.” Notice that the transfer switch does not have to be automatically operated; manual transfer is permitted.

A multipole, double-throw enclosed switch may be used as a transfer switch if it is marked for this application. This information is taken from the 2006 edition of “Guide Information for Electrical Equipment” (White Book) published by Underwriters Laboratories Inc. under the title “Switches, Enclosed (WIAX):” Double-throw switches that have been investigated for switching a common load from a normal supply to an optional standby system are marked “Suitable for Use in Accordance with Article 702 of the National Electrical Code.”

Hermatic compressor disconnect
Q: May a fused pullout switch be used as a disconnect for an air conditioning compressor motor?

A: Pullout switches with adequate horsepower ratings may be used as the disconnecting means required by 440.12. To obtain the equivalent horsepower, the nameplate rated-load current or branch-circuit selection current, whichever is greater, and the locked-rotor current are compared with the appropriate Tables in 430.248, 430.249, 430.250, 430.251(A) or 430.251(B). The equivalent horsepower obtained from these tables is used to determine the minimum horsepower rating required for the pullout switch.

Fuses in the pullout switch are permitted to be sized for 175 percent of compressor rated-load current or branch-circuit selection current, whichever is higher. If this value of overcurrent protection does not allow the compressor to start, time-delay fuses or fuses with a rating of 225 percent of rated-load current or branch-circuit selection current are permitted by 440.22(A).

Receptacle location in bathroom
Q: Does a receptacle on the right side and in back of a person standing in front of a bathroom wash basin satisfy the requirement of 210.52(D) if the receptacle is within 3 feet of the edge of the basin? Mirrors are on three sides of the basin.

A: The only location requirement in 210.52(D) is that a receptacle be installed adjacent and within 3 feet of the outside edge of the basin. An exception allows the receptacle to be located on the side or face of the basin cabinet provided it is not more than 12 inches below the basin counter top.

Securing Type NM-B cables
Q: Type NM-B cables are run on ceiling joists in the attic of a two-family dwelling unit. The cables are supported at intervals less than 4.5 feet. Do they have to be secured in place? If they do, are bent-over nails an acceptable means of securing the cable?

A: Nonmetallic sheathed cable must be supported and secured to comply with 334.30. Part of the language in 334.30 recognizes securing the cable with staples, cable ties, straps, hangers or similar fittings designed and installed so as not to damage the cable. In my opinion, bent over nails do not qualify as a means of securing Type NM-B cable. The cable must be secured every 4.5 feet and within 12 inches of every outlet or junction box, cabinet or fitting.

Where the attic space is not accessible by permanent stairs or ladders, Type NM-B cable is permitted to run across the top of ceiling joists provided it is protected from physical damage within 6 feet of the nearest edge of a scuttle hole. The rules for this requirement are in 334.23 and 320.23.

Supporting luminaires from nonmetallic boxes
Q: Is it permissible to support ceiling-mounted luminaires from nonmetallic outlet boxes? If the answer is yes, are these boxes specially marked?

A: Outlet boxes are allowed to support ceiling-mounted luminaires by 314.27. This is the requirement in part (A) of 314.27: “Boxes used at luminaire (lighting fixture) or lampholder outlets shall be designed for the purpose. At every outlet used exclusively for lighting, the box shall be designed or installed so that a luminaire (lighting fixture) may be attached.” Part (B) limits the weight of a luminaire to 50 pounds on an outlet box that is installed in accordance with 314.23.

A nonmetallic box with or without a bracket or bar hanger that is suitable for support of a luminaire weighing 50 pounds or less is marked “For Fixture Support” on the carton containing the boxes. Additional information on the proper application of nonmetallic outlet boxes may be found under guide (QCMX), beginning on page 193 of the 2006 White Book.

Grounding conductor numbers
Q: We are using rigid, nonmetallic conduit as the wiring method for receptacle branch circuits in a drug store. Where multiple 20-ampere, 120-volt branch circuits are installed in the same raceway, is an equipment-grounding conductor required for each branch circuit? If a single equipment-grounding conductor is provided, what size is required?

A: An equipment-grounding conductor for each branch circuit is not required by 250.122(C). This single equipment-grounding conductor must be sized for the largest overcurrent device protecting the branch circuit conductors in the raceway.

Branch-circuit receptacle rating

Q: The NEC allows 15-ampere, 125-volt receptacles to be connected to 20-ampere branch circuits. Are 20-ampere receptacles permitted on 15- or 30-ampere branch circuits?

A: Where two or more receptacles are supplied by a single branch circuit, 15-ampere receptacles are allowed on a 15- or 20-ampere branch circuit. Where a single receptacle is supplied by an individual 20-ampere branch circuit, the receptacle must have a rating of 20 amperes. However, there are two exceptions to this requirement [see 210.21(B)(1)].

Table 210.21(B)(3) lists receptacle ratings in amperes for various size branch circuits. This table applies to branch circuits supplying two or more receptacles and contains the following information: for a 15-ampere branch circuit receptacle, ratings cannot exceed 15-amperes; for a 20-ampere branch circuit receptacle, ratings can be 15 or 20 amperes; and for a 30-ampere branch circuit, the ampere rating of the receptacle must be 30 amperes. A summary of branch circuit requirements appears in Table 210.24. A note to this table states that receptacles with ampere ratings that are at least 125 percent of the full load current of electric-discharge, mogul-base luminaires that they supply are permitted to be connected to 50-ampere branch circuits. Generally, 20-ampere receptacles can be supplied only from 20-ampere branch circuits, and 30-ampere receptacles from 30-ampere branch circuits.                EC

, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720.

About the Author

George W. Flach

Code Q&A Columnist

George W. Flach was a regular contributing Code editor for Electrical Contractor magazine, serving for more than 40 years. His long-running column, Code Q&A, is one of the most widely read in the magazine's history. He is a former chief electrical...

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