Twist-Lock Receptacles, Conductors and More

By Dec 15, 2006
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Included articles: Article 210—Branch Circuits; Article 230—Services; Article 250—Grounding and Bonding; Article 300—Wiring Methods; Article 310—Conductors for General Wiring; Article 314—Outlet, Device, Pull, and Junction Boxes; Conduit Bodies; Fittings; and Handhole Enclosures; Article 320—Armored Cable: Type AC; Article 430—Motors, Motor Circuits, and Controllers; Article 518—Assembly Occupancies; Article 695—Fire Pumps; Portions of the 2005 edition of the General Information for Electrical Equipment Directory (White Book) published by Underwriters Laboratories are also mentioned.

Twist-lock receptacles for cooking

Q: Some commercial electric cooking equipment is provided with twist-lock attachment caps, but I have not been able to locate GFCI protected twist-lock receptacles. Am I permitted to remove these plugs and replace them with standard straight-blade attachment caps?

A: If the cord caps are rated 15 or 20 amperes, 125 volts, 210.8(B)(2) requires that they be ground-fault circuit interrupter (GFCI) protected. Before removing the twist-lock cord caps, consideration should be given to the use of GFCI circuit breakers or standard 15- or 20-ampere, 125-volt GFCI receptacles installed on the line side (ahead) of the cooking appliances.

If the twist-lock attachment caps are rated 30 amperes or more or other than 125-volts, single phase, GFCI protection is not required.

Same raceways, different services

Q: Does the National Electrical Code (NEC) permit branch circuits from different services to be installed in the same electrical metallic tubing? I am wiring an apartment building, and combining branch circuits from different services would be economical.

A: Yes, 300.3(C)(1) permits this arrangement. This part of the NEC reads as follows: “(C) Conductors of Different Systems. (1) 600 Volts, Nominal, or Less. Conductors of circuits rated 600 volts, nominal, or less, ac circuits, and dc circuits shall be permitted to occupy the same equipment wiring enclosure, cable, or raceway. All conductors shall have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the enclosure, cable, or raceway.”

Remember, that where more than three current-carrying conductors are installed in a single raceway, derating as required by Table 310.15(B)(2)(a) applies.

Terminating grounding electrodes

Q: Is it permitted to terminate the grounding-electrode conductor from a buried metal water pipe in a lug bolted to the outside of the service-disconnect switch enclosure?

A: No, the grounding electrode conductor must be connected to the grounded-service conductor or the equipment-
grounding terminal, bar or bus. These rules appear in 250.24(A)(1) and (4). The basic requirement is for the grounding-electrode conductor to be connected to the grounded-service conductor. Part (A)(1) requires that the connection be made at any accessible point from the load end of the service drop or service lateral to and including the terminal or bus to which the grounded-service conductor is connected at the service-disconnecting means.

Part (A)(4) of 250.24 allows the grounding-electrode conductor to be terminated on the equipment-grounding terminal bar or bus where the main bonding jumper is connected to the system-grounded conductor terminal bar or bus and the equipment-grounding terminal bar or bus in the service-disconnecting means.

Also, 250.12 requires that nonconducting coatings such as paint, lacquer and enamel be removed to ensure good electrical continuity between the connected parts. However, removing the protective coating on an electrical enclosure installed outdoors violates 300.6, which requires corrosion protection of ferrous metal enclosures installed outdoors.

Service equipment manufacturers provide kits for connection of the grounded-circuit conductor to the grounding-electrode conductor where these parts are not factory installed.

Fire pump branch circuits

Q: We are installing a 100-horsepower, 480-volt, three-phase fire pump in an existing building. The branch-circuit conductors for the fire pump are sized for the locked rotor current of the fire pump motor. Is this the way to size the conductors? Also, what is the minimum ampere rating of the feeder overcurrent protection?

A: If you size the conductors for the locked-rotor current of the motor, they will probably be too large to fit into the lugs in the fire-pump controller. To comply with 695.6(C)(2), the branch-
circuit conductors for the fire pump must have a minimum ampacity of 125 percent of the fire pump motor full-load current, but they must also be sized to limit the voltage drop to 15 percent of controller rated voltage under normal starting conditions and 5 percent of motor rated voltage when the motor is operating at 115 percent of full-load current. Therefore, fire pump motor conductor size may have to be increased slightly above 125 percent to limit voltage drop.

According to Tables 430.250 and 430.251(B) the full-load current for a 100-hp, 480-volt, three-phase motor is 124 amperes and the locked rotor current is 725 amperes. The minimum branch-circuit conductor ampacity is (1.25 x 124) 155 amperes. Therefore, the minimum conductor size is 2/0 AWG Type THWN copper.

Where service conductors run directly to the fire pump controller, no overcurrent protection is required. The only overcurrent protection for the service conductors is provided by the circuit breaker in the fire pump controller, which is set to trip at 300 percent of motor full load current in not less than 8 or more than 20 seconds.

If overcurrent protection at the service for the fire pump is provided, the rating of the disconnect and overcurrent protection cannot be less than 725 amperes. Since a 725-ampere fuse or circuit breaker is not a standard size [See 240.6(A)], 800 ampere fuses or an 800 ampere circuit breaker is allowed.

Floor boxes in dwelling units

Q: What types of floor outlet boxes are suitable for installation of receptacles in dwelling units? Are nonmetallic boxes permitted?

A:Floor boxes must be listed for this application. 314.27(C) reads, “Floor Boxes. Boxes listed specifically for this application shall be used for receptacles located in the floor.” The exception to this part (C) does not apply to this application.

Metallic and nonmetallic floor boxes are listed by Underwriters Laboratories, Inc. under guide (QCIT) for metallic boxes and under (QCME) for nonmetallic boxes. These boxes are provided with covers and gaskets to exclude surface water and sweeping compounds that may be used for floor cleaning. Where the box and cover are in separate packages, installation instructions are included in both packages. Additional information is found on pages 89, 90 and 91 in the 2005 edition of the General Information for Electrical Equipment Directory (White Book) published by Underwriters Laboratories Inc.

Meter socket supply-side disconnect

Q: Is it permissible to install a non-fused disconnect switch ahead of a 400-ampere, 480Y/277-volt, meter socket? The utility company is requesting this switch to allow their service personnel to remove the meter from the socket while it is de-energized. Grounding and bonding will be done in a 400-ampere circuit breaker enclosure.

A: Yes, this disconnect is permitted by Item 3 of 230.82 provided that it has an adequate short-circuit current rating. The title for 230.82 is “Equipment Connected to the Supply Side of Service Disconnect” and part (3) reads, “Meter disconnect switches nominally rated not in excess of 600 volts that have a short-circuit current rating equal to or greater than the available short-circuit current, provided all metal housings and service enclosures are grounded.”

The 400-ampere circuit breaker (service disconnect) must comply with all requirements in 230.70 and grounding and bonding must comply with Parts IV and V of Article 250.

Type AC cable as a wiring method

Q: Is type AC cable a suitable wiring method for receptacle and lighting circuits in an elementary school with an auditorium that can accommodate 500 people?

A: The answer is generally yes. Uses permitted are listed under 320.10, which includes a fine print note that states, “The ‘Uses Permitted’ is not an all-inclusive list.” See 320.12 for where Type AC cable is not permitted.

If the auditorium is required to be fire- rated construction by the local building code, 518.4(A) requires the use of Type AC cable with an insulated equipment-grounding conductor sized to comply with Table 250.122.

Where Type AC cable is used to wire classrooms, office space, library, cafeteria, etc. that do not accommodate 100 or more people, Type AC cable without an insulated equipment-grounding conductor satisfies the NEC as a wiring method where installed to comply with Article 320.

Secondary neutral conductor

Q: Flexible metal conduit is the wiring method for conductors from a single-phase, 120/240-volt secondary of a transformer. Bonding and grounding of the neutral conductor is accomplished at the secondary terminal in the transformer enclosure. What grounding or bonding is required at the secondary disconnecting means?

A: The neutral conductor must terminate on a bus or lug that is insulated from the secondary-disconnecting means enclosure. 250.30(A) states, “A grounding connection shall not be made to any grounded circuit conductor on the load side of the point of grounding of the separately derived system except as otherwise permitted in this article.”

A system-bonding jumper must be installed from the secondary neutral conductor terminal at the transformer to the equipment-grounding bus in the secondary disconnect. All equipment-grounding conductors from the secondary circuits are terminated on this equipment-grounding terminal strip.

The bonding jumper is sized to comply with 250.28, which requires the use of Table 250.66 or where the secondary conductors are larger than 1,100 Kcmil, copper, the bonding conductor, cannot be less than 12.5 percent of the largest phase conductor.

Flexible metal conduit is not recognized as an equipment-grounding conductor where its length is greater than 6 feet or where overcurrent devices more than 20 amperes protect conductors contained in the flexible metal conduit; however, the bonding conductor installed in the raceway provides the grounding continuity required by 250.118.            EC

Flach, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720.







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