Article 90 Introduction
Article 110 Requirements for Electrical Installations
Article 250 Grounding
Article 404 Switches
Article 430 Motors, Motor Circuits and Controllers
Article 440 Air Conditioning and Refrigerating Equipment
Routing grounding electrode conductor
Q: We have to install a 50 kVA, singlephase, dry-type transformer (480 volts to 120/240 volts) in an area that is about 45 feet from the service switchboard. Is it permissible to include the grounding electrode conductor for the secondary neutral with the 480-volt supply conductors? The wiring method is metal conduit and the circuit originates at the service distribution panel. Secondary conductors are 3/0 Type THWN copper and terminate in a lighting and appliance panelboard with a 200-ampere main circuit breaker.
A: Yes, the grounding electrode conductor can be installed in the same raceway as the primary supply conductors. Here are the rules that apply: 250.26; 250.28; 250.30; 250.64; 250.66; 250.70; 250.92; 250.97; 250.104.
The neutral conductor of a single-phase, three-wire system is required to be grounded by 250.26. The grounding electrode conductor cannot be smaller than 4 AWG copper if the grounding electrode for the 480Y/277 volt service is a metal water pipe, or effectively grounded metal frame of the building. The reference for this requirement is 250.30(A)(2). An exception to 250.30(A)(4) allows the grounding electrode conductor for the separately derived system to terminate on the equipment ground bus in the service equipment provided that it is of sufficient size for the separately derived system.
It may be necessary to bond the grounded conductor of the separately derived system to the interior metal water piping at the nearest available location. Part (4) of 250.104 and the exception provide additional information.
A bonding jumper not smaller than 4 AWG copper or 2 AWG aluminum must be installed from the neutral terminal in the transformer to the ground bus in the lighting and appliance panelboard. The secondary neutral cannot be grounded in the panelboard. The neutral bus in the panel must be insulated from the enclosure.
Bonding metal raceways that enclose grounding electrode conductors are covered by 250.64(E). And bonding metal raceways that contain conductors operating at over 250 volts to ground are found in 250.97.
Toggle switches as motor disconnects
Q: Are toggle switches that are usually installed flush mounted in device boxes for control of lighting fixtures suitable for use as disconnecting means for motor operated appliances such as kitchen waste disposers, whirlpools, swimming pool pump motors and similar motor-operated equipment?
A: Yes, they are under limited conditions. These devices are referred to as “Snap Switches” in the NEC and their permitted uses are described in 404.14. AC general use snap switches are suitable for use on ac circuits only. They are tested for control of resistive, inductive, and tungsten-filament lamp loads up to the ampere rating marked on the switch. They are also tested at 80 percent of their ampere rating for motor loads. This means that a 15-ampere, single-pole, 125-volt switch may serve as the disconnecting means for a single-phase motor with a full load current of 12 amperes or less. The full load current for a 1/2 horsepower, single-phase, 120-volt motor is 9.8 amperes according to Table 430.148. This is probably the largest motor that can be disconnected by this switch, although 430.109(C) allows motors of 2 horsepower or less and 300 volts or less to be controlled by properly rated AC general-use snap switches.
Another form of toggle switch is the AC-DC general use snap switch. This switch is suitable for use on both AC and DC circuits. Inductive loads including motor loads cannot exceed 50 percent of the ampere rating of the switch unless the switch is marked with a horsepower and voltage rating that exceeds the 50 percent limitation. A 15- ampere, 125-volt, single pole AC-DC snap switch cannot be used to control a motor with a full load current greater than 7.5 amperes, or about 1/3 horsepower, unless marked with a higher horsepower rating.
Disconnecting means for motors
Q: Am I allowed to use two three-pole unfused safety switches to meet the new requirement in Section 430.102(B)? The motor is three-phase, 208 volts, wye-delta start and is located about 250 feet from the motor control center.
A: This question is related to the requirement for a disconnecting means within sight of the motor, even though there is a disconnect capable of being locked in the off position that is within sight of the controller. The requirement for a disconnecting means within sight of the motor is new in 2002 edition of the NEC.
Two three-pole disconnects are not acceptable. To disconnect all of the motor leads, a six-pole disconnect is required because 430.103 says that no pole of the disconnecting means can operate independently. This is the way part of 430.103 reads: “The disconnecting means shall open all ungrounded supply conductors and shall be designed so that no pole can be operated independently.”
Six-pole safety switches are catalog items, but delivery may take some time. Some of these switches are provided with auxiliary contacts that open the starter control circuit before opening the power wiring to the motor.
Part-winding, wye-delta, and reduced voltage start motors may qualify under the exception to 430.102(B). Part (A) of the exception gives some relief to the requirement for a disconnect within sight of the motor and reads like this: “Where such a location of the disconnecting means is impracticable or introduces additional or increased hazards to persons or property.” The electrical inspector may rule that a within sight disconnecting means for a wye-delta start motor is not practical. Fine Print Note No. 1 gives some guidance for implementation of this requirement and the exception. According to the Fine Print Note, some examples of increased or additional hazards are motors in excess of 100 horsepower, multimotor equipment, submersible motors, motors associated with variable frequency drives and motors located in hazardous (classified) locations.
Location of service panelboard
Q: Am I permitted to install a service panelboard in a closet that is not to be used for storage and includes no combustible materials? The closet will have a sign on the door that says “Electric Equipment Not for Storage.”
A: Electrical equipment rooms are permitted to contain distribution panels, overcurrent devices and other electrical equipment. Clearances in front of and around the service panelboard must conform to 110.26. If the door to the electric closet is locked, the key should be in possession of the tenant(s). Working space in front of and on the sides of the panelboards must conform to 110.26(A). Part (B) requires that the working space not be used for storage, and temporary barriers may be necessary if the working space extends into a passageway while the service panelboard is being inspected or maintained.
Illumination must be provided for the working spaces and equipment. The minimum headroom for the working space is 6.5 feet or the height of the equipment if it is taller than 6.5 feet. Dedicated electrical space must extend from the floor to six feet above the panelboard or to the structural ceiling if it is lower. Nothing foreign to the electrical installation is permitted in this space.
These are the basic requirements for an electrical room that contains service equipment, switch boards, panelboards, or motor control centers. A thorough review of 110.26 will provide the information needed to evaluate the closet.
Room air conditioner requirements
Q: Our jurisdiction has been enforcing the 2002 edition of the NEC since January which brings up this question: Are all cord-and-plug connected window air conditioners that are being sold now required to have AFCI or LCDI protection? Is this requirement based on the date of manufacture of the air conditioner or the date the permit is issued?
A: The requirement for Leakage Current Detection and Interruption (LCDI) or Arc Fault Circuit Interrupter (AFCI) protection for single-phase, cord-and-plug-connected room air conditioners appears in 440.65 and is new in the 2002 NEC. It may be awhile before room air conditioners with this feature appear in appliance stores. This is recognized by a paragraph in 90.4, which reads: “This Code may require new products, constructions, or materials that may not yet be available at the time the Code is adopted. In such event, the authority having jurisdiction may permit the use of the products, constructions, or materials that comply with the most recent previous edition of this Code adopted by the jurisdiction.”
If, in your area, electrical products have to be tested and listed by a recognized independent testing laboratory, you will soon see room air conditioners with LCDI or AFCI protection in the supply cords; therefore, I would not be concerned about the date of manufacture of the air conditioner or the date that a permit was issued for the wiring.
Size of grounding electrode conductor
Q: What size grounding electrode conductor is required for a three-phase, 208Y/120-volt service that consists of seven sets of 500-kcmil aluminum conductors? Is a 3/0-AWF copper grounding electrode conductor adequate for this 2000-ampere service? What size conductor “copper or aluminum” should be used for the main bonding jumper?
A: Where the grounding electrode is ten-feet or more of buried metal pipe, or the metal frame of a building that is effectively grounded, or both, the grounding electrode conductor cannot be smaller than 3/0-AWG copper or 250-kcmil aluminum. The size of the grounding electrode conductor is obtained from Table 250.66. Where an aluminum grounding electrode conductor is used outdoors, it cannot be terminated within 18 inches of the earth. This restriction appears in 250.64(A).
The bonding jumper connects the service enclosure and equipment grounding bus to the circuit grounded conductor, and the size must comply with 250.28. For an aluminum main bonding jumper, Part (D) requires that the conductor not be smaller than 12.5 percent of the largest phase conductor. This results in an aluminum bonding jumper of (7 x 500 x 0.125) 437.5 kcmil. Therefore, the main bonding jumper must be at least 500-kcmil. If copper wire is used for the main bonding jumper, it must sized based on the assumed use of copper phase conductors with the same ampacity as the aluminum conductors. According to Table 310.16, 350-kcmil copper conductors with 75-degree Celsius insulation have the same ampacity as 500-kcmil aluminum conductors with 75-degree insulation. The calculations (7 x 350 x 0.125) indicate that a single 350-kcmil copper main bonding jumper is necessary. EC
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.254.2132.