Wiring Air Conditioning and More


Article 210 Branch Circuits

Article 250 Grounding

Article 300 Wiring Methods

Article 310 Conductors for General Wiring

Article 334 Nonmetallic Sheathed Cable: Types NM, NMC and NMS

Article 422 Appliances

Article 518 Places of Assembly

Article 700 Emergency Systems

Wiring air conditioning equipment

Q:Does the National Electrical Code permit the installation of Type NM-B (Nonmetallic Sheathed Cable) in flexible metal conduit to supply an outdoor remote condensing unit exposed to the weather? The wiring extends from a weatherproof disconnect switch to the air conditioning unit. If flexible metal conduit is not acceptable, what about liquidtight flexible metal conduit or electrical metallic tubing?

A:Nonmetallic sheathed cable cannot be used in any of these wiring methods because it is permitted in dry locations only. Uses not permitted for the installation of Type NM-B cable are covered by 334.12. And item (10)d prohibits the use of this cable by these words: “Where exposed or subject to excessive moisture or dampness.”

Although liquidtight flexible metal conduit and electrical metallic tubing can be installed to keep rain, ice and snow out of these raceways, the definition for “wet location” in Article 100 indicates that unprotected locations exposed to the weather are wet. Here is the definition: “Location, Wet. Installations underground or in concrete slabs or masonry in direct contact with the earth; in locations subject to saturation with water or other liquids, such as vehicle washing areas; and in unprotected locations exposed to weather.”

Types of loads permitted on branch circuit for central heating

Q:The NEC requires an individual branch circuit for central heating equipment. Does the Exception to 422.12 allow an attic light and receptacle to be connected to this circuit since they are provided to permit servicing of the furnace?

A:Although a receptacle is required within 25 feet and on the same level as the heating equipment, and a switch controlled lighting outlet is required near the furnace by 210.63, and 210.70(c), I am not sure that these items are covered by the exception to 422.12. The exception reads like this: “Auxiliary equipment, such as a pump, valve, humidifier or electrostatic air cleaner directly associated with the heating equipment shall be permitted to be connected to the same branch circuit.” Because the exception gives examples of auxiliary equipment that are permitted on the same branch circuit, I would not allow a work light and receptacle on the heater circuit; however, the authority having jurisdiction should be consulted for his/her interpretation.

If the AHJ allows the receptacle and luminaire (lighting fixture) on the branch circuit with the heater, the circuit cannot exceed 20A, 125V and the heater load cannot exceed 10A to comply with 210.23(A)(2).

Fixed wiring in assembly occupancies

Q:Am I allowed to use rigid nonmetallic conduit (Schedule 40) and electrical nonmetallic tubing in assembly occupancies?

A:Not always, it depends on the size of the structure, or the number of people that the structure can accommodate, the type of construction (fire rated or not fire rated) and the classification of the occupancy. If the building or structure is not designed or intended for occupancy of 100 or more persons, the wiring methods mentioned may be installed in accordance with their appropriate Articles, either 352 or 362. If the building or structure can accommodate 100 or more persons, and is required to be of fire rated construction by the Building Code, Article 518—Places of Assembly applies. In all places of assembly that are covered by Article 518, nonmetallic raceways encased in at least 2 inches of concrete are permitted by 518.4(A).

Additionally, rigid nonmetallic conduit and electrical nonmetallic tubing may be installed concealed in walls, floors, and ceilings that provide a thermal barrier that has at least a 15-minute finish rating in the following occupancies: club rooms, college and university classrooms, conference and meeting rooms in hotels and motels, courtrooms, drinking establishments, dining facilities, restaurants, mortuary chapels, museums, passenger stations and terminals of air, surface, underground, and marine public transportation facilities, libraries and places of religious worship. All of these occupancy classifications and detailed information on the installation of rigid nonmetallic conduit and electrical nonmetallic tubing can be found in 518.4(C).

Finally, if the building or structure can accommodate 100 or more persons and is not required to be fire rated construction, Article 518 does not apply, and any appropriate wiring method in Chapter 3 may be installed.

Wiring a storage tank water heater

Q: I have to wire a 50-gallon storage tank water heater that has 4,500W interlocked heating elements and a voltage rating of 240 single phase. Do I use 10 AWG copper branch circuit conductors or can I use 12 AWG copper conductors with an ampacity of 25 as shown in Table 310.16? Is a 25A circuit breaker the right size for this load?

A:The full-load current for this water heater is 18.75A (4,500 divided by 240). A storage-type water heater with this size tank must be supplied by a branch circuit with a rating of not less than 125 percent of the water heater’s full-load current. Therefore, the minimum rating of the overcurrent device is 25A. If there is no maximum size overcurrent device marked on the water heater nameplate, a 30A 2-pole circuit breaker may be used. This larger size circuit breaker is permitted by 422.11(E)(3).

Notice that the ampacities for 14, 12, and 10 AWG copper conductors shown in Table 310.16 are followed by an asterisk which references 240.4(D) at the bottom of the table, and this part (D) limits overcurrent protection to 15A for 14 AWG, 20A for 12 AWG, and 30A for 10 AWG copper conductors. The branch circuit conductors cannot be smaller than 10 AWG copper.

Loads supplied from an emergency generator

Q:In a new grocery store or supermarket, an on-site generator and automatic transfer switch are provided to supply required exit signs and emergency lighting. The store manager would like to add cash register receptacles and branch circuits for the check out stands. Is this permitted by the National Electrical Code? If not, how can this be accomplished without purchasing another generator?

A:Non-emergency loads cannot be connected to the emergency panelboard. Fine Print Note 3 in 700.1 gives some examples of types of electrical loads that may be connected to the emergency system, and check out counters in supermarkets are not mentioned. This is the type of load that is covered by Article 702—Optional Standby Systems.

Automatic transfer switches are covered by 700.6 for emergency systems. They must be electrically operated and mechanically held, and the transfer switch must supply only emergency loads.

Assuming the on-site generator has sufficient capacity to supply the emergency system and some or all of the cash registers simultaneously, an automatic or manual transfer switch can be installed for the non-emergency load. All wiring for the emergency loads must be independent and separate from all other wiring in the building. The cash register wiring cannot be mixed with the emergency wiring, but can occupy the same raceways that contain wiring for any other electric equipment.

Nonmetallic sheathed cable installed in metal studs

Q:Where the holes in metal studs are less than 1 1/4 inches from the edge of the studs, are steel plates required to protect nonmetallic sheathed cable from physical damage?

A:The flange on the metal studs should provide protection for the nonmetallic sheathed cable.

The requirements in 334.17 state that protection for the cable must satisfy 300.4, and grommets must be installed in the holes. They must remain in place while the cable is being pulled in and must be listed for the purpose of cable protection.

A steel plate that is at least 1/16-inch thick must be provided where nonmetallic sheathed cable is installed in bored holes in wood framing members where the nearest edge of the hole is less than 1 1/4 inches from the edge of the wood framing member. This requirement also applies where NM cable is installed in notches in wood framing members, but these requirements do not apply to nonmetallic sheathed cable installed in metal framing members.

Part (B) of 300.4 provides requirements for protection of NM cable where the cable passes through holes in metal studs. Subpart (1) requires grommets in all holes that nonmetallic sheathed cable passes through. And subpart (2) requires a steel plate at least 1/16-inch thick at all points where nails or screws are likely to penetrate the cable. It should be noted that there are no dimensions given for the requirement for a steel plate where metal framing members are involved.

Grounding multiple services

Q:A small commercial building has four occupants. Each tenant is supplied by a separate service connected to a single service drop. One service is 300A, 120/240V. Service entrance conductors are 300 kcmil Type THWN copper in 2-inch rigid metal conduit. The remaining 3 services are 200A, 120/240V with three 3/0 Type THWN copper conductors in three 1 1/2 inch conduits. These four disconnects (one 400 and three 200) are grouped to comply with 230.71 and 230.72.

I ran a 2/0 AWG copper grounding electrode conductor from the 400A switch to the metal water pipe, then made taps to this conductor with 4 AWG copper grounding electrode conductors from each of the 200A services. Properly sized split-bolt connectors were used for these taps. The inspector said that I should have used irreversible compression-type connectors or exothermic welds. I could not find a requirement for irreversible compression-type connectors or exothermic welding in 250.64. Am I missing something?

A:The main grounding electrode conductor (in this case 2/0 AWG conductor) must be continuous without splice or joint. If this conductor is spliced, irreversible compression-type connectors or exothermic welding must be used at the splice. There is no requirement in 250.64 that specifies the method for joining the tap conductors to the grounding electrode conductor. Properly sized and applied split-bolt connectors satisfy the rules for grounding electrode conductor taps in 250.64(D).

Another method for grounding this service is to run individual grounding electrode conductors from each service to the water pipe. This would involve a 2 AWG copper conductor from the 400A switch and three 4 AWG grounding electrode conductors, one from each of the 200A switches. 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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