Article 110 Requirements for Electrical Installations
Article 230 Services
Article 240 Overcurrent Protection
Article 250 Grounding
Article 310 Conductors for General Wiring
Article 400 Flexible Cords and Cables
Article 406 Receptacles, Cord Connectors and Attachment Plugs (Caps)
Article 430 Motors, Motor Circuits and Controllers
Article 511 Commercial Garages, Repair and Storage
Article 695 Fire Pumps
Entrance doors for large electrical equipment
Q:In an electrical equipment room there is a 1,600A service main that is 3 feet wide. Next to this disconnect switch is a motor control center that is supplied from 3 feet of 1,600A bus duct. The motor control center is 5 feet wide. This arrangement of service switch, bus duct and motor control center occupies about 11 feet of wall space. The working space in front of this equipment is a little over 3 feet. The voltage is 208Y/120V. Are two doors required for this electrical equipment room to comply with the NEC?
A:No, two doors are not required by the wording in 110.26(C)(2), but two doors meeting the requirements of this rule should be provided where it can be accomplished without major change to the layout of the equipment room.
My reason for the “No” answer is there are two separate electrical enclosures, and the words in 110.26(C)(2) imply that the maximum width of a single enclosure must be at least 6 feet.
Where only one entrance to the electrical equipment room is required by the authority having jurisdiction (AHJ), the direction of opening of the door and special door opening hardware are not required as indicated in (C)(2) of 110.26.
Lubrication rooms with pits
Q:What is the classification of a lubrication room where oil changes are made, but there is no provision for dispensing gasoline or other types of vehicle fuels?
A:Any pit that is used for lubrication or oil changes for automobiles or trucks that contain gasoline is classified as a Class I, Division 2 location. This includes the entire volume of the pit, the area above the pit to a height of 18 inches, and extending for a distance of 3 feet horizontally from the edges of the pit. Although 511.3(B)(3) considers a pit to be a Class I Division 2 area, Exception No. 2 allows a pit to be classified in accordance with Table 514.3(B)(1), where lubrication and service rooms are located in buildings that do not have facilities for dispensing flammable liquids.
A pit may be unclassified (an ordinary location) where exhaust ventilation that provides at least one cubic foot per minute of exhaust air for each square foot of pit floor area during all times that the building is occupied or when vehicles are parked over the pit. The intake for the exhaust air must be located within 12 inches of the pit floor.
For a pit that has a floor area of 4 feet by 12 feet, an exhaust fan must have a minimum capacity of 48 cubic feet per minute, and must operate at all times whenever a vehicle is parked over the pit.
Number of service disconnects
Q:I added a 480V service with three service disconnects to an existing building that is supplied by a 208Y/120V service with five disconnect switches. Both services are located in the same area. The inspector is concerned because there are more than six service disconnects at one location. I told him that the Code allows a maximum of six disconnect switches for each service. Am I right or wrong?
A:You are right. More than one service to a building is allowed by 230.2(D). Although 230.71(A) limits the number of service disconnects to six at each location, this number applies to each service; therefore, 12 service disconnecting means are permitted; six on each service.
Disconnecting means for each service must be identified to indicate the area served. A plaque or directory must be provided at each service to specify the location of all power sources supplying the building. This requirement is in 230.2(E).
Nonmetallic sheathed cable feeder for one-family dwelling
Q:Is it permissible to use NM cable containing 4 AWG copper conductors as a feeder for a 100A service supplying a one-family dwelling unit? The service-entrance conductors are 4 AWG copper as permitted by Table 310-15(B)(6).
A:The referenced Table permits 4 AWG copper conductors to supply a 100A, 3-wire, single-phase, 120/240V service for dwelling units.
Since there are no insulation temperature ratings associated with Table 310.15(B)(6), 4 AWG copper conductors with Type TW insulation can be used for a 100A service for dwelling units. And 310.6(B)(6) allows the feeder conductors to be the same size as the service-entrance conductors. This is the way part of 310.6(B)(6) reads: “The feeder conductors to a dwelling unit shall not be required to be larger than their service-entrance conductors.”
Although Type NM-B nonmetallic sheathed cable has conductors with 90 C insulation, its ampacity is limited to 60 C. Still, it may be used as the feeder for a dwelling unit with a 100A, 3-wire, single-phase service with 4 AWG service-entrance conductors.
Transformer size for a fire pump motor
Q:What size (kVA) transformer is needed for a 50hp, 230V, 3-phase fire pump motor? The supply voltage to the building is 480Y/277V.
A:The full-load current for a 50hp, 230V, 3-phase induction motor is listed in Table 430.150 as 130A. Because 695.5(B) does not allow secondary overcurrent protection for a transformer that supplies a fire pump, and the primary overcurrent protection must be large enough to allow locked-rotor current of the fire pump motor to flow indefinitely, the transformer must be oversized.
For a 50hp motor, the locked rotor current obtained from Table 430.151(B) is 725A. The primary current under locked rotor condition is 347.4A (725 x 230/480). Since transformer primary overcurrent protection is permitted to be 125 percent of rated transformer primary current, the rated primary current of the transformer is approximately 275A. Therefore, a 225kVA, 3-phase, 480V transformer with 350A overcurrent protection satisfies the requirement for transformer overcurrent protection on the primary only and provides locked rotor current indefinitely for the 50hp fire-pump motor.
Tap to a tap
Q:I recently increased the service capacity from 200 to 400A in a grocery store. The service entrance conductors are 500 kcmil copper with Type THWN insulation. From a junction box about 3 feet below the 400A fused switch, I ran 3/0 AWG copper conductors with Type THWN insulation to a new panelboard with a 200A main circuit breaker. The length of each conductor is 25 feet or less. The supply wires from the 400A switch to the junction box are 500 kcmil copper with Type THWN insulation. At this point, I made a tap to supply the new panelboard and reconnected the existing panelboard. The local inspector turned the job down with the comment that I cannot tap-a-tap. I do not think that I made a tap to a tap. What is your opinion?
A:You cannot tap-a-tap because of 240.21, but you are not in violation of the part of 240.21 which prohibits this. The tap conductors are not more than 25 feet long; therefore, you must comply with part (B)(2) of 240.21. To comply with part (B)(2), the tap conductors cannot be more than 25 feet long, the ampacity of the tap conductors must be at least one-third the ampere rating of the overcurrent device ahead of the tap, and the tap conductors must be protected from physical damage or they must be installed in a raceway.
The 500 kcmil conductors connected to the load side of the 400A disconnect switch are not tap conductors. Here is the definition as it appears in 240.21: “Tap Conductors. As used in this Article, a tap conductor is defined as a conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4.” Because the 500 kcmil conductors that are connected to the load side of the 400A switch have an ampacity of 380, they are protected by the 400A fuses and they are not tap conductors. The tap begins at the point where the 3/0 AWG conductors are spliced to the 500 kcmil conductors. Although the length of each phase conductor and neutral are more than 25 feet long, it is stated in the question that the 3/0 AWG conductors are not longer than 25 feet.
Assuming that the tap conductors (3/0 AWG) are protected from physical damage, the requirements for a 25-foot tap as outlined in 240.21(B)(2) are satisfied.
Receptacles above suspended ceilings
Q:Does the National Electrical Code permit 125V, 15A receptacles to be installed above a suspended ceiling in a bank for connection of surveillance cameras? It is intended to install the cameras below the ceiling, notch the lay-in ceiling tiles and plug each camera into the receptacle located above the camera.
A:Although there is nothing in Article 406—Receptacles, Cord Connectors, and Attachment Plugs (Caps) that would prevent such an installation, a receptacle above a suspended ceiling serves no purpose. This is because 400.8 prohibits flexible cords above suspended ceilings in two places. Item 2 of 400.8 prohibits flexible cords where run through holes in walls, structural ceilings, suspended ceilings, dropped ceilings or floors. Item 5 has a similar restriction and does not allow flexible cords to be concealed by walls, floors or ceilings, or located above suspended or dropped ceilings.
Restrictions on the use of flexible cords in suspended ceilings was first introduced in the 1999 NEC. Part of the reason for this revision is the language that has been in the Code for may years in 410.30(C) that permits cord-and-plug connection of electric-discharge luminaires (fixtures) where the flexible cord is visible for its entire length. EC
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720.