Overcurrent Protection, Disconnecting Means and Mo

Disconnecting means

Q: Disconnecting means for circuits supplied by a generator are required where the circuits enter a building or structure. Are these disconnects from an outdoor generator required to meet the rules in Article 225? Is there a maximum number of disconnects permitted?

A: An outdoor generator and its wiring must satisfy parts of Article 445—Generators and Article 225—Outside Branch Circuits and Feeders. It is assumed the generator will supply one or more of the loads mentioned in 225.26(A).

The maximum number of disconnects is covered by 225.33(A) and reads like this: “(A) General. The disconnecting means for each supply permitted by 225.30 shall consist of not more than six switches or six circuit breakers mounted in a single enclosure in a group of separate enclosures, or in or on a switchboard. There shall be no more than six disconnects per supply grouped in any one location.”

The locations of these disconnects must comply with 225.32 and the exceptions require the disconnecting means to be readily accessible and located nearest the point of entrance of the conductors. Where this requirement cannot be met, they must be encased in at least 2 inches of concrete or comply with one of the other methods mentioned in 230.6.

Overcurrent protection for the generator must comply with 445.12; however, overcurrent protection is not required if the authority having jurisdiction determines that the generator is vital to the operation of the electrical system and the generator should operate to failure to prevent a greater hazard to persons. In this case overload sensing devices are permitted to be connected to an alarm that is supervised by authorized personnel instead of disconnecting the generator.

Circuits that supply or pass through a building(s) must be provided with disconnecting means that are marked “Suitable for Use as Service Equipment” to satisfy 225.36. A generator that supplies one or more of the following: a fire pump, emergency loads, legally required standby loads, and interconnected power production systems must also meet the additional requirements that are mentioned in Articles 695, 700, 701 and 705.

Fire pump overcurrent

Q: I did the wiring for a 75-horsepower, 480-volt, three-phase fire pump motor with a nameplate full-load current rating of 94 amperes. The feeder conductors to the fire pump are 1 AWG copper with THWN insulation in 1.25-inch rigid metal conduit and liquid-tight flexible metal conduit. Feeder overcurrent protection consists of 300-ampere fuses in a 400-ampere disconnect switch. The electrical inspector turned the job down with this comment: “Feeder overcurrent protection must be increased to 600-ampere.” I followed the rules in 430.52 and Table 430.52 to size the overcurrent protection, why should the overcurrent protection be increased to 600 amperes?

A: Fire pump motors must be installed to comply with Article 695—Fire Pumps. These requirements modify the rules in Article 430.

Where the fire pump controller or fire pump transfer switch is not supplied directly from the service, overcurrent devices are permitted, provided that they are selected or set to carry locked-rotor current of the fire pump motor indefinitely.

There is a table of locked rotor currents for various motor horsepowers in NFPA 20-2003, “Standard for the Installation of Stationary Pumps for Fire Protection,” that is within one or two amperes of the values for every horsepower shown in Table 430.251(B) in the National Electrical Code (NEC). The intent of this requirement is to allow the motor to start under very severe starting conditions. The fire pump controller also has unique features. Among other things, the controller cannot open the branch circuit to the fire pump in less than 8 seconds and must open the circuit in not more than 30 seconds at 300 percent of motor full-load current.

According to Table 430.251(B) the locked rotor-current for a 75-horsepower, 460-volt, three-phase motor is 543 amperes; therefore, a 600-ampere disconnect switch with 600-ampere fuses or a 600-ampere circuit breaker is the minimum-size overcurrent protection allowed for this installation.

Apartment outdoor receptacles

Q: Outdoor receptacles are required for multifamily buildings by a new paragraph in 210.52(E). How many receptacles are required for an eight-unit apartment building? May the outdoor receptacles be supplied from the “house” meter or must a receptacle outlet be supplied from each tenant meter?

A: Where each dwelling unit is at grade level and each has an individual exterior entrance, an outdoor receptacle that is not more than 6 feet above grade must be installed. If one receptacle can be located on the exterior that is accessible to all occupants the requirement in 210.52(G) is satisfied. The one or more outdoor receptacles should be supplied from the “house” meter unless individual receptacles are supplied from each tenant meter. Connecting one or more receptacles to the “house” meter assures that the outdoor receptacle(s) will be energized at all times.

This new paragraph in the 2005 NEC also references 210.8(A)(3) to point out that GFCI protection is required.

Luminaires in suspended ceilings

Q: Where a two-by-four lay-in luminaire is supported independent of the ceiling grid, is it necessary to secure the fixture to the grid? Some manufacturers specify additional support wires from the luminaire (lighting fixture) to the structural member above, and provide tabs or clips to secure the fixture to the ceiling grid. Are both methods required by the NEC?

A: It appears that the luminaire (lighting fixture) manufacturer wants to make sure that the luminaire does not fall out of the ceiling should the suspended ceiling fail. This is the way part (C) of 410.16 reads: “Suspended Ceilings. Framing members of suspended ceiling systems used to support luminaires (fixtures) shall be securely fastened to each other and shall be securely attached to the building structure at appropriate intervals. Luminaires (fixtures) shall be securely fastened to the ceiling framing member by mechanical means such as bolts, screws, or rivets. Listed clips identified for use with the type of ceiling framing member(s) and luminaire(s) (fixture(s)) shall also be permitted.”

Although 410.16(C) does not require both support wires and clips for installing lay-in luminaires (lighting fixtures), 110.3(B) has to be considered. This part requires the installation of electrical equipment in accordance with instructions supplied by the manufacturer. If support wires and clips are required by the manufacturer, they must be used.

Determining lighting track load

Q: How is the branch-circuit load determined for lighting track? How is the feeder load calculated for 120 to 240 volt three-wire lighting track?

A: No load is added for track lighting in dwelling units and guest rooms or suites in hotels or motels. Track lighting is included in the branch-circuit and feeder calculations for these occupancies at volt-amperes per square foot as listed in Table 220.12. For all other occupancies, 220.43(B) requires a load calculation of 150 volt-amperes for each 2 feet of track or fraction thereof. This load calculation is for sizing feeder and service conductors and overcurrent protection.

The ampere rating of the lighting track determines the size of the overcurrent device and branch-circuit conductor ampacity. For example, 125-volt, 20-ampere lighting track requires 12 AWG copper branch circuit conductors and 20-ampere overcurrent protection.

If 50 feet of 125-volt, 20-ampere lighting track is required for a job, a load of (50 divided by 2 times 150Va) 3,750 volt-amperes must be added to the feeder and service calculations. For the multiwire branch-circuit track (120-240 volt), 50 feet of track would require that 7,500 volt-amperes be added to the service and feeder calculations.

There is no limit to the length of track permitted on a branch circuit; however, the ampere rating of the overcurrent device protecting the branch circuit cannot exceed the ampere rating of the track. Construction and installation rules are in 410.100 through 410.105 for lighting track.

Outdoor receptacles locations

Q: I installed one receptacle outlet at the front and back of a one-family dwelling. These receptacles are 15-ampere, 125-volt devices located about 3 feet above the ground. The central air conditioning unit is located on the roof of the carport. The inspector said I need a receptacle at the air conditioning unit even though one of the outdoor receptacles is not more than 20 feet from the unit. Is this additional receptacle required by the Code?

A: Yes it is. 210.63 requires a 15- or 20-ampere, 125-volt receptacle. This receptacle must be located on the same level and within 25 feet of the air conditioning equipment. Although at least one of the receptacles required by 210.52(E) is within 25 feet of the air conditioning unit, it is not at the same level. Therefore, the additional receptacle requested by the inspector must be provided.

EMT in Class II locations

Q: Is electrical metal tubing (EMT) with compression fittings permitted in Class II Division 2 locations? The contained conductors supply 480-volt branch circuits.

A: Article 502 applies to all voltages in Class II Division 1 and 2 locations, and electrical metallic conduit is a permitted wiring method in Class II Division 2 locations. Boxes and fittings are required to be dusttight. All wiring and equipment must be grounded and bonding jumpers are required at all raceway terminations (see 502.30).     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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