Article 225-Outside Branch Circuits and Feeders; Article 240-Overcurrent Protection; Article 250-Grounding; Article 400-Flexible Cords and Cables; Article 430-Motors, Motor Circuits, and Controllers; Article 445-Generators; Article 450-Transformers and Transformer Vaults. NFPA-54 National Fuel Gas Code is also mentioned.
Clearance from gas meter
Q: In the August 1999 issue of Electrical Contractor someone asked for the minimum clearance from a gas meter. Here is the question: "How close to 120/240-volt service equipment (meter, disconnect switch, and panelboard) can a gas meter and gas pipes be installed?
A: My short answer mentioned the minimum clear working spaces required by Section 110-26 of the National Electrical Code (NEC), and also mentioned NFPA 54-National Fuel Gas Code saying that the gas meter must be located at least three feet from sources of ignition.
This answer prompted a response from the codes and standards coordinator for a gas utility company.
Here is his letter: "An item on page 32 of your August issue, regarding clearance between gas meters and electrical equipment, contains several statements that need clarification. The author's conclusion that 'electric service equipment should not be installed within three feet of a gas meter' properly applies to non-utility gas meters and electric service equipment that can create an arc when operated. However, this does not generally apply to the clearance between an electric service meter and a gas utility meter."
The author wrote that NFPA 54 of the National Fuel Gas Code requires that a gas meter be located three feet from a source of ignition. This is correct, but omits an important point. This provision, and all of Section 2.7, is applicable only to 'premises-owned gas meters'-for instance, customer-owned submeters. Meters and other service equipment used by the gas utility are specifically excluded from the scope of the National Fuel Gas Code.
All portions of a gas utility distribution system up to the outlet of the meter are regulated by 49 CFR Part 192, under the U.S. Department of Transportation. These regulations require a three-foot clearance from indoor meters to sources of ignition, but have no comparable requirement when the meter is located outdoors.
There is also misunderstanding about the term 'source of ignition.' Electrical equipment is a source of ignition if it creates an arc or spark during operation.
Electric wiring and junction boxes, for instance, are not considered sources of ignition. But most switches, panelboards, relays, and other devices that can create an arc when operated are treated as ignition sources.
"The National Fuel Gas Code Handbook," in its discussion of the above cited gas meter clearance, states that electric meters (except those that incorporate switching relays) are not considered a source of ignition.
In summary, the three-foot clearance cited in the National Fuel Gas Code is not directly applicable to the clearance between gas and electric utility meters. Gas utilities are likely to have their own company requirements and restrictions on the proximity of their gas meters to sources of ignition, including arc-producing electrical equipment. The gas utility responsible for the meter should be consulted for any such clearance requirements before installing electric service near a gas meter." The last sentence provides good advice. The utilities involved should be consulted before locating an electric service in the immediate vicinity of a gas meter.
Flexible cord to supply motors
Q: I have to wire a 10-horsepower motor for an air compressor in a shop that sells and repairs automobile tires. The interior of the building is not classified. To reduce the transmission of vibration from the compressor to the metal raceway system and building structure, I would like to use about three feet of Type S00 cord to the motor. Is No. 8 Type S00 cord suitable for this application? The voltage is 208Y/120, three-phase.
A: Section 400-7(7) appears to allow the use of flexible cord to prevent the transmission of noise or vibration. However, the wire size for the flexible cord is not adequate.
Although Type S00 cord is suitable for extra hard usage and damp locations, and is oil resistant, the ampacity of 4 No. 8 Type S00 cord is only 35 amperes. This ampacity is obtained from Column A of Table 400-5(A). This is based on the fact that there are three current-carrying conductors in the flexible cord assembly. An equipment grounding conductor is the fourth wire in the cable assembly.
According to Table 430-150, the full-load current of a 10-horsepower, 208-volt motor is 30.8 amperes. Since the motor branch circuit conductors must have an ampacity of 125 percent of motor full-load current (Section 430-22), the minimum branch circuit conductor ampacity is 38.5. Therefore, No. 6 copper conductors must be used for the short length of flexible cord.
I would also find out from the electrical inspector if there is any objection to using flexible cord for this application.
Overcurrent protection for a generator
Q: Are overcurrent protective devices and a disconnecting means required at the point where the emergency feeder conductors connect the output terminals of a 100 kVa, 208Y/120-volt, three-phase, emergency generator that is located outdoors, or may the disconnecting means and overcurrent protection be provided at the building served?
A: Section 445-10 requires a disconnecting means that will disconnect the generator and all protective devices and control apparatus from the circuits supplied by the generator unless the driver for the generator can be readily shut down. I assume the generator prime mover is not operating in parallel with another power source and, therefore, can be shut down. If this is the case, a disconnect at the generator is not required.
This is no requirement in Article 445-Generators, or Article 240-Overcurrent Protection for overcurrent devices to be located a certain distance from the generator. However, the feeder conductors from the generator terminals to the first overcurrent device must have an ampacity of at least 115 percent of the nameplate current rating of the generator. Also, Section 445-4(a) says, "Constant Voltage, except AC generator exciters, shall be protected from overloads by inherent design, circuit breakers, fuses, or other acceptable overcurrent protective means suitable for the conditions of use."
Where the emergency feeder enters the building, various sections in Article 225-Outside Branch Circuits and Feeders come into play; in particular, "Part B More than One Building or other Structure."
The disconnecting means for the emergency generator must be at a readily accessible location nearest the point of entrance to the conductors. The location selected for the disconnect must be remote from the normal supply disconnecting means to minimize the possibility of simultaneously interrupting supplies. Also, the disconnecting means for the emergency generator must be suitable for use as service equipment and as such identified with a permanent plaque or directory. For a complete list of requirements, Sections 225-30 through 225-40 should be reviewed.
The short answer is that overcurrent protection is not required at the generator terminals.
Grounding service equipment
Q: On an outdoor overhead service, a 200-ampere disconnect switch is installed below the meter base. The meter base is connected to the service disconnect via metal nipple that goes into a knockout in the bottom of the meter base and into the weatherproof disconnect through a threaded hub. The nipple is bonded to the meter base with a bonding locknut. The neutral is connected to the center terminal on the three-wire single-phase meter base and also at the grounded neutral bus in the 200-ampere switch. Is this arrangement contrary to Section 250-21 (objectionable current flow over grounding conductor) because the neutral conductor and conduit nipple are now in parallel?
A: The answer is "No." Section 250-92 requires that all noncurrent-carrying metal parts of service equipment be bonded. Section 250-24 requires that the grounded conductor (neutral) be connected to one or more grounding electrodes by using a grounding electrode conductor. The connection of the grounding electrode conductor to the neutral may be made at any accessible point from the load end of the service drop or lateral to and including the neutral bus in the service disconnecting means. Although different connection points will cause division of neutral currents, with some current flowing through the raceway and some flowing through the neutral conductor, these currents are not considered objectionable as mentioned in Section 250-21.
Secondary overcurrent protection
Q: Where a three-phase delta-wye 480-volt to 208Y/120-volt dry type transformer feeds a power panelboard, is secondary overcurrent protection required for the panelboard or secondary conductors? The primary is protected by 80-ampere time-delay fuses in a 100-ampere fused switch. The kVa rating is 50.
A: According to Section and Table 450-3(b), secondary overcurrent protection for the transformer is not required because primary overcurrent protection is within 125 percent, or the next larger standard overcurrent device is used. The full-load current of 50-kVA, 480-volt, three-phase primary is 60 amperes, and 125 percent increases this figure to 75. According to Section 240-6, the next larger standard-size fuse or circuit breaker is 80; therefore, secondary overcurrent protection for the transformer is not required.
If the power panelboard has supply conductors that include a neutral, and more than 10 percent of its overcurrent devices protect branch circuits rated 30 amperes or less, an overcurrent device with a rating that is not greater than the ampere rating of the panelboard must be provided to protect the panelboard.
Overcurrent protection is generally required with secondary conductors. for Article 240-Overcurrent Protection, Section 240-21(c)(2) lists the requirements for overcurrent protection for secondary conductors that are not more than 10 feet long, and 240-21(c)(3) lists requirements that must be met for secondary conductors that are more than 10 feet but not more than 25 feet long in industrial installations.