Article 110—Requirements for Electrical Installations
Article 240—Overcurrent Protection
Article 300—Wiring Methods
Article 384—Switchboards and Panelboards
Article 620—Elevators, Dumbwaiters, Escalators, Moving Walks, Wheelchair Lifts, and Stairway Chair Lifts
Article 685—Integrated Electrical Systems
Article 700—Emergency Systems
NFPA 20—1999 Standard for the Installation of Stationary Pumps for Fire Protection is also mentioned.
Conduits in bottomless switchboards
Q: Does Section 300-10 permit stub-ups of metal conduits in the bottom of bottomless switchboards without physically attaching the conduits to the switchboard enclosure? If this method is permitted, is there any limit as to how far the conduits can rise above the concrete pad that supports the switchboard? Would your answer be different if non-metallic raceways were used?
A: Although it appears that raceways must be connected to all boxes, fittings, and cabinets to comply with Section 300-10, the last sentence in the section does not require this where permitted elsewhere in the National Electrical Code (NEC). Conduits are permitted to enter the bottom of a switchboard where they meet the requirements of Section 384-10.
This section limits the length of conduits and their end fittings to 3 inches above the bottom of the switchboard. There is also a Table that shows the minimum spacing between bus bars and the bottom of the switchboard enclosure. For uninsulated bus bars, the minimum space is 10 inches. Conduit stub-ups must be arranged to allow sufficient bending space to terminate the contained conductors.
Metallic conduits that contain service lateral conductors or underground service entrance conductors must be grounded to comply with Section 250-80. The size of the bonding jumper is given in Table 250-66, where service-entrance conductors are in parallel in separate raceways, the size of the bonding jumper is based on the size of the conductors in each raceway provided that each raceway is individually bonded to the switchboard grounding bus.
If the bonding jumper is daisy-chained from one conduit to the other, the bonding jumper must be sized for the total cross-sectional area of one of the phase conductors. This means that the bonding jumper must be sized to 12.5 percent of the largest phase conductor if the total area exceeds 1,100kcmil copper or 1,750 kcmil aluminum.
Bonding of metallic feeder conduits that enter the bottom of the switchboard must comply with Section 250-102(d). The bonding jumper’s size is taken from Table 250-122.
Section 384-10 height restrictions apply to nonmetallic conduits, but the bonding and grounding requirements do not.
Connecting electric drive fire pumps
Q: A fire sprinkler system is to be installed in an existing four-story office building. Is it permissible to make a tap ahead of the service disconnecting means in a current transformer (CT) enclosure that is part of the service switchboard? In your opinion, does such a connection meet the requirements and intent of Section 695-3(a)(1)? Is a limited service fire pump controller acceptable for a 30-horsepower fire pump motor?
A: Where the CT enclosure has a separate barrier (metal on all four sides); there are no switches or circuit breakers in the compartment with the CTs, the CT section is at the end of the switchboard, and there is nothing in the enclosure with the CTs except a set of service-entrance conductors and a set of load conductors that feed the line bus in the switchboard.
I would accept this method of making a connection ahead of the service disconnecting means and consider this arrangement as meeting the intent of Section 695-3(a)(1).
Some utility companies do not allow more than one set of service-entrance conductors in a CT enclosure; therefore, check with the distribution and metering departments before making the connection.
Limited service fire pump controllers are recognized in NFPA 20-1999 Standard for the Installation of Stationary Pumps for Fire Protection. The maximum horsepower rating for a limited service controller is 30. Its use is subject to approval by the authority having jurisdiction. This authority may be vested in the State Fire Marshall or the Building Official.
These controllers are generally defined in Section 7.7 of NFPA 20 as automatic controllers for across-the-line starting of squirrel-cage motors rated 30-horsepower or less. They do not have an isolating switch, and locked rotor overcurrent protection is provided by an inverse-time nonadjustable circuit breaker that has a rating between 150 and 250 percent of motor full-load current.
Considerable savings can be realized by using a limited service controller, but remember that the use of a limited service fire pump controller must be approved by the authority having jurisdiction, and he/she may require a fully rated controller except for special situations.
Series rated and fully rated
Q: Will you explain the advantages and disadvantages of a Series-Rated Distribution System versus a Fully Rated System?
A: A series-rated system is generally described as one that has a circuit breaker with a current interrupting rating that is lower than the available fault current, but is protected by a line side overcurrent device that has an interrupting rating that is greater than the maximum fault current at the point of application. In simple language, the line side overcurrent device opens fast enough to protect the load side circuit breaker from overcurrent that is in excess of its interrupting rating.
This is in contrast to a fully rated system in which each overcurrent protective device has adequate interrupting rating to open the circuit without depending on any help from a line side overcurrent device.
In a series-rated system, overcurrent protective devices must be used that have been tested together. This is part of the requirement as it appears in Section 110-22: “Where circuit breakers or fuses are applied in compliance with the series combination rating marked on the equipment by the manufacturer, the equipment enclosure(s) shall be legibly marked in the field to indicate the equipment has been applied with a series combination rating.” The marking shall state the following:
“CAUTION: SERIES COMBINATION SYSTEM RATED ___ AMPERES. IDENTIFIED REPLACEMENT COMPONENTS REQUIRED.”
The blank space must be filled in with the current interrupting rating of the series-rated system.
It should be noted that recognized independent testing laboratory certification is required for series-rated systems, and these series-rated systems cannot be used in some applications.
For example, according to Section 240-86(b), series-rated systems cannot be used where the sum of the full load currents of motors exceeds 1 percent of the interrupting rating of the lower-rated circuit breaker. This means that the full-load currents of motors connected to the line side of a circuit breaker with a 10,000 ampere interrupting rating cannot exceed 100 amperes.
Series-rated service overcurrent protection and feeder overcurrent protection are not permitted where ground-fault protection is required for a service to a health care facility. In fact, Section 517-17(b) requires a minimum six-cycle separation between the service and feeder ground-fault tripping bands to assure selectivity.
Series-rated systems are not permitted on some elevator installations. This requirement appears in Section 620-62: “Selective Coordination, where more than one driving machine disconnecting means is supplied by a single feeder, the overcurrent protective devices in each disconnecting means shall be selectively coordinated with any other supply side overcurrent protective devices.” Series- rated systems may not be appropriate for Integrated Electrical Systems—Article 685 where an orderly shutdown is required for safety.
Finally, a Fine Print Note (FPN) in Section 700-25 points out the fact that selective coordination of overcurrent devices in an emergency system will increase system reliability.
The advantage of a series-rated system over a fully rated system is cost. The installed cost for a series-rated system is less than for a fully rated system. However, a fully rated system can be selectively coordinated and limit an outage to a small part of the electrical system.
Single-pole circuit breakers supplying 240-volt loads
Q: Is a handle tie required on two single-pole, 30-ampere circuit breakers that supply a water heater? Is a two-pole, common trip circuit breaker required for this application? The heater operates at 240 volts, single-phase and a separate disconnect is installed at the heater.
A: The NEC does not require a two-pole common trip circuit breaker. Section 240-20(b)(2) permits single-pole circuit breakers with handle ties to supply line-to-line single-phase loads. Also, a disconnect within sight of the water heater is not required for a heater with a thermostat that has a marked “off” position. Section 422-33 allows a unit switch with an off position to serve as one of the disconnecting means where the other disconnecting means is within the dwelling unit or on the same floor of the dwelling unit in multifamily dwellings.
For two-family dwellings, the other disconnecting means is permitted to be inside or outside of the dwelling. The service disconnect is recognized as the other disconnecting means in one-family dwellings. These options appear in Section 422-33(a), (b), and (c).
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at (504) 254-2132.