Grounding Devices, Wiring and Grounding a Transformer and More

Article 200—Use and Identification of Grounded Conductors; Article 220—Branch-Circuit, Feeder and Service Calculations; Article 240—Overcurrent Protection; Article 310—Conductors for General Wiring; Article 314—Outlet, Device, Pull and Junction Boxes; Conduit Bodies; Fittings; and Handhole Enclosures; Article 404—Switches; Article 422—Appliances

The White Book published by Underwriters Laboratories Inc. also is mentioned.

Circuit breakers for switching luminaires (lighting fixtures)

Q: Are circuit breakers marked “SWD” suitable for switching incandescent as well as fluorescent luminaires? Are circuit breakers marked “HID” suitable for switching high-intensity discharge, fluorescent and incandescent lighting fixtures? Is an HID-marked circuit breaker suitable for switching HID, fluorescent and incandescent luminaires that all are on the same branch circuit?

A: Circuit breakers marked “SWD” are permitted for switching fluorescent and incandescent lighting loads. Circuit breakers marked “HID” may be used for switching high-intensity discharge, fluorescent and incandescent lighting loads. Circuit breakers marked “HID” may be used to switch any combination of high-intensity discharge, fluorescent and incandescent lighting loads. The requirement for additional marking on circuit breakers used to switch fluorescent and high-intensity discharge lighting is found in 240.83(D).

Service-entrance conductors for apartment buildings

Q: The calculated load for an apartment building is 665 amperes, 120/240 volts. May the service-entrance conductors be two sets of parallel 350 Kcmil Type THWN copper conductors in separate raceways? I am using Table 310.15(B)(6) and doubling the ampacity of 350 Kcmil copper conductors to obtain an ampacity of 700?

A: The special or elevated ampacities of conductors permitted by Table 310.16(B)(6) stops at 400 amperes. It does not include elevated ampacities for service-entrance conductors in parallel.

The minimum size service-entrance conductors for a calculated load of 665 amperes is two sets of 400 Kcmil Type THWN copper or other copper insulated conductors with 75°C or 90°C insulation in two separate raceways. These conductors may be protected by a 700-ampere overcurrent protective device.

Identifying grounded circuit conductor in cable

Q: Does the National Electrical Code (NEC) permit a black conductor in a multiconductor cable to be marked with white paint at its terminations and used as a neutral conductor (grounded circuit conductor) in a feeder?

A: Although this method of identifying a grounded circuit conductor (may be a neutral) is acceptable for sizes larger than 6 AWG conductors in a raceway, it is not acceptable for multiwire cables.

For sizes larger than 6 AWG installed in a raceway, this is what Item 3 of 200.6(B) allows: “At the time of installation, by a distinctive white or gray marking at its terminations. This marking shall encircle the conductor or insulation.”

This requirement appears in part (E) of 200.6: “Grounded Conductors of Multiwire Cables. The insulated grounded conductors of a multiconductor cable shall be identified by a continuous white or gray outer finish or by three continuous white stripes on other than green insulation along its entire length. Multiconductor flat cable 4 AWG or larger shall be permitted to employ an external ridge on the grounded conductor.

There are two exceptions to this rule: one for qualified persons and the other for varnished-cloth-insulated cable.

Transformer wiring and grounding

Q: Our firm has a job to wire a 112.5 kVA transformer. We would like to know if this proposed installation satisfies the requirements in the NEC. Primary voltage—480, three-phase; secondary voltage—208Y/120, 112.5 kVA; primary wire size—1/0 AWG copper with THWN insulation; primary overcurrent protection—150 amperes; secondary conductors—four-500 Kcmil copper type THWN protected by a 400-ampere circuit breaker in a panelboard. A 1/0 AWG copper conductor will be used as the grounding-electrode conductor and will be run from the 400-ampere circuit breaker to the structural steel of the building.

A: The primary full load current for this transformer is 135.5 amperes. This full load current is within 90 percent of the ampere rating of the primary overcurrent protective device, and you may get nuisance tripping when you energize the transformer. For this reason, I suggest the primary conductors and overcurrent protection be increased to 2/0 AWG copper with Type THWN insulation and a 175-ampere overcurrent device.

Secondary full load current is (112,500 ÷ 208 x 1.73) 312 amperes. Therefore, 400 Kcmil copper conductors with THWN insulation protected by a 350-ampere overcurrent device provide adequate protection while allowing full use of the kVA rating of the transformer.

A 1/0 AWG copper grounding-electrode conductor is required for 500 Kcmil or 400 Kcmil copper secondary conductors. The grounding-electrode conductor may be connected to the grounded-circuit conductor (neutral) at the transformer or in the secondary overcurrent protective device enclosure. If connected to the neutral bus in the transformer enclosure, a system-bonding jumper must be installed at the transformer neutral bus and the equipment-grounding bus in the secondary panelboard. The neutral bus in this panelboard must be insulated (isolated) from the enclosure.

The secondary neutral conductor may be reduced in size depending on the line to neutral load; however, it cannot be reduced below the size required for the grounding-electrode conductor as given in Table 250.66.

Ceiling fan support outlet boxes

Q: Are all listed outlet boxes marked “Acceptable for Fan Support” suitable for supporting a (paddle) fan that weighs approximately 45 pounds?

A: No, they are not. Ceiling-suspended (paddle) fans must be supported either independent of an outlet box or by a listed outlet box that is identified for this use, according to 422.18.

Requirements for outlet boxes used as the sole support of ceiling fans must also conform with 314.27(D). This is the way 314.27(D) reads: “Boxes at Ceiling Suspended (Paddle) Fan Outlets. Outlet boxes or outlet box systems used as the sole support of a ceiling-suspended (paddle) fan shall be listed, shall be marked by their manufacturer as suitable for this purpose, and shall not support ceiling-suspended (paddle) fans that weigh more than 32 kg (70 lbs). For outlet boxes or outlet box systems designed to support ceiling-suspended (paddle) fans that weigh more than 16 kg (35 lbs), the required marking shall include the maximum weight to be supported.”

The Guide Information for Electrical Equipment Directory (2006 edition of the White Book) published by Underwriters Laboratories Inc. has this information under the titles “Metallic Outlet Boxes (QCIT)” and “Nonmetallic Outlet Boxes (QCME)” under the heading Ceiling Suspended Fan Support: “A box, a box with bracket, or bar hanger intended for support of a ceiling-suspended (paddle) fan weighing 35 lbs or less is provided with a marking on the product ‘ACCEPTABLE FOR FAN SUPPORT.’ A box, a box with bracket, or bar hanger for support of a ceiling-suspended (paddle) fan weighing more than 35 lbs but not more than 70 lbs is provided with a marking on the product ‘ACCEPTANCE FOR FAN SUPPORT UP TO 70 LBS.’”

A fan support box with a marking of 70 pounds must be used for this installation.

Electric range branch-circuit wires

Q: In a new one-family dwelling, an 8 AWG copper branch circuit was roughed in for the electric range. The nameplate on the range has this information: 240-volts, single-phase, 13.7 kW. Does the wire size have to be increased?

A: The minimum branch-circuit conductor ampacity for this load is calculated using 220.55 and Table 220.55. According to Note 1 of the table, a range with a nameplate not exceeding 12 kW is permitted to use branch-circuit conductors and overcurrent protection based on an 8 kW load. For a 13.7 kW range, the demand load is (8,000 + 2 x 400) = 8,800 watts. Dividing 8,800 by 240 results in a current of 37 amperes; therefore, the wire size does not have to be increased. In fact, the nameplate kW rating on a range can be as high as 16.5 kW on an 8 AWG copper branch circuit protected at 40 amperes.

Equipment connected to 20A bathroom circuit

Q: May a heater-vent-light unit with a full-load current of 11.1 amperes be supplied from the required 20-ampere receptacle branch circuit in the bathroom of a dwelling unit?

A: A combination heater-vent-lighting fixture usually is installed in the bathroom ceiling as a fixed appliance. Since this appliance has a full-load current that exceeds 50 percent of the branch-circuit ampere rating, it is not permitted on the 20-ampere branch circuit that supplies the receptacle. Although the exception to 210.11(C)(3) permits other loads to be supplied from this 20-ampere branch circuit provided the branch circuit supplies an individual (single) bathroom, 210.23(A)(2) limits the load of utilization equipment that is fastened in place to 50 percent of the ampere rating of the branch circuit.

Different voltage grounding devices
Q: Does the NEC permit a 277-volt snap switch to be installed in a two-gang device box with a 120-volt receptacle?

A: Yes, this may be done with specially designed device boxes. Part (B) of 404.8 allows this arrangement and reads: “(B) Voltage Between Adjacent Devices. A snap switch shall not be grouped or ganged 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.” 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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