Article 225—Outside Branch Circuits and Feeders
Article 310—Conductors for General Wiring
Article 320—Armored Cable: Type AC
Article 430—Motors, Motor Circuits, and Controllers
Article 518—Places of Assembly
Article 695—Fire Pumps Insulating bushings and MC cable
Q: Are insulating bushings required to be installed in metal clad (Type MC) cable containing No. 12 copper conductors just the same as armored cable? I have noticed a package of anti-short bushings are included with some types of metal clad cable, but can’t find anything in the National Electrical Code (NEC) that requires their use. Are anti-short bushings required with MC cable?
A: An insulating bushing or equivalent protection must be provided for all armored cable terminations. This requirement appears in 320.40 of the 2002 NEC. However, a similar requirement does not appear in Article 330—Metal Clad Cable: Type MC. Manufacturers of metal clad cable supply anti-short bushings with each coil or reel of interlocked armor Type MC cable to enhance the safety of the electrical wiring installation. Along with the package of bushings is a recommendation that they be used at all cable terminations. Although recommendations cannot be enforced by the electrical inspector, he/she should suggest to the installing electrician that they be used as recommended.
Cable connectors with insulated throats that are suitable for use with either Type AC or Type MC cable are a satisfactory substitute for the insulating bushings. If this type of fitting is used, the cable should be fully inserted into the connector so that the cut end of the armor is in contact with the insulating bushing in the connector.
On-site generator wiring
Q: Are the conductors supplied from an on-site generator treated as service conductors? Are the requirements in Article 230 applicable to these conductors?
A: No, they are not. A few definitions in Article 100 clear up this issue. First is the definition of a service: “The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served.” Notice the words “… serving utility… .”
The second definition is of a service point: “The point of connection between the facilities of the serving utility and the premises wiring.”
And the service conductors are defined as: “The conductors from the service point to the service disconnecting means.”
The conductors supplied from a generator are either feeder or branch circuit conductors. They must be sized at 115 percent of the nameplate current rating of the generator. Other requirements for the generator are found in Article 445. If the generator is installed outdoors, some of the rules in Article 225 apply, especially those that deal with disconnecting means, such as Section 225.31 through 225.39.
Assuming that this generator is being installed at the owner’s request, it is assumed to be an optional standby system and must comply with the rules in Article 702—Optional Standby Systems. Transfer equipment must be provided to comply with 702.6 to prevent inadvertent interconnection of normal and alternate sources of power. If the transfer equipment switches the neutral or grounded circuit conductor a separately derived system is created and the requirements in 250.30 come into play.
As can be seen from this short discussion, many rules are involved in the installation of an on-site generator that is connected to a premise wiring system.
Temporary wiring in exhibition halls
Q: Does the NEC allow the use of cable trays for support of flexible cords such as Type SO or Type SJT for temporary wiring that supplies exhibit booths for a trade show? The building is approximately 200 feet by 600 feet and must comply with the rules in Article 518—Places of Assembly for electric wiring.
A: Temporary wiring for an exhibition hall must comply with 518.3(B). This part allows flexible cords and cables approved for hard usage or extra-hard usage to be laid on floors where protected from contact by the general public. Also, the exception allows the installation of hard and extra-hard usage cords and cable in cable trays where supervision and maintenance ensure that only qualified persons will service the installation. However, the cable trays cannot be used for any other wiring; flexible cords and cable must be installed in a single layer; a permanent sign must be attached to the cable tray at intervals not exceeding 25 feet that reads: “CABLE TRAY FOR TEMPORARY WIRING ONLY.”
Table 400.4 indicates that Type SJT flexible cord is listed for “Hard Usage” and Type SO is classified for “Extra Hard Usage.” Therefore, these types of cords or cables may be installed in cable trays for temporary wiring in large exhibition halls under the requirements of the Exception in 518.3(B).
Q: Should the horsepower rating or the nameplate full-load current be used to determine the branch circuit conductor ampacity, overload protection, and rating of the disconnecting means for a listed motor-operated appliance?
A: The full-load current marked on the nameplate should be used. Exception No. 3 to 430.6(A)(1) indicates that the full-load current marked on the nameplate shall be used instead of the horsepower rating shown on the nameplate to determine the rating of the disconnecting means, the size of the branch circuit conductors, the branch circuit short-circuit and ground-fault protection, and any separate overload protection.
Let’s assume we have a motor-operated appliance with a nameplate full-load current of 17 amperes, 230 volts, single phase, and 5 horsepower. Referring to the exception, the nameplate full-load current is used to obtain a branch circuit conductor size. According to 430.22(A), the branch circuit conductor ampacity cannot be less than 125 percent of full-load current. This results in a conductor ampacity of (17 x 1.25) 21. According to Table 310.16 and the asterisk note, No. 12 copper wire is adequate. The motor overload device, if required, could be set to trip at 19.55 amperes or higher, depending on the service factor or temperature rise marked on the motor nameplate. Thirty-ampere dual-element time-delay fuses in a 30-ampere safety switch with at least a 3-horsepower rating satisfy 430.109(A)(1). If the 5-horsepower rating was used, the branch circuit conductors would have to be increased to No. 8 copper with 60 degrees Celsius insulation. The disconnecting means would have to be increased to 60 amperes with 50-ampere or lower ampere rating dual-element time-delay fuses, and the switch would need to have at least a 5-horsepower rating.
It is my understanding that nationally recognized testing laboratories usually pay no attention to the horsepower rating on the nameplate of a motor-driven appliance, but do make sure that the full-load current shown on the nameplate is accurate. For this reason, the current is used to size all of the components in the circuit. Inflated horsepower ratings are used for marketing purposes and are not realistic.
Reliable electric supply for a fire pump
Q: In the 2002 edition of the NEC, 695.3 requires electric motor-driven fire pumps to have a reliable source of power. Which of the following jurisdictions determines the reliability of a utility power source? Is it the chief building inspector, chief electrical inspector, fire-prevention inspector, or anyone else?
A: Article 695 in the NEC is based on requirements in NFPA 20—Standard for the Installation of Stationary Pumps for Fire Protection. Therefore, the agency that adopts and enforces NFPA 20 is usually the Authority Having Jurisdiction (AHJ) for deciding on approval of the installation and equipment. This agency also determines the reliability of the power supply for the fire pump motor.
In some jurisdictions, the state fire marshal is the AHJ; in others, it is the superintendent of the fire department or the chief of the fire prevention bureaus.
The electrical inspector is responsible for the installation under the rules in Article 695 but he must also work with the AHJ where the requirements are based on reliability, approval, and judgment. NFPA 20 is the parent document for Article 695; therefore, the agency that adopts and enforces NFPA 20 is usually the AHJ. The electrical inspector should cooperate with the AHJ and allow him/her to make decisions that are beyond the scope of the inspector’s authority, such as reliability of the electrical service, approval of feeder sources as permitted in 695.3(B)(2), and the location of a fire pump transfer switch and fire pump controller in relation to the location of the fire pump motor as outlined in 695.12(A).
Q: Is a Code-compliant grounding connection made between the metal yoke of a receptacle (15 amperes, 125 volt) and a metal box if the fiber or paper device screw retainers are not removed before installation?
A: If the receptacle is to be installed on a surface-mounted metal box without a bonding jumper provided between the box and the grounding terminal on the receptacle, the answer is no. For connecting a receptacle grounding terminal to a surface mounted metal box, 250.146(A) applies. To eliminate the bonding jumper, direct metal-to-metal contact between the device yoke and metal box is required. This means that the nonconductive retainers on the mounting screws must be removed before installing the receptacles. EC
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at (504) 254-2132.