Grounding of Receptacles, Overcurrent Protection for a Feeder and More

Article 210 Branch Circuits

Article 240 Overcurrent Protection

Article 250 Grounding

Article 310 Conductors for General Wiring

Article 430 Motors, Motor Circuits and Controllers

Branch-circuit size for motor load

Q: I have to install a 11/2 horsepower, single-phase, 120V motor with a nameplate full-load current of 17.5 amperes. May the branch circuit conductors be 14 AWG copper with Type THHN insulation?

A: The minimum branch circuit conductor ampacity is required to be 25 by Table 430.148 unless the motor is a shaded-pole or permanent-split capacitor-type used on a fan or blower. The exception for fan and blower motors is found following 430.6(A)(1). If the motor does not qualify under any of the exceptions in 430.6(A)(1), the motor full-load current shown in Table 430.148 must be used and the minimum branch circuit conductor ampacity cannot be less than 125 percent of this table value. Therefore, the minimum conductor ampacity is 1.25 x 17.5, about 22 amperes.

Although 14 AWG copper with Type THHN insulation has an ampacity of 25 according to Table 310.16, it cannot be used at this ampacity because of 110.14(C). Terminations at motors are suitable for 75 C rated conductors, and the branch-circuit overcurrent protective device terminals, as well as the terminals on the motor controller, must also be marked for 75 C insulated conductors to use the 75 C column in Table 310.16.

The minimum branch circuit conductor size for this application is 12 AWG copper with 60 C or 75 C insulation.

The asterisks appearing with 14,12 and 10 AWG in Table 310.16 refer users of the Code to 240.4(D), which limits the overcurrent protection for these copper wires to 15 for 14 AWG, 20 for 12 AWG and 30 for 10 AWG. But 240.4(D) also allows the overcurrent devices protecting these conductors to exceed these values where permitted for electrical equipment shown in Table 240.4(G); motors are included in the table.

Number 12 AWG copper branch circuit conductors protected by a 40-ampere circuit breaker or 50-ampere nontime delay fuse satisfies the Code for this motor installation.

Service-entrance cable exposed outdoors

Q: We installed service entrance cable in rigid metal conduit as part of a service for a small commercial building. It was necessary to remove part of the jacket to allow the wires to fit into the individual holes in the insulating bushing, which is part of the service head. There is no marking on any of the wires to indicate that the insulation is “sunlight resistant.” About three feet of the conductors are exposed to the weather and will be in the sun part of the time. Is this a problem?

A: I don't think so. Sub part (D) to 310.8 requires insulated conductors to be listed for sunlight resistance or marked “sunlight resistant,” therefore, the individual conductors do not have to be marked.

This information appears under the title “Service Entrance Cable (TYLZ)” in the 2003 edition of the General Information for Electrical Equipment Directory (White Book) published by Underwriters Laboratories Inc: “Service entrance cable rated 600 V is listed in sizes 14 AWG and larger for copper, and 12 AWG and larger for aluminum or copper-clad aluminum. Type SE cable contains Type RHW, RHW-2, XHHW, XHHW-2, THWN, or THWN-2 conductors. Type USE cable contains conductors with insulation equivalent to RHW or XHHW. Type USE-2 contains insulation equivalent to RHW-2 or XHHW-2 and is rated 90 C wet or dry. The cable is designated as follows:

“Type SE-Indicates cable for above ground installation. Both the individual insulated conductors and the other jacket or finish of Type SE are suitable for use where exposed to sun.” Also, Types USE and USE-2 are suitable for use where exposed to sun.

Service-entrance conductors in parallel

Q: What size grounding electrode conductor is required for a service that consists of 6-500 Kcmil copper conductors per phase? The conductor insulation is Type THWN. A 1,200-ampere feeder must also be installed. The plan is to use five conductor-Type MC cable. Each cable consists of 4-500 Kcmil and one AWG aluminum equipment-grounding conductor for the phase wires, neutral and equipment ground. Is interlocked armor Type MC cable with one AWG aluminum equipment-grounding conductor suitable for this application?

A: The grounding-electrode conductor for the service does not have to be larger than 3/0 copper or 250 Kcmil aluminum. It could be six AWG copper if the only grounding electrodes are rods, pipes or plates. Rod, pipe and plate electrodes must meet the requirements of 250.52(A)(5) and (6). When the grounding electrode is at least 10 feet of buried metal water pipe or building steel, the grounding-electrode conductor is required to be 3/0 copper or 250 Kcmil aluminum.

The equipment-grounding conductors in the MC cable are not large enough. They have to be at least 3/0 AWG copper or 250 Kcmil aluminum to comply with part (F) of 250.122. Each parallel equipment-grounding conductor must be sized on the basis of the ampere rating of the overcurrent device protecting the circuit conductors in the cable in accordance with Table 250.122. For a 1,200-ampere overcurrent device, a 3/0 AWG copper or 250 Kcmil aluminum is required.

There is at least one Type MC cable manufacturer that produces cables with oversize equipment-grounding conductors. These catalog item products allow paralleling of Type MC cable, and they can be obtained without long delays in shipment or excessive cost.

Concrete-encased electrode

Q: When a concrete-encased electrode is used to ground the service for a neighborhood bakery, does the steel reinforcing rod have to be an unbroken length of 20 feet or more, or may rods be spliced together so that the electrode is at least 20 feet in length? May the connection for the grounding electrode conductor be made to the reinforcing steel outside of the concrete?

A: Concrete-encased grounding electrodes are permitted by 250.52(A)(3), and specifications for their installation are also included.

The reinforcing rod used as a grounding electrode must be encased in at least two inches of concrete that is in contact with the earth. The rod may be bare or coated with an electrically conductive material or may be galvanized. The minimum rod diameter is 1/2 inch, and the minimum length is 20 feet-a length that may be obtained by splicing short lengths together with steel tie wires.

The grounding-electrode conductor does not have to be any larger than four AWG copper, and the connection of the grounding electrode to the grounding-electrode conductor does not have to be accessible. The connection does not have to be embedded in concrete. The size of the grounding-electrode conductor is given in 250.66(B). The accessibility requirement for the connection of the grounding electrode to the grounding-electrode conductor is waived by the exception to 250.68(A).

Bonding flexible metal conduit

Q: A 480Y/277 volt service panel supplies a transformer to derive a 208Y/120V system for lights, receptacles and 3-phase, 208V motors. The electrical inspector is requiring that we provide a bonding conductor on the inside or outside of the flexible metal conduit that is used on the primary and secondary of the transformer even though the lengths are less than six feet. Is the electrical inspector making the right call?

A: If the contained conductors in each length of flexible metal conduit are protected by overcurrent devices rated greater than 20 amperes, the answer is yes. To qualify as an equipment-grounding conductor, the flexible metal conduit must satisfy all of the requirements in 250.118(5) or (6). According to the 2003 edition of the General Information for Electrical Equipment Directory (White Book) published by Underwriters Laboratories Inc. under the title Flexible Metal Conduit (DXVZ) the following sentence appears: “Flexible metal conduit no longer than 6 ft and containing circuit conductors protected by overcurrent devices rated at 20 amperes or less is suitable as a grounding means.” Therefore, item (5) in 250.118 cannot be used at this time, and Item 6 must be applied with these restrictions: (1) fittings must be listed for grounding; (2) the circuit conductors contained in the conduit are protected by overcurrent devices rated at 20 amperes or less; (3) the total length of flexible metal conduit and other connected flexible metal raceways does not exceed six feet, and (4) the conduit is not installed for flexibility.

Permission to install the bonding jumper on the outside or inside of the flexible metal conduit is given in 250.102(E).

Laundry room in the basement

Q: Is it permissible to use the GFCI protected receptacle in an unfinished basement of a one-family dwelling to provide power to a washing machine and gas clothes dryer?

A: If this is the only receptacle in the basement, the answer is no. A receptacle protected by a GFCI is required by 210.8(A)(5), and 210.52(G) requires at least one receptacle outlet in addition to any provided for laundry equipment. The receptacle(s) for the laundry equipment may be supplied by the same branch circuit that supplies the GFCI receptacle. The receptacle(s) for the washer and dryer may be connected to the line side of the GFCI protected receptacle as allowed by Exception 2 of 210.8(A)(5) or may be connected to the load side of the GFCI receptacle or to the GFCI circuit breaker that supplies the basement receptacle. 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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