Article 225 Feeders
Article 230 Services
Article 250 Grounding
Article 408 Switchboards and Panelboards
Article 445 Generators
Article 680 Swimming Pools, Fountains and Similar Installation Systems
NFPA 20 1999 Standard for the Installation of Stationary Pumps for Fire Protection
Ground-fault protection for an on-site generator
Q: Members of our design team are discussing the requirement for ground-fault protection for an on-site generator that is rated 1,000 kVA, 480Y/277 volts and protected by a 1,200-ampere circuit breaker. The generator will supply power to the electric-drive fire pumps, exit and emergency lighting, elevators, ventilation and smoke-removal equipment, emergency communications, a fire-alarm system, and battery chargers for batteries that provide power for a telephone exchange in a high-rise office building. The requirements in 215.10 apply to a feeder rated over 1,000 amperes on a solidly grounded wye system where the voltage is over 150 to ground but not over 600. Exception No. 2 to 215.10 eliminates the requirement for ground-fault protection of equipment where the load is a fire pump, and Exception No. 3 does not require GFP where this protection is provided on the feeder's supply side. However, this protection does not protect the generator feeder when the normal source is off and the transfer switches have connected the load to the generator. The generator manufacturer does not want the 1,200-ampere circuit breaker removed. What do you suggest?
A: An on-site generator used to supply this variety of loads must satisfy the appropriate rules in Article 445,Generators; Article 695,Fire Pumps; Article 700,Emergency Systems; Article 701,Legally Required Standby Systems; and Article 702,Optional Standby Systems.
The generator neutral must be grounded to comply with 250.20(B)(2) and in a manner permitted by 250.30.
The conductors from the generator terminals to the 1,200-ampere circuit breaker must be sized at not less than 115 percent of the generator nameplate full-load current to comply with 445.13.
Four-pole automatic-transfer switches should be provided for each fire pump, the emergency system and the legally required standby system. These transfer switches must be electrically operated and mechanically held. A manual transfer switch is satisfactory for the optional standby system.
Ground-fault protection of equipment is not permitted on the output of the generator, but ground-fault sensing is required on the emergency system. Therefore, the ground-fault sensor should be located at, or ahead of, the 1,200-ampere circuit breaker. Instructions have to be posted at the sensor location that explain what should be done if the alarm activates because of a ground fault. This requirement appears in 700.7(D). There is no requirement for ground-fault protection for legally required systems.
Selective-load pickup, load shedding, and peak-load shaving are permitted by 700.5(B) provided that power to the fire pumps and emergency loads is adequate and available within 10 seconds. Time delay for energizing the legally required standby circuits is permitted because 701.11 allows these loads to be without power for one minute. Energizing each transfer switch at different times allows the generator load to increase gradually, thereby assuring increased reliability.
On-site fuel supply for the engine that drives the generator must be sufficient for at least eight hours of fire pump operation at 100 percent of rated pump capacity, plus two hours of fuel for the emergency loads, plus a two-hour supply for all legally required standby loads, plus whatever amount of fuel is needed to supply optional standby loads for a period of time selected by the owner. The eight-hour on-site fuel supply for the fire pumps is required by 6-6.2 in NFPA 20,1999 Standard for the Installation of Stationary Pumps for Fire Protection.
I have discussed some of the important rules that apply to an on-site generator set that supplies various and different electrical loads. If the Articles mentioned earlier are reviewed and applied correctly, a reliable, Code-compliant installation will result.
Grounding-electrode conductor for a dry-type transformer
Q: May the grounding-electrode conductor for the secondary of a dry-type transformer installed indoors about 25 feet from the service equipment be run in the same raceway as the primary conductors? The primary voltage is 480, three-phase; the secondary voltage is 208Y/120, and the transformer is 75 kVA. The primary circuit originates at the service switchboard and the service neutral is grounded to a metal water pipe and building steel. Overcurrent protection on the primary is 125 amperes and secondary overcurrent protection is 250 amperes. Secondary conductors are 4/0 AWG Type THWN copper.
A: I am assuming that the service-entrance conductors are 4/0 AWG copper or larger. If this is a correct assumption, the grounding-electrode conductor from the secondary neutral terminal of the transformer to the grounded neutral bus in the service switchboard is a permitted connection. It is necessary to assume that the service-entrance conductors are at least equal to or larger than 4/0 AWG copper because the grounding-electrode conductor cannot be smaller than 2 AWG copper for the secondary neutral of the transformer and the neutral bus or equipment-grounding bus in the service switchboard has to be grounded with a grounding-electrode conductor that is not smaller than 2 AWG copper to comply with the second exception in 250.30(A)(4). And a 2-AWG grounding-electrode conductor is required by Table 250.66 where the service-entrance conductors are larger than 3/0 AWG.
Enclosures for grounding-electrode conductors are covered by 250.64(E), and methods of bonding metal raceways that contain conductors that operate at over 250 volts to ground are mentioned in 250.97.
Notes 3 and 8 to Table 1 in Chapter 9 indicate that the dimensions of bare conductors as shown in Table 8 may be used for raceway fill calculations. If the primary conductors are three 2-AWG Type-THWN insulated-copper conductors and the grounding-electrode conductor is a bare 2-AWG copper conductor, the minimum-size metal conduit for this combination is (3 x 0.1158 + 0.067) 1-inch rigid-metal conduit or schedule 40 or 80 rigid-nonmetallic conduit.
Swimming pool wiring
Q: We have a small job to wire an in-ground swimming pool that is part of a one-family residence and would like to know if a 240-volt, 15-ampere, branch circuit for the swimming pool pump that is protected by a two-pole, GFCI circuit breaker can be installed in the same nonmetallic raceway with a 120-volt, 15-ampere branch circuit that supplies a GFCI-protected receptacle located about 15 feet from the swimming pool. Is a 240-volt single-phase pump permitted to be cord-and-plug connected?
A: Conductors connected to the load side of a GFCI circuit breaker can occupy the same raceway as conductors that are not protected by a GFCI where the loads are a swimming pool pump motor and a GFCI-protected receptacle. However, this arrangement is not permitted where one of the loads is an underwater luminaire. Separation of GFCI-protected conductors from those without GFCI protection where underwater luminaires are involved is found in 680.23(F)(3).
GFCI protection for the pool pump motor is in 680.22(A)(5), and cord-and-plug connections for pool-associated motors are permitted by 680.21(A)(5). The cord length is limited to three feet, and the cord must include an equipment-grounding conductor that conforms to 250.122. The pump-motor receptacle may be located not less than five feet from the pool's inside walls if it complies with the following: it must be a single receptacle with a locking configuration and must be a grounding type and have GFCI protection. If the receptacle does not meet these requirements, it must be installed at least 10 feet from the pool's inside walls. These requirements are found in 680.22(A)(1).
Grounding the neutral at a service
Q: We are electricians in a city of about 10,000 people. The utility company is owned by the city. Last winter we had a severe ice storm that damaged many residential services. The city building inspector met with electrical contractors and said that services to the homes would be inspected under the new electrical code. He also wanted us to ground the service and meters all in the same manner. In most cases, the meter is mounted on the house and supplied from an overhead service drop. From the meter enclosure, a conduit nipple supplies the main disconnect panel below the meter. A neutral conductor is run from the meter socket through the nipple to the neutral bus in the disconnect. A bonding screw bonds the neutral bus to the panel enclosure, and a bonding jumper connects the neutral bus to the bonding bushing on the conduit nipple. A 6-AWG copper grounding-electrode conductor runs from the neutral bus to the ground rods. The inspector said he wants us to run another 6-AWG bare-copper wire from the neutral terminal in the meter to the neutral bus in the service disconnect panel. We think this is wrong because the extra bare 6-AWG conductor parallels the neutral conductor and the conduit nipple. We have called this to his attention twice, but he thinks that this is the way it should be done. There have been many problems in the past from losing the neutral at the service. The city was using square meter sockets with several wires under one clamp or terminal. The city changed meter socket manufacturers and this problem has nearly vanished. My question is, should we make the installation as he says, or should we continue to try to change his decision?
A: You are right. Installing the 6-AWG copper conductor in the conduit nipple and connecting it to the neutral terminal in the meter can and the neutral bus in the service disconnect puts it in parallel with the system neutral-service conductor that is also connected to the neutral terminal in the meter base and neutral bus in the disconnect enclosure. There are now three parallel paths for neutral current between these two enclosures. One is through the service-entrance neutral conductor, the second is through the 6-AWG conductor required by the inspector, and the third is through the conduit nipple.
Although most services for one-family dwellings will not be larger than 1/0 AWG copper with a neutral not smaller than 2 AWG, paralleling this size neutral conductor with 6-AWG wire is not a good idea. This conductor combination certainly does not satisfy the requirements for parallel conductors in 310.4.
You said that you have had trouble with open neutrals in the past, and that often more than one conductor was installed in a single terminal. If the 6-AWG conductor is added as directed by the inspector, make sure that both wires are connected to separate terminals. Individual terminals for grounded circuit conductors are required by 408.21. I suspect that many of the open neutrals mentioned in the question are the result of more than one wire in a single lug or terminal. I don't think I would question the inspector's interpretation any more, but I would make sure that each wire terminates in its own terminal or lug. EC
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.254.2132.