Article 90 Introduction;
Article 110 Requirements for Electrical Installations;
Article 210 Branch Circuits;
Article 220 Branch-Circuit, Feeder and Service Calculations;
Article 250 Grounding;
Article 300 Wiring Methods;
Article 310 Conductors for General Wiring;
Article 400 Flexible Cords and Cables;
Article 518 Places of Assembly;
Article 760 Fire Alarm Systems. Burial depth for UF cable
Q: What is the minimum burial depth for UF cable that is used to supply outdoor lighting along the driveway for a one-family residence? The cable will be run under a concrete walkway, lawn area and driveway for a two-car garage.
A: Although Underground Feeder and Branch-Circuit Cable (Type UF) is not suitable for installation where it is subject to physical damage, if it is buried in accordance with the minimum burial depths mentioned in Table 300.5 and 300.5, you should have a Code-compliant installation.
The minimum burial depth under lawn areas is 24 inches. This dimension decreases to 18 inches under a concrete walkway and at the driveway. However, these burial depths can be decreased to 12 inches where the lighting branch circuit overcurrent protection is 15 or 20A, the voltage is 120 and the branch circuit is protected by a ground-fault circuit-interrupter. This reduction in burial depth is permitted under column 4 of Table 300.5.
Grounding parking lot lighting poles
Q:I have a job to install parking lot lighting on metal poles. The specifications call for ground rods at each pole. Are two ground rods required at each pole if the resistance to earth is more than 25 ohms? If two ground rods are driven at each pole, is an equipment-grounding conductor required to be run with the branch circuit conductors?
A:A properly sized equipment-grounding conductor must be run with the circuit conductors. It must be sized to comply with Table 250.122.
The driven rod and grounding electrode conductor at each lighting pole is classified as a supplementary grounding electrode in 250.54 and reads: “Supplementary Grounding Electrodes. Supplementary grounding electrodes shall be permitted to be connected to the equipment grounding conductors specified in 250.118 and shall not be required to comply with the electrode bonding requirements of 250.50 or 250.53(c) or the resistance requirements of 250.56, but the earth shall not be used as the sole equipment grounding conductor.” Notice that the length, diameter and type of material of the ground rod are not specified, neither is the size of the grounding electrode conductor. This information is usually mentioned in the specifications. Also notice that these supplementary grounding electrodes do not remove the requirement for the installation of an equipment grounding conductor along with the circuit conductors. Both of these grounding conductors terminate on the grounding terminal in the lighting pole.
These supplementary grounding electrodes and conductors are usually specified to limit the effects of lightning.
Dwelling unit bedroom branch circuits
Q:How many ceiling lighting fixtures (luminaires), receptacles and smoke detectors are permitted on a single 15A branch circuit that supplies residential bedrooms? Some electrical contractors are wiring as many as four bedrooms on a single 15A branch circuit because of the requirement for arc-fault circuit-interrupter protection in 210.12(B).
A:A 15A, 120V branch circuit can supply (15 x 120) 1,800VA. Since the minimum load permitted in Table 220.3(A) is 3VA per square foot, one 15A general purpose branch circuit is allowed to supply a 600 square foot floor area. This means that four 12-by-12 foot bedrooms could be supplied by a single 15A general purpose branch circuit. More than four could be served if they were smaller, and less if they were larger. The limiting factor is 600 square feet of floor area.
Although it is possible to wire four bedrooms on one branch circuit, there are some parts of the Code that mention the shortcomings of the Code about adequacy and efficiency. In 90.2(B) there is a sentence that says compliance with these standards may not provide an efficient, convenient, or adequate electrical installation. Connecting four bedrooms to a single AFCI will make it difficult and time-consuming to troubleshoot a failure, and all bedrooms will be without electric power until the source of trouble is located and corrected.
Abandoned fire alarm cables
Q:I have a job to install a new fire alarm system in an existing commercial building. Does the NEC have any requirements for removal of all unused cables in the furred down ceiling space?
A:Fire alarm cables that are no longer in use (abandoned) must be removed. There is a definition for Abandoned Fire Alarm Cable in 760.2: “Installed fire alarm cable that is not terminated at equipment other than a connector and not identified for future use with a tag.” Part (A) of 760.3 has the title: “Spread of Fire or Products of Combustion.” The sentence under this title says that accessible portions of abandoned fire alarm cables cannot remain in place.
In 760.71(A), there is a requirement to remove all abandoned fire alarm cables that are installed in ducts, plenums and other spaces used for environmental air. Part (B) requires the removal of abandoned cables installed in risers.
Generally, the accessible portions of cables that are not terminated in fire alarm equipment or tagged for future use must be removed.
Q:I have to install two, three-wire, 3-phase 480V branch circuits to supply two 30A, non-continuous loads near the ceiling of a boiler room where the temperature rises to 104 F. Both branch circuits can be installed with the same metal conduit, but I must use Type THW copper wire to comply with the specifications. What is the minimum size wire permitted for this?
A:Because the Ampacity Table for insulated conductors (Table 310.16) is based on not more than three current-carrying conductors in a raceway or cable assembly, and an ambient temperature of 30 C (86 F), the conductors in this question must be derated twice.
Adjustment factors for more than three conductors in a raceway are given in Table 310.15(B)(2)(a), and for six current-carrying conductors in a single raceway the contained conductors must be derated to 80 percent of the value shown in Table 310-16. Also a correction factor of 0.88 must be applied because of the elevated ambient temperature. This results in a conductor ampacity of (30 divided by 0.8) 37.5 for six conductors in a raceway, and (37.5 divided by 0.88) 43 as the final minimum ampacity. The second derating calculation is for the elevated ambient temperature. Although the ampacity calculations result in an ampacity of 42.6, the information in Annex 8 Examples allows dropping of any fractions that are less than 0.5 or adding a whole number for any fractions that are 0.5 or more. Therefore, the minimum conductor ampacity is 43A, and the conductor size is 8 AWG with Type THW insulation.
Additional items that must be checked are the wire terminals on the equipment at both the supply and the load. These must be identified as suitable for termination of 75 C insulated copper conductors.
Temporary wiring in assembly occupancies
Q:Are flexible cords and cables permitted to be installed in cable trays for temporary wiring in an exhibition hall that will accommodate more than 300 people?
A:It is assumed that this building is within the scope of Article 518—Places of Assembly. Flexible cords and cables approved for hard or extra-hard usage are permitted in exhibition halls for temporary use. Where placed in cable trays, there must be supervision and maintenance to ensure that only qualified persons will handle the flexible wiring in the cable tray and the tray is marked: “Cable Tray for Temporary Wiring Only” at intervals not exceeding 25 feet. This information is in the Exception to 518.3(B).
Type letters for hard usage and extra hard usage cords and cables can be found in Table 400.4.
Three-pole circuit breakers for use with single-phase 120/240V loads
Q:Are there any rules in the National Electrical Code that prohibit the use of a 3-pole circuit breaker on a 120/240V multiwire branch circuit?
A: I am not aware of any requirements that specifically prohibit the use of 3-phase circuit breakers for protection of single-phase loads. However, 90.7 and 110.3 require that electrical materials and equipment be installed in accordance with instructions supplied with the product by the manufacturer. And 110.3(B) requires that electrical equipment be installed to comply with any requirements that are included in the listing or labeling of the product.
These parts of the NEC suggest that the product directory published by Underwriters Laboratories Inc. be reviewed. In the General Information for Electrical Equipment Directory (White Book) on page 14 under the category “Circuit Breakers, Molded-Case and Circuit Breaker Enclosures (DIVQ)” the following sentence appears: “Three-pole circuit breakers are suitable for use only on three-phase systems unless marked to indicate otherwise.”
The answer to the question is no, unless the circuit breaker is marked. EC
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720.