Protection for Type MC cable


Q:
Why does the National Electrical Code (NEC) require physical protection for Type MC cable where installed less than 1.25 inches from the surface for runs that are parallel to metal framing members, but this clearance is not required where the cable is pulled through prepunched holes in metal framing members?

A:The cable must be protected from physical damage where installed less than 1.25-in. from the nearest edge of the framing member. Although 300.4(A), (C) and (D) do not mention the need for physical protection of Type MC cable that is installed through framing members, 330.17, “Through or Parallel to Framing Members,” reads: “Type MC cable shall be protected in accordance with 300.4(A), (C) and (D) where installed through or parallel to framing members.”

The problem is there are no rules in 300.4 that apply to wiring methods that are installed in holes or slots in metal framing members. However, it is reasonable to assume that the 1.25-inch clearance that applies to Type MC that runs parallel to metal framing members also applies to cable that is installed through framing members.

Wiring receptacles in patient areas

Q:Does the Code permit surface metal raceway as the wiring method for patient care areas of a hospital where a green insulated conductor is installed in the surface metal raceway and the raceway is used as the other equipment-grounding conductor? These branch circuits supply “hospital grade” 20-ampere, 125-volt ¬receptacles.

A:Although there are no specific rules for grounding surface metal raceways in Article 386—Surface Metal Raceways, 300.10, requires electrical continuity of metal raceways, requires that raceways be mechanically joined together to form a continuous electrical conductor, and states they must be connected to all boxes, fittings and cabinets to ensure electrical continuity.

In the General Information for Electrical Equipment Directory (White Book) 2005 edition, published by Underwriters Laboratories Inc., under the title “Raceways (RGKT),” a list of raceway types appears including surface metal raceways and fittings. There are six different kinds of raceways included under this title.

At the end of the list, this paragraph appears: “Metallic raceways and associated fittings installed in accordance with the product markings and manufacturer’s instructions are suitable for use as equipment grounding conductors.”

Properly installed surface metal raceway containing an insulated equipment-grounding conductor may be used to supply receptacles in patient care areas of a hospital as outlined in 517.13 (A) and (B).

P.A. system branch circuits

Q:What types of wiring methods are permitted for 125-volt branch circuits that supply a public address system in an auditorium that is required to be of fire-rated construction and accommodates more than 100 people? Is type NM-B cable permitted for these units?

A:Branch circuits connected to the premise electrical system must be installed to comply with any of the wiring methods permitted in Article 518—Assembly Occupancies. Although this equipment is covered in Article 640— Audio Signal Processing, Amplification and Reproduction Equipment, 640.3(E) reads: “Assembly Occupancies. Equipment used in assembly occupancies shall comply with Article 518.”

Wiring methods that are acceptable for the power wiring for the public address system in the auditorium are metal raceways; flexible metal raceways; nonmetallic raceways encased in not less than 2 inches of concrete; and Types MI, MC or AC cable containing an insulated equipment-grounding conductor sized in accordance with Table 250.122.

Class 2 and Class 3 wiring associated with the public address system may be wired to comply with Article 725—Class 1, and Class 2 and Class 3 Remote Control, Signaling and Power-Limited Circuits.

Legally required standby systems

Q:What are the major differences in the requirements for an emergency system and legally required standby system?

A:The Scopes of the Articles (700—Emergency Systems and 701—Legally Required Standby Systems) should be reviewed to separate one system from the other. An emergency system is intended to automatically supply illumination and power to designated areas and equipment when the normal source of power fails.

It supplies power and illumination essential for safety to human life. Legally required standby systems are intended to supply, distribute and control electricity to required facilities such as ventilation, smoke removal systems, sewage disposal and communications. For the emergency system, a portable or temporary alternate source must be available whenever the emergency generator is out of service for major maintenance or repair. The automatic transfer switch must be identified for emergency use.

Ground-fault indication is required for emergency systems operating at more than 150 volts to ground and overcurrent devices are rated at 1,000 amperes or more. Wiring for emergency circuits must be kept entirely independent of all other wiring unless required to be associated with normal source wiring.

All boxes and enclosures containing emergency wiring must be permanently marked. Feeder circuits for emergency systems must be protected by an automatic fire suppression system, a one-hour fire rating or embedded in 2 inches of concrete or a cable listed to maintain circuit integrity for not less than one hour in assembly, educational, residential, correctional, business and mercantile occupancies that can accommodate 1,000 or more people in buildings that are above 75 feet in height.

The emergency power source must have protection as described for feeders in occupancies described above. These requirements are unique to the emergency system. A legally required standby system must energize the load in 60 seconds compared to 10 seconds for the emergency system. All legally required standby wiring may occupy the same raceways and boxes with the normal wiring. A connection ahead of the service disconnecting means is permitted for the legally required standby system where acceptable to the authority having jurisdiction. These are some of the important differences between the two systems.

Delta circuit breaker

Q:What is a “delta” breaker that is mentioned in 408.36(E)?

A:A delta breaker is a specially designed three-pole circuit breaker that provides three-phase power to three-phase loads from a single-phase, three-wire distribution panel or load center. Two poles of the circuit breaker connect to the buses in the three-wire single-phase panelboard and the “stringer” or high leg of the three-phase, four-wire delta power supply at the service is connected to a lug or terminal on the delta breaker. This arrangement supplies three-phase equipment that usually was air-conditioning equipment in a residential occupancy.

The problem is back feed to the single-phase bus in the panelboard when the two-pole main circuit breaker is open and the three-phase load is energized with the delta breaker in the closed position.

For this reason, 408.36 prohibits their use on new construction. This is the way part (E) reads: “Delta Breakers. A three-phase disconnect or overcurrent device shall not be connected to the bus of any panelboard that has less than three-phase buses. Delta breakers shall not be installed in panelboards.”

To provide replacement delta breakers in existing panelboards, UL lists delta breakers “for replacement only.” This information appears under the title: “Circuit Breakers, Molded Case and Circuit Breaker Enclosures” (DIVQ) in the 2005 Edition of the General Information for Electrical Equipment Directory (White Book): “Three-pole circuit breakers having provision for two poles to be connected to a bus structure and a third isolated pole (commonly referred to as delta breakers) are marked ‘for replacement use only.’”

Installing ground rods

Q:Does the NEC allow a ground rod to be placed in a 3-inch diameter hole that is about 9 feet deep then backfilled with dirt that was removed from the hole?

A:The NEC in 250.53(G) requires that ground rods be driven unless rock prevents driving the rod in the earth. Part (G) says: “Rod and Pipe Electrodes. The electrode shall be installed such that at least 2.44 m (8 ft.) of length is in contact with the soil. It shall be driven to a depth of not less than 2.44 m (8 ft.) except that where rock bottom is encountered, the electrode shall be driven at an oblique angle not to exceed 45 degrees from the vertical or, where rock bottom is encountered at an angle up to 45 degrees, the electrode shall be buried in a trench that is at least 750 mm (30 inches) deep.”

Notice that the rule uses the word “driven” in two places. The only time it can be placed in the earth and backfilled is where rock is encountered close to the earth surface.

GFCI protection for clothes washer

Q:A clothes washer and gas-fired clothes dryer are located in the bathroom of an efficiency apartment. Do the 15-ampere, 125-volt receptacles that supply these appliances have to be protected by ground-fault circuit interrupters (GFCIs)?

A:Yes, they do. All 15- and 20-ampere, 125-volt receptacles installed in bathrooms must have GFCI protection for personnel.

Although these receptacles may not be readily accessible because they are behind appliances, there is no exception that removes the requirement.

Unlike garages and accessory buildings, which have exceptions that remove the requirement where receptacles are not readily accessible or where receptacles supply appliances in dedicated space, there is no similar relief for bathroom receptacles.

If there is a laundry tub in the bathroom, 210.8(A)(7) requires a ground-fault circuit interrupter protected receptacle within 6 feet of the outside edge of the tub. EC

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