Room AC Branch Circuits, Outlet Box Knockouts and More

Article 210 Branch Circuits; Article 215 Feeders; Article 240 Overcurrent Protection; Article 250 Grounding and Bonding; Article 310 Conductors for General Wiring; Article 314 Outlet, Device, Pull, and Junction Boxes; Conduit Bodies; Fittings; and Handhole Enclosures; Article 406 Receptacles, Cord Connectors, and Attachment Plugs (Caps); Article 410 Luminaires, Lampholders, and Lamps; Article 440 Air-Conditioning and Refrigeration Equipment; Portions of the Guide Information for Electrical Equipment (White Book) published by Underwriters Laboratories Inc. also are mentioned.

Replacing receptacles with GFCIs

I’m currently working on a 55-year-old house with an original 50-amp push-o-matic service panel. The house has two-prong receptacles, which are not grounded. My understanding is that short of rewiring the house and installing a 100-amp service upgrade, I can replace the two-prong receptacles with a ground-fault circuit interrupter (GFCI) receptacle and the homeowner would be protected. If this is true, would the one GFCI receptacle, protect all receptacles down line on the same circuit, allowing the two-prong receptacles to be replaced with three-prong duplex receptacles?

Yes, you can replace two-prong receptacles with GFCI-protected receptacles. If the GFCI receptacle is installed at the first outlet on the branch circuit, all downstream receptacles will be protected by the GFCI receptacle installed at the beginning of the branch circuit. Nongrounding-type receptacles are permitted as replacements in accordance with Section 406.3(D). Also grounding-type receptacles are permitted as replacements without an equipment ground where they are marked “GFCI Protected” and “No Equipment Ground.”

Room AC branch circuit

Does the National Electrical Code (NEC) permit a cord-connected room air conditioner to be connected to a 120-volt receptacle branch circuit in a bedroom of a one-family dwelling unit?

Yes, it does. In Section 440.62, there are restrictions on the power supply for room air conditioners. 1) The unit must be cord-and-plug-connected. 2) The air conditioner is not more than 40 amperes and 250-volts. 3) The total rated load current is shown on the room air conditioner nameplate rather than individual motor currents. 4) The rating of the branch-circuit short-circuit ground-fault protective device does not exceed the ampacity of the branch-circuit conductors or the rating of the receptacle, whichever is less.

Where the branch-circuit supplies lighting outlets and general use receptacles, the cord-and-plug-connected room air conditioner full-load current cannot exceed 50 percent of the branch-circuit rating. For example, an air conditioner with a full-load current of 10 amperes or less may be installed on a 20-ampere branch circuit that supplies other loads. The receptacle that supplies the air conditioner should be rated for 20-amperes.

There also is permission in Section 440.62(C) to install a cord-and-plug air conditioner with a full-load current of 80 percent of the branch-circuit rating where the circuitry is interlocked to prevent simultaneous operation of the room air conditioner and energization of other outlets on the same branch circuit. This arrangement allows a room air conditioner with a full-load current of 16 amperes on a 20-ampere branch circuit.

Show window receptacle

Is it permissible to locate a show window lighting receptacle in the center of a show window but above the ceiling tile?

At least one receptacle outlet is required within 18 inches of the top of a show window, which is 6 feet or longer measured horizontally according to Section 210.62. However, Section 400.8(5) prohibits the use of flexible cords or cables “… where concealed by walls, floors, or ceilings or located above suspended dropped ceilings.” This section has other restrictions on the use of flexible cords and cables.

Ground size in Type MC cable

The electrical inspector says that a 1 AWG aluminum conductor for an equipment ground in 5-conductor 500 kcmil aluminum Type MC cable is too small for a run of 2 Type MC cables in parallel. This is for a 600-ampere feeder for an office building. Is he correct?

Yes, he is. The equipment-grounding conductor cannot be smaller than 2/0 AWG aluminum to comply with Section 250.122(F) and Table 250.122.

At least one manufacturer produces aluminum sheathed Type MC cable with aluminum conductors with oversize equipment-grounding conductors for parallel installations. These cables with oversize equipment-grounding conductors are catalog items and comply with the requirements for equipment-grounding conductors mentioned in 250.122(F).

GFCI protection marking

Does the NEC require marking of GFCI-protected receptacles connected downstream of a GFCI-protected branch circuit?

All 15- and 20-ampere receptacles installed in dwelling units at these locations must have GFCIs for the receptacles: bathrooms, garages, outdoors, crawl spaces, unfinished basements, kitchens, laundry, utility and wet bar sinks, and boathouses. These requirements apply to new construction and are in Section 210.8(A).

In existing dwelling units, replacement receptacles can be grounding- or nongrounding-type receptacles to comply with Section 406.3(D). Part (2) of this section requires GFCI-protected receptacles where other parts of the NEC require them. Where there is no equipment-grounding conductor in the receptacle box, a nongrounding-type receptacle is permitted.

Where a nongrounding receptacle is being replaced at a location that requires a GFCI-protected receptacle and an equipment-grounding conductor is not in the receptacle box, the GFCI receptacle must be marked “No Equipment Ground” to comply with Section 406.3(D)(3)(b).

Grounding-type receptacles may be used as replacements for nongrounding receptacles on GFCI-protected branch circuits where the receptacles are marked “GFCI Protected” and “No Equipment Ground.” See part (C) of Section 406.3(D)(3)(c).

Conductor overcurrent protection

Is it permissible to terminate 12 AWG nonmetallic cable Type NM-B on a 15-ampere overcurrent device? The homeowner supplies 12 AWG copper conductors and 15-ampere overcurrent devices for branch-circuit protection. The language in Sections 240.3 and 310.15 recommends protection at the assigned capacities.

Yes, this is permissible. In fact it is often required to comply with the NEC. For example, where there are seven current-carrying conductors in a single raceway, the ampacity of a 12 AWG copper conductor is reduced to 14 amperes (0.70 20), and 15-ampere overcurrent protection is required. Where compensating for voltage drop, larger size wires are selected and overcurrent protection is based on the load served. Fine Print Note No. 4 in Section 210.19(A)(1) recommends a maximum voltage drop of not more than 5 percent for feeders and branch circuits to provide reasonable efficiency of operation. A similar Fine Print Note No. 2 appears in Section 215.2(A)(3). Both FPNs recommend a total voltage drop on branch circuits and feeders of not more than 5 percent. This will require oversize conductors on some branch circuits.

Grounding parking lot lighting poles

Does the NEC require a ground rod in addition to an equipment-grounding conductor run with the branch-circuit conductors to each metal lighting pole in a parking lot? The ampere rating of the branch circuit is 50 amperes.

Each metal lighting pole must be grounded to comply with Section 250.96 and 250.110.

Grounding of the metal lighting poles is required by Part V of Article 410. The equipment-grounding conductor must be of the type specified in Section 250.118 and sized to comply with Section 250.122. According to Section 250.122, the minimum size equipment-grounding conductor cannot be smaller than 10 AWG copper or 8 AWG aluminum.

Where a separate equipment-grounding conductor and ground rod at each pole is specified, it must be provided in addition to the grounding required by Article 250. This additional grounding of each lighting pole is probably provided to limit damage should any lighting pole be struck by lightning.

The grounding electrode should be at least 8 feet long and 0.5 inch in diameter. The grounding conductor should be at least 10 AWG copper or 8 AWG aluminum. The ground rod should be driven to the surface or below the surface of the earth.

It is noted that a 50-ampere branch circuit is used for these luminaires. Where 50-ampere circuits are used, mogul base screw-shell lampholders are required by Section 410.62(C)(2).

Outlet box knockouts

Does the NEC allow more than one nonmetallic cable entry in a single knockout in a one-, two- or three-gang outlet box?

For a single-gang box, this information is furnished from the Guide Information for Electrical Equipment Directory published by Underwriters Laboratories Inc.: “A box nominally 2.25 by 4 inch or smaller is intended for one or more nonmetallic sheathed cables to enter through a single or multiple stage knockout opening.” For boxes marked “Nonmetallic Sheathed Cable Only,” clamps have been tested for securing only one cable per clamp, except multiple section clamps are considered suitable for securing one cable under each section of the clamp, each entering a separate knockout.

An exception to Section 314.17(C) permits multiple cable entries through a single knockout for boxes not larger than 2.25-by-4 inches. It is not necessary to secure the cable to the box where the cable is secured within 8 inches of the box and the cable sheath extends at least 0.25-inch into the box.

The last sentence in Section 314.17(C) requires that nonmetallic sheathed cable be secured to the box except for the single gang box covered by the exception.

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

Editor’s Note: This edition of Code Q&A is the last written by George Flach. He has been composing this column for ELECTRICAL CONTRACTOR since the 1960s. He passed away on Aug. 2, 2009.

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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