Voltage Limits, Hall Area Lighting and More

Article 210 Branch Circuits; Article 310 Conductors for General Wiring; Article 404 Switches; Article 430 Motors, Motor Circuits, and Controllers; Article 695 Fire Pumps; Article 702 Optional Standby Systems; Chapter 9 Tables; The 2006 edition of the Guide Information for Electrical Equipment published by Underwriters Laboratories Inc. also is mentioned.

Voltage limits in the same box

Q: Does the National Electrical Code (NEC) permit the installation of a 277-volt switch to control lighting fixtures and a 125-volt, 15-ampere duplex receptacle to be installed side-by-side in a two-gang outlet box?

A: Yes, provided they can be installed to comply with 404.8(B), which reads: “Voltage Between Adjacent Devices. A snap switch shall not be grouped in enclosures with other snap switches, receptacles, or similar devices, unless they are arranged so that the voltage between adjacent devices does not exceed 300 volts, or unless they are installed in enclosures equipped with identified, securely installed barriers between adjacent devices.” Listed boxes with barriers that slide in place are manufactured for this purpose.

Apartment common hall lighting

Q: Is it acceptable to supply a common hall for four apartments from each apartment—four lighting fixtures, each supplied from one apartment—or must a “house” meter be installed to supply these common area lighting fixtures?

A: Wiring from individual apartments cannot supply loads outside of the apartment. The requirement for supplying common area loads is found in 210.25. This is the rule: “Common Area Branch Circuits. Branch circuits in dwelling units shall supply only loads within that dwelling unit or loads associated only with the dwelling unit. Branch circuits required for the purpose of lighting, central alarm, signal, communications, or other needs for public or common areas of a two-family or multifamily dwelling shall not be supplied from equipment that supplies an individual dwelling unit.” This requirement usually results in the installation of a “house” meter for these and other common area loads.

RMC sleeves for nonmetallic cable

Q: Does the National Electrical Code have any requirements for the size of rigid metal conduit used as a sleeve for nonmetallic-sheathed cable? If the answer is yes, what size conduit is required for a 12/2 NM-B cable with oblong dimensions that measure ¼ inch by 7/16 inch? The conduit sleeve is about 10 feet long and has a 90-degree bend at each end.

A: Yes, there is a minimum size for this rigid metal conduit. Note 9 of Chapter 9 Tables provides this information: “A multiconductor cable of two or more conductors shall be treated as a single conductor for calculating percentage conduit fill area. For cables that have elliptical cross sections, the cross-sectional area calculation shall be based on using the major diameter of the ellipse as a circle diameter.”

To be able to use the tables, the fractional dimension of the cable must be converted to a decimal equivalent. Therefore, 7/16 inch becomes 0.4375 inch, and in Table 4—Article 344—Rigid Metal Conduit (RMC) under the 1 Wire column, 53 percent fill is allowed.

This results in a ¾-inch rigid metal conduit sleeve for the 12/2 nonmetallic-sheathed cable.

Bathroom receptacle branch circuit

Q: May the 20-ampere receptacle branch circuit required by 210.52(D) also supply the heater-vent-light in the bathroom of a dwelling unit?

A: Part (D) of 210.52 requires a receptacle within 3 feet of the outside edge of each basin in dwelling units. And, 210.11(C)(2) requires a 20-ampere receptacle branch circuit to supply bathroom receptacles in dwelling units. This branch circuit cannot supply any other outlets unless it supplies a single bathroom.

If the 20-ampere branch circuit supplies a single bathroom, other loads in the bathroom may be connected to it, but the heater-vent-light appliance nameplate full-load current cannot exceed 10 amperes to conform with 210.23(A)(2). This part of 210.23 limits the load on fixed appliances to 50 percent of the branch circuit rating where the branch circuit also supplies cord-and-plug-connected appliances such as hair dryers, shavers and electric toothbrushes.

Conductor derating

Q: A single metal raceway contains four three-wire, 120/240-volt, single-phase branch circuits. The conductors are 12 AWG Type THHN copper and supply general-purpose receptacles in an office building. What is the adjusted ampacity of these conductors? Is this insulation suitable for a damp location?

A: Since there are more than three current-carrying conductors in a raceway, derating to comply with 310.15 must be accomplished. Although there are 12 conductors in the raceway, 310.15(B)(4) allows omitting the neutral conductors from the calculations because they carry only the unbalanced current. This results in eight current-carrying conductors when applying the derating factor in Table 310.15(B)(2)(a), which is 70 percent.

According to Table 310.16, the ampacity of 12 AWG copper with Type THHN insulation is 30. Therefore, the corrected ampacity is (0.7 x 30) 21. Since the 60°C ampacity cannot be exceeded, the corrected ampacity is 20.

Type THHN insulation is suitable for dry and damp locations as noted in 310.8(B) and Table 310.13.

Fire pump generator disconnect

Q: Where a fire pump is supplied from a utility service and an on-site emergency generator, is an outdoor-housed generator allowed to have a readily accessible disconnecting means at the generator?

A: Where a generator is used to comply with 695.3, it must have capacity to allow normal starting and running of the fire pump motor while supplying all other simultaneously connected loads. A tap ahead of the on-site generator disconnecting means is not required by 695.3(B)(1); therefore, a disconnect at the generator is acceptable.

Switching optional standby loads

Q: Is an enclosed, double-throw, multipole switch acceptable for controlling a generator for an optional standby system?

A: Two requirements in Article 702—Optional Standby Systems must be satisfied. One is in 702.4, which covers equipment approval and reads, “All equipment shall be approved for the intended use.” The other requirement is in 702.6, which states “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 both of these rules use the words “intended use,” and the word “approved” appears in 702.4. A proper size multipole, double-throw enclosed switch should be acceptable to the authority having jurisdiction, but he or she may require a switch that is marked “Suitable For Use in Accordance With Article 702 of the National Electrical Code.” The 2006 edition of the Guide Information for Electrical Equipment (White Book) published by Underwriters Laboratories has this information 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.’”

Cord-and-plug motor disconnect

Q: Where an attachment plug and receptacle are used as the disconnecting means for a motor-operated appliance, what size and type of receptacle and attachment plug are required for a 0.5 horsepower, 120-volt, single-phase motor?

A: The answer to this question is in 430.109(F). This part requires a horsepower-rated attachment plug and receptacle with ratings not less than the motor rating. Horsepower ratings for NEMA configuration receptacles are listed in the Guide Information for Electrical Equipment Directory published by Underwriters Laboratories Inc. under the title, “Receptacles for Plugs and Attachment Plugs (RTRT).” The list shows a 125-volt, 15-ampere, three-wire, two-pole receptacle with a 0.5 horsepower rating. A similar list appears under the title, “Attachment Plugs Fuseless (AXUT),” which shows the horsepower ratings for NEMA configurations of attachment plugs.

Service conductors defined

Q: What is the difference between service conductors and service-entrance conductors, and who has jurisdiction over each?

A: Definitions for these phrases appear in Article 100. Service conductors are defined as the “Conductors from the service point to the service disconnecting means.” There are two definitions for service-entrance conductors: one is service-entrance conductors, overhead system; the other is for underground systems. An overhead system is defined as “the service conductors between the terminals of the service equipment and a point usually outside the building, clear of building walls, where joined by tap or splice to the service drop.” An underground installation is defined as “the service conductors between the terminals of the service equipment and the point of connection to the service lateral.”

Jurisdiction for the conductors begins where the serving utility terminates their conductors. This usually is described as the service point, which is defined as “the point of connection between the facilities of the serving utility and the premises wiring.” The authority having jurisdiction is responsible for inspection of the service conductors. EC

FLACH, 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 in...

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