Metal Roofing, Oil-Change Buildings and More

Article 210 Branch Circuits; Article 250 Grounding and Bonding; Article 300 Wiring Methods; Article 314 Outlet, Device, Pull, and Junction Boxes, Conduit Bodies; Fittings; and Handhole Enclosures; Article 330 Metal-Clad Cable: Type MC; Article 334 Nonmetallic Sheathed Cable: Types NM, NMC, and NMS; Article 350 Liquidtight Flexible Metal Conduit: Type LFMC; Article 511 Commercial Garages, Repair and Storage; Article 700 Emergency Systems; Article 760 Fire Alarm Systems; The Guide Information for Electrical Equipment (White Book), published by Underwriters Laboratories Inc., also is mentioned.

Metal roof cable spacing

We have a job to install the wiring in a commercial building with a metal roof. How much space from the underside of the roof is required to protect the metal clad cable from nail penetration?

A new part (E) in Section 300.4 in the National Electrical Code (NEC) requires a minimum of 1.5 inches clear space between the metal corrugated sheet roof decking and the outer surface of the metal-clad cable.

Fluorescent luminaires and LFMC

We have to install some wet location luminaires in a car wash. What are the requirements for grounding these luminaires where the wiring method is liquidtight flexible metal conduit (LFMC)?

The answer varies depending on the load, conduit size and length. Where flexibility is not required after installation, the cable grounding must comply with Section 250.118(6). Grounding and bonding of liquidtight metal conduit is required by 350.60, and the bonding and grounding requirements are located in Section 250.118(6). For the raceway to serve as an equipment ground: (1) the conduit must be terminated in listed fittings, (2) where the trade size does not exceed ¾ inch, the contained conductors must be protected by overcurrent devices rated at 20 amperes or less, and (3) the length of LFMC in the same ground return path must not exceed 6 feet. If the conduit installation does not meet these requirements, an equipment-grounding conductor is required.

Bedroom fire alarm system branch-circuit connection

I installed a burglar and fire alarm system with a plug-in transformer in the bedroom of a single-family residence. Does the receptacle have to be protected by an arc-fault circuit interrupter to comply with Section 210.12(B) of the NEC?

Arc-fault circuit interrupter protection is not permitted for the fire alarm system by Section 760.41(B) for nonpower-limited fire alarm circuits and Section 760.121(B) for power-limited fire alarm circuits. The power supply must be installed in rigid metal conduit, intermediate metal conduit, electrical metallic tubing or steel armored cable, meeting the requirements of 250.118 with metal outlet and junction boxes.

An individual branch circuit must be provided, and a single receptacle must be used to terminate the branch circuit. This is required for two reasons. First, the two sections in Article 760 of the NEC require an individual branch circuit for a fire alarm. Second, the objective is to prevent a spare receptacle that allows the occupant to plug in floor or table lamps or other electrical appliances that would not be protected by an arc-fault circuit interrupter.

Article 100 in the NEC includes a definition for an individual branch circuit that reads as follows: “Branch Circuit, Individual. A branch circuit that supplies only one utilization equipment.”

Quick oil-change buildings

What is the hazardous area classification for a quick oil-change building? Is any area in the building classified as a Division 1 or 2 location?

The requirements in Article 511, Commercial Garages, Repair and Storage, were rewritten in the 2008 edition of the NEC. This material was extracted from NFPA 30A-2008, Code for Motor Fuel Dispensing Facilities and Repair Garages.

Garages are classified as major or minor repair facilities with definitions for each in Section 511.2. The oil change building would be classified as a minor repair garage if services are limited to lubrication, inspection and minor automotive repair work, such as engine tune ups; parts replacement; fluid changes that include oil, antifreeze, transmission fluid, brake fluid and air conditioning refrigerants; brake system repairs; tire rotation; and similar routine maintenance work. Also included are associated floor space used for offices, parking or showrooms.

Any pit, below-grade work area or subfloor work area is a Class 1 Division 2 location that extends up to the floor level, where ventilation is not provided. For the area to be unclassified, ventilation of at least 1 cubic foot per minute of the below-grade floor area must be provided during the time the building is occupied or when vehicles are parked in or over this area. The air intake for the below-grade work area must be within 12 inches of the subfloor work area.

Unclassified areas include those adjacent to classified areas, such as stock rooms, switchboard rooms and other similar locations in which flammable liquids or vapors are not likely to be released.

Grounding-electrode conductor size

What is the minimum size grounding-¬electrode conductor permitted for a 3-wire, 120/240-volt, single-phase service that is 150 amperes and consists of three 1/0 AWG copper conductors with Type THW insulation where the grounding electrode is concrete encased with two ground wires?

According to Table 250.66, the minimum size grounding-electrode conductor is 6 AWG copper. Although not required by the NEC, the minimum size grounding-electrode conductor to the two ground rods also is a 6 AWG copper. This answer assumes that the concrete-encased electrode consists of at least 20 feet of 0.5-inch or larger reinforcing rods encased in not less than 2 inches of concrete.

A 6 AWG copper grounding-electrode conductor that is not subject to physical damage is permitted to run along the surface of the building without installation in a raceway where it is securely fastened to the construction; otherwise, it must be protected by installation in a metal raceway, rigid nonmetallic raceway or cable armor (See 250.64(B)).

GFCIs for air conditioning

Is a ground-fault circuit interrupter-protected receptacle required to be installed behind a fenced and locked area for servicing air conditioning equipment in a commercial building?

Yes, it is required that a GFCI-protected receptacle be installed at this location. This answer assumes that the receptacle is located outdoors. The requirement for a GFCI receptacle is in Section 210.8(B)(4) and the exception does not apply.

A 125-volt, single phase, 15- or 20-ampere receptacle must be provided within 25 feet for servicing the air conditioning equipment by Section 210.62. This receptacle must be accessible and cannot be connected to the air conditioning disconnecting means.

Emergency transfer switch

I installed a new transfer switch for an emergency system in a three-story commercial building. The electrical inspector has rejected this installation because the transfer switch is not marked for “emergency systems.” Is there a special marking for transfer switches that are marked “emergency systems?”

Yes, there are differences in transfer switches and those that are acceptable for use in emergency systems are marked “Automatic Transfer Switch for Emergency Systems.” This information is obtained from the 2008 edition of the directory for Guide Information for Electrical Equipment (White Book) published by Underwriters Laboratories Inc.

There are a number of different types of transfer switches, and some are not suitable for installation on emergency systems. For this reason, a sentence was added to Section 700.6(C) in the 2008 NEC that reads: “Automatic transfer switches rated 600V AC and below, shall be listed for emergency system use.”

Outdoor Type MC cable

Is Type MC cable permitted to be used between a disconnect switch and an air conditioning unit installed outdoors and exposed to the weather?

Sections 330.12(3) and (11) and 225.10 permit Type MC cable for this installation. Where installed outdoors in a wet location, the cable must have a metallic covering that is moisture-proof or a lead sheath or moisture-proof jacket that is provided under the metal covering.

According to the definition in Article 100, unprotected locations exposed to the weather are wet locations; therefore, conductor insulation suitable for wet locations must be in the cable assembly.

Nonmetallic sheathed cable

Is nonmetallic sheathed cable an acceptable wiring method for a three-unit apartment building that is being constructed as a Type II building even though the building code permits Type III construction?

Where Type III construction is permitted but not used, Section 334.10(2) allows the use of nonmetallic sheathed cable as the wiring method. Under item (2) nonmetallic sheathed cable is allowed in multifamily dwellings permitted to be of Types III, IV and V construction, and the question indicates that Type III construction is permitted but the owner has decided to use Type II construction on the building.

Nonmetallic boxes with metal raceways

Does the NEC permit nonmetallic outlet boxes in metal raceway systems?

Two exceptions in Section 314.3 permit nonmetallic boxes. The first exception permits nonmetallic boxes where internal bonding is provided between all entries to the box. The second exception permits nonmetallic boxes where integral bonding means is provided for attaching an equipment-bonding jumper inside the box between all threaded entries in the box. Boxes must be listed for the purpose.

Editor’s Note: We are saddened to inform our readers that George Flach passed away on Aug. 2, 2009. He was ELECTRICAL CONTRACTOR’s longest running columnist. He will be dearly missed.

FLACH, was a regular contributing Code editor, is a former chief electrical inspector for New Orleans. Questions can be sent to

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...

Stay Informed Join our Newsletter

Having trouble finding time to sit down with the latest issue of
ELECTRICAL CONTRACTOR? Don't worry, we'll come to you.