AFCI Circuit Breaker, UF Cable Temperature Rating and More

Article 210—Branch Circuits; Article 240—Overcurrent Protection; Article 250—Grounding and Bonding; Article 334—Nonmetallic-Sheathed Cable: Types NM, NMC, and NMS; Article 340—Underground Feeder and Branch-Circuit Cable: Type UF; Article 406Receptacles, Cord Connectors, and Attachment Plugs (Caps); Article 702—Optional Standby Systems; “Guide Information for Electrical Equipment (White Book)” published by Underwriters Laboratories Inc. also is mentioned.

AFCI circuit breaker

Q: Will a single-pole arc-fault circuit interrupter (AFCI) circuit breaker perform as intended on a 120/240-volt, three-wire branch circuit? Also, will a single-pole AFCI circuit breaker work on a two-wire without ground 120-volt branch circuit? The wiring in the existing residence is two-wire NM cable.

A: A single-pole AFCI circuit breaker will not function as intended on a multiwire branch circuit. An AFCI circuit breaker has a 30 or lower milliampere ground-fault circuit interrupter (GFCI) as part of the electronics. This will cause the circuit breaker to trip when the unbalanced current between the ungrounded (hot) conductor and the neutral (grounded) branch-circuit conductor exceeds the milliampere trip level of the GFCI.

A single-pole AFCI circuit breaker will detect arc faults and trip where connected to two-wire branch circuits without an equipment-grounding conductor.

The 2005 edition of the National Electrical Code (NEC) requires AFCI protection in the form of branch/feeder type until Jan. 1, 2008. After that time, a combination type AFCI is required. The major difference between these two types is the sensitivity to the amount of arc-fault current. The branch/feeder type is tested to detect parallel arcing and series arcing in branch-circuit wiring and parallel arcing in cord sets, extension cords and supply cords.

The combination type AFCI will interrupt all of the arcs mentioned for the branch/feeder AFCI plus series arcs in extension cords, cord sets and power supply cords. The “Guide Information for Electrical Equipment Directory (White Book)” published by Underwriters Laboratories Inc. describes the various types of AFCIs.

UF cable temperature rating

Q: Where Type UF cable is used as a substitute for nonmetallic sheathed cable as the wiring method for a one-family dwelling unit, the conductor insulation must have a rating of 90°C. What method is used to identify the insulation on the wires in Type UF cable?

A: The requirement for 90°C insulation on conductors in Type UF cable where used for interior wiring of a one-family residence is located in 340.112.

To identify the temperature rating of the conductor insulation, it may be necessary to refer to the 2006 edition “Guide Information for Electrical Equipment (White Book).” Under the heading of “Underground Feeder and Branch Circuit Cable (YDUX),” the following information is written: “Some multiconductor cable is surface marked with the suffix ‘B’ immediately following the type letters to indicate the usage of conductors employing 90°C rated insulation.”

Even though the conductor insulation is rated 90°C, its ampacity cannot exceed the value under the 60°C ampacity column in Table 310.16. Derating may be applied to the 90°C ampacity provided that the final adjusted ampacity does not exceed the 60°C ampacity shown in the Table. Article 334.80 permits this method of derating.

Transformer secondary grounding

Q: Should the secondary of a 480-volt, three-phase to 208Y/120-volt transformer be grounded at the source or first disconnecting means?

A:  Either at the secondary neutral terminal of the transformer or at the first secondary disconnecting means and overcurrent protection is acceptable.

Where the grounding-electrode con­ductor is connected to the neutral-conductor terminal in the transformer, it must be sized to comply with Article 250.66 and Table 250.66. With this connection, the neutral bus in the disconnect must be insulated from the enclosure, and a system-bonding jumper must be installed between the transformer XO terminal and the equipment-grounding lug or bus in the disconnecting means.

This system-bonding conductor must not be smaller than shown in Table 250.66 for the largest derived phase conductor. Where one or more phase conductors is larger than 1,100 kcmil copper or 1,750 kcmil aluminum, the system-bonding jumper cannot be less than 12.5 percent of the cma of the largest phase conductor. These figures assume that the grounding electrode is a buried metal water pipe or grounded structural metal of a building.

To eliminate installing a system-bonding jumper in the raceway along with the derived phase conductors, ground the secondary neutral conductor at the first disconnect on the secondary side of the transformer. Use a disconnect switch that is marked “Suitable for Use as Service Equipment” to bond the neutral bus to the enclosure. Connect the grounding-electrode conductor to the bonded neutral bus or lug in the disconnect and the structural metal of the building or grounded metal water pipe. The neutral-conductor terminal in the transformer is not grounded and must be isolated from any grounded surface when using this arrangement.

These requirements for grounding separately derived systems are found in 250.28, 250.30 and other sections in Article 250.

Secondary transformer conductors

Q: What is the minimum size copper conductor with Type THHW/THWN insulation required to supply a 400-ampere circuit breaker from the secondary of a transformer?

A: Transformers with a two-wire (single-voltage) or three-wire delta secondary may not require overcurrent protection where protection is provided by primary overcurrent protection [see 240.21(C)(1)].

In dry, damp and wet locations, the minimum conductor size is 600 kcmil copper or 3/0 AWG copper with two conductors per phase in separate raceways. These are the minimum conductor sizes because of the requirements in 240.21(C), which prohibits increasing the overcurrent protection device to the next larger standard size where the conductor ampacity does not match a standard overcurrent device ampere rating.

Lighting whip color code

Q:  Must lighting fixture whips be identified by color or other means where two different voltage systems are present in the same building?

A:  Yes, the conductors must be identified to denote the voltage system to which they are connected. This requirement is written in 210.5(C): “(C) Ungrounded Conductors. Where the premises wiring system has branch circuits supplied from more than one nominal voltage system, each ungrounded conductor of a branch circuit, where accessible, shall be identified by system. The means of identification shall be permitted to be by separate color coding, marking tape, tagging, or other approved means and shall be permanently posted at each branch-circuit panelboard or similar branch-circuit distribution equipment.” If the fixture whip includes a grounded branch-circuit conductor, additional rules for identifying the grounded conductor are in 200.6, 400.22 and 402.8.

Motel room receptacles

Q: Does the Code permit receptacles to be spaced more than 12 feet apart in a guest room of a motel? On electrical plans for a new motel, one wall is 15 feet long with only one receptacle on that wall. Other receptacles are placed on both sides of the bed and at various locations around the room.

A: The spacing requirements of 210.52(A)(1) for receptacles in a dwelling unit do not apply to guest rooms in motels, but the number of receptacles required by 210.52(A) must be provided. If the measurement around the walls and openings for doors requires 10 receptacles to comply with 210.52, this number of receptacles must be provided in each guest room.

Also, at least two receptacle outlets must be readily accessible. Receptacle outlets may be placed convenient to the furniture layout. The only difference in the requirements in 210.60 and 210.52 is the spacing of receptacle outlets and the requirement that at least two receptacle outlets be readily accessible.

Wet location receptacles

Q: At a self-service car wash, a 15-ampere, 250-volt receptacle is installed in each car wash stall. Is it required to have a cover that is weather­proof whether an attachment plug is inserted or not?

A: This area is defined as a wet location by definition in Article 100, and 406.8(B) requires all 15- and 20-ampere 125- and 250-volt receptacles installed in wet locations to be provided with an enclosure that is weatherproof, whether an attachment cap is inserted or not. Therefore, the answer is yes.

Standby generator connection

Q: I have a 7.5 kVA generator to connect to the wiring in a single-family dwelling. I plan to use two 30-ampere, double-pole circuit breakers with interlocks to connect the panelboard to the generator. This will prevent the generator from operating in parallel with the utility service.

The local inspector has requested a separate panelboard for the branch circuits that will be supplied by the generator. He is concerned about overloading the generator. I have made a list of the branch circuits that can be energized at one time when the generator is delivering power. A separate panelboard for the generator load will eliminate the flexibility that I have provided to the owner. Is a separate panelboard required by the NEC for the generator?

A: It appears that you have satisfied the requirements of 702.5 and 702.6. These are the Sections in Article 702—Optional Standby Systems that directly apply to the question.

Article 702.5 states the optional standby generator must have adequate capacity and rating for the supply of all equipment intended to be operated simultaneously. It also states that the user of the optional standby system is permitted to select the loads connected to the generator. With the load schedule you have provided, the owner should be able to select loads that will not overload the generator.

The interlocked circuit breakers appear to meet the requirement in 702.6, which requires transfer equipment. Part of this section requires that the transfer equipment be suitable for the intended use and designed and installed to prevent inadvertent interconnection of normal and alternate sources of supply.

If you outline the steps you have taken to prevent overloading of the generator, the inspector probably will not require a separate panelboard.  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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