Article 110—Requirements for Electrical Installations
Article 240—Overcurrent Protection
Article 384—Switchboards and Panelboards
Article 450—Transformers and Transformer Vaults
Article 680—Swimming Pools, Fountains, and Similar Installations
Q: Is overcurrent protection required on the secondary side of a 45kVA, three-phase, 480-volt delta to three-phase, four-wire 208Y/120 volt dry-type transformer? The transformer is protected on the primary side by a three-pole, 70-ampere circuit breaker. The secondary conductors are four No. 1/0 Type THWN copper conductors that are connected to the secondary terminals of the transformer and the main lugs in a 150-ampere, 30-circuit panel board. Is a main required in the panel board?
A: The primary full-load current is (45,000 ÷ 480 x 1.73 =) 54 amperes. According to Section and Table 450-3(b), secondary overcurrent protection for the transformer is not required where the primary overcurrent protection does not exceed 125 percent of rated primary current. However, Note 1 to the Table allows an increase to the next larger standard rating of fuse or circuit breaker where 125 percent of the rated primary current does not correspond to a standard fuse or circuit breaker rating.
One hundred and twenty-five percent of 54 amperes results in a current of 67.5 amperes. According to National Electrical Code (NEC) Section 240-6, the next higher ampere rating above 67 is 70 amperes. Therefore, a 70-ampere circuit breaker and No. 4 Type THWN copper conductors are satisfactory. It is possible to use three No. 6 Type THWN copper conductors to supply the primary if the circuit breaker terminals and transformer primary terminals are marked for 75 degrees Celsius. (See Section 110-114(c).)
Overcurrent protection for the secondary winding of the transformer is not required, but the secondary conductors must have overcurrent protection as required by various Sections in Article 240—Overcurrent Protection. Section 240-21(c) covers requirements for overcurrent protection of secondary conductors. For indoor installations, overcurrent protection must be provided at the transformer, within 10 feet of the transformer, or for industrial installations within 25 feet of the transformer.
A 150-ampere circuit breaker must be installed at the transformer or within 10 feet to protect the No. 1/0 Type THWN conductors. For industrial applications that meet the requirements in part (3) of Section 240-21(c), the length of the secondary conductors may be increased to 25 feet before providing overcurrent protection.
I assume this is a lighting and appliance panelboard as described in Section 384-14(a). That is, more than 10 percent of its overcurrent devices supply lighting and appliance branch circuits. A lighting and appliance branch circuit is defined as one that has a connection to the neutral and overcurrent protection rated 30 amperes or less. This explanation is given because Section 384-16(a) requires overcurrent protection for this type of panelboard and the overcurrent protection cannot exceed the ampere rating of the panelboard.
The 150-ampere circuit breaker in the feeder that supplies the panelboard satisfies this rule and also protects the secondary conductors.
Access to working space
Q: In order to get to the service switchboard in a mechanical equipment room you have to squeeze through a 16-inch opening between a water heater and a boiler. There is adequate working space around the switchboard after getting through the narrow passageway. Does this arrangement satisfy the NEC? Is this size opening satisfactory for accessibility to the grounding electrode conductor connection to the water pipe and ground rods?
A: Although 16 inches of space between two heaters may be acceptable to a thin (skinny) electrical inspector, it would be inadequate for an average-sized one.
In my opinion, the clear space between the boiler and water heater should be at least 24 inches wide. If the surfaces of the water heater and boiler are hot, the clear space will have to be increased to 30 to 36 inches.
I selected 24 inches as the minimum width because this dimension is used in Section 110-26(c) for equipment rated 1,200 amperes or more and over six feet wide. For lower-amperage equipment, the requirement reads: “At least one entrance of sufficient area shall be provided to give access to the working space about electrical equipment.”
The grounding electrode conductor connection to the grounding electrode is required to be accessible by Section 250-68(a). I think that the 16-inch opening satisfies this requirement for accessibility, but this is no longer an issue if the opening is increased to 24 or more inches.
Q: Why is a neutral bar kit required for a 480-volt, three-phase service that supplies only motor loads? There is no neutral load.
A: If the power supply from the utility is 480-volts, three-phase, three-wire, ungrounded, there is no need for the neutral bar kit. For an ungrounded system, Section 250-24(d) only requires that the grounding electrode conductor that is connected to the grounding electrode to be connected to a metal enclosure for the service conductors at any accessible point from the load end of the service drop or lateral to the service disconnecting means.
However, the utility supply is probably from a transformer with a three-phase, four-wire, 480Y/277 secondary with the center tap grounded. If this is the case, a neutral (four wires) must be brought in to the service equipment. This requirement appears in Section 250-24(b).
The neutral conductor must be in the same raceway or cable assembly as the phase conductors, and cannot be smaller than shown in Table 250-66. Additionally, where the phase conductors are larger than 1,100kcmil copper or 1,750kcmil aluminum, the neutral conductor must have a cross sectional area that is at least equal to 12.5 percent of the largest service-entrance phase conductor.
The neutral bus must be installed in the service disconnect, and the neutral conductor connected to the bus. The main bonding jumper and grounding electrode conductor is then installed in the usual manner. A grounding electrode system meeting the requirements of Part C of Article 250 must be connected to the grounding electrode conductor.
The neutral conductor does not have to extend beyond the service equipment. However, it is necessary to bring the neutral conductor into the service equipment and install the main bonding jumper to establish a low impedance path to clear any ground fault that may occur on the premises wiring system. This is the reason why the NEC requires that the grounded conductor of a supply system be brought in to the service equipment.
Lighting fixtures above a pool
Q: If lighting fixtures are installed more than 12 feet above the maximum water level for an indoor swimming pool, are totally enclosed lighting fixtures required? Is ground fault circuit interrupter (GFCI) protection required for the fixtures?
A: The answer to both questions is no. Section 680-6(b)(1) and (3) cover requirements for lighting fixtures installed above swimming pools. Part (1) covers outdoor pools and requires lighting fixtures and paddle fans to be located at least 12 feet above the maximum water level in the pool. Fixtures mounted at this height are not required to be totally enclosed, or protected by a GFCI. Where this height cannot be obtained for an indoor pool, it is permitted to be reduced to 7 feet 6 inches above the maximum water level. Because of this reduction in height, the lighting fixtures must be totally enclosed and protected by GFCIs.
Q: Does the Code allow the use of a double-pole AC general-use snap switch for control of a 120-volt receptacle circuit and a 277-volt fluorescent lighting circuit?
A: Generally, no. Although not specifically covered in the NEC, Section 380-8(b) limits the voltage between switches grouped in a single enclosure to 300 volts, and the voltage across these two circuits could exceed that value.
The General Information for Electrical Equipment Directory (White Book), published by Underwriters Laboratories, Inc. contains this information under the category “Snap Switches (WJQR): Multipole, General-Use Snap Switches have not be investigated for more than single circuit operation unless marked ‘2-circuit’ or ‘3-circuit.’”
To comply with Section 110-3(b), snap switches must be installed in accordance with instructions furnished with the switches. These instructions will probably not recognize or allow the proposed installation.
Branch circuits for elevators
Q: Are there any requirements in the NEC for separate circuits for elevator car lighting, machine room lighting, and hoistway pit lighting?
A: Yes. There are rules in Article 620 for lighting in these locations. Section 620-22 requires a separate branch circuit to supply car lights, receptacles, auxiliary lighting power source, and ventilation on each elevator car. A separate branch circuit is required to supply lighting and receptacles in the elevator machine room or machine space. A dedicated branch circuit is required for the air conditioning and heating units on each elevator car.
A separate branch circuit is required for hoistway pit lighting and receptacles. These requirements appear in Sections 620-22, -23, and -24.
All 15- and 20-ampere, 125-volt receptacles installed in hoistway pits, on the tops of elevator cars, and in machine rooms must have GFCI protection. The lighting at these locations cannot be connected to the load side of a GFCI.
A single disconnecting means is required for each elevator car to control car lighting, receptacles, and ventilation equipment.
This disconnecting means must be located in the machine room and be capable of being locked in the open position. This requirement appears in Section 620-53. A similar rule in Section 620-54 applies to heating, ventilation, and air conditioning (HVAC) equipment for the elevator car.
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at (504) 254-2132.