Published In November 2001
CODE CITATIONS Article 110—Requirements for Electrical Installations Article 240—Overcurrent Protection Article 250—Grounding Article 450—Transformers and Transformer Vaults Working space around electrical equipment Q: Section 110-26 requires working space for electrical equipment rated 600 volts or less where it is necessary to service the equipment while it is energized. How flat is the standing area required to be? Is the surface in front of the electrical equipment required to be level and smooth? A: Since the rule requires working space in front of the equipment, the surface that the electrician must stand on should be relatively smooth and level. He or she should not have to hold on to something to maintain balance or avoid slipping. The electrical inspector who must make the decision should feel comfortable while standing in front of the electrical equipment. For indoor installations a floor that slopes to a floor drain should be satisfactory, and for outdoor installations a flat grass area should provide firm footing for an electrician. This is a judgment call for the authority having jurisdiction (AHJ) to make. Overcurrent protection for transformers and secondary conductors Q: Does this installation comply with the National Electrical Code (NEC)? A three-phase, 480-volt, delta-connected 45kVA transformer protected by a 70-ampere circuit breaker supplies two feeders not more than 25 feet in length. The secondary is 208/120 volts wye. Three No. 2s and one No. 6 neutral feed a 100-ampere power panel containing a 100-ampere main. The other feeder is four No. 6 copper conductors that supply a lighting panel with a 60-ampere main. Is secondary overcurrent protection required for the transformer? Do the two taps connected to the secondary comply with the NEC? A: To eliminate secondary overcurrent protection, the primary overcurrent device cannot exceed 125 percent of rated primary current. Rated primary current is (45,000 divided by 480 times 1.73) approximately 54 amperes. Multiplying this figure by 1.25 results in about 68 amperes. Note 1 to Table 450-3(b) allows an increase to the next-larger size fuse or circuit breaker. Therefore, a 70-ampere circuit breaker or fuses provides all of the protection that is needed for the transformer. Now that the requirements in Article 450 are satisfied, it is time to look at overcurrent protection for the secondary conductors. Tap conductors must comply with Section 240-21(b) (2) and (3). The maximum reflected current in the secondary of the transformer is (480 divided by 208 times 70) 161 amperes. The tap conductors must have an ampacity that is not less than one-third the current that can be supplied by the transformer. One-third of 161 amperes is 53 amperes. Both sets of tap conductors have ampacities that are greater than 53 amperes. If the taps meet the other requirements in Section 240-21(b) for 25-foot taps, then the installation satisfies the NEC. Grounding separately derived systems Q: Grounding of a separately derived system, such as a transformer 480Y/277 to 208Y/120, requires a grounding electrode as provided for in Section 250-30(3). What would be wrong with running an unbroken grounding electrode conductor (sized per 250-66) back to the main service with the phase conductors and landing it on the equipment ground bar. The ground bar is properly sized; the neutral, building steel, and water electrodes are all tied together at this point; and a main bonding jumper is provided. My understanding has always been that all grounding electrodes at a structure are to be bonded together to form one low-impedance path. Effectively grounded structural steel has not usually been a problem, but water pipe due to future maintenance and modification is questionable, short of connecting to the first 5 feet. At this point, you are doing just about the same thing that I am proposing. What is your opinion? A: This method of grounding a separately derived system is not recognized in Section 250-30(a)(3) of the 1999 edition of the NEC, where an effectively grounded building structural metal member or the first 5 feet of grounded metal water pipe is available because part of (a)(3) reads: “The grounding electrode shall be the nearest one of the following:” structural metal and water pipe are then mentioned. What you suggest has been submitted as a proposal to change the 1999 NEC. The proposal allows a No. 3/0 copper or 250 kcmil aluminum continuous grounding electrode conductor to be run through the building. Grounding electrode conductors from separately derived systems could than be connected to this continuous grounding electrode by irreversible compression connectors or exothermic welding. This seems to be a good idea. The proposal was modified and expanded during the Public Comment period. Two significant changes include the addition of the words “or structure” after “building” and removal of the minimum size of the continuous grounding electrode conductor. Adding the word structure makes this change applicable to both buildings and structures. By removing the grounding electrode conductor size, the revision allows the continuous grounding electrode conductor to be sized as required by Section and Table 250-66. Although this change will allow a smaller continuous grounding electrode conductor, installing a No. 3/0 copper or 250 kcmil aluminum continuous grounding electrode conductor will allow all separately derived systems in the building or structure to be connected to this one continuous grounding electrode conductor. Where there are more than three or four separately derived systems, one No. 3/0 copper or 250 kcmil aluminum continuous grounding electrode conductor may be more economical than installing separate grounding electrode conductors for each separately derived system. Outdoor panelboards Q: A rainproof panelboard used for service equipment is installed outdoors. Am I permitted to install electrical metallic tubing with a weatherproof connector in the top of the panelboard cabinet? Do I need a hub or can sealing (gasketed) lock nuts for this application? May I come out of the bottom of the panelboard with a standard (not weatherproof) EMT connector? The EMT connector is far enough from the front edge of the panelboard enclosure that rain and snow will not contact it, and the bottom of the enclosure is about 3 ½ feet above the ground. A: Sealing locknuts may not be suitable for this application. In the 2001 edition of the Electrical Construction Equipment Directory (Green Book) published by Underwriters Laboratories Inc. the following information appears under “Sealing Locknuts” in the category “Conduit Fittings (DWTT)”: “Sealing locknuts are intended for use with threaded rigid metal conduit and intermediate metal conduit with one sealing locknut in the outside or the inside and either an ordinary locknut or sealing locknut on the opposite side of the enclosure for wet locations or liquid-tight applications. Sealing locknuts may also be used with Listed wet location or liquid-tight fittings where so marked on the fitting carton.” Therefore, if the carton containing the EMT connectors is not marked to indicate that a sealing locknut(s) will provide a liquid-tight connection, locknuts and connectors cannot be used as indicated. An ordinary EMT connector can be used in the bottom of the panelboard enclosure if installed as suggested, since the panelboard enclosure probably has prepunched knockouts in the bottom. Conductor terminations Q: Since Section 110-14 does not specifically require the use of an oxide-inhibitor paste on aluminum conductor terminations, is it necessary to use an inhibitor for all aluminum conductor terminations? Is there a standard for these compounds, and are any listed by a nationally recognized testing laboratory? A: You are correct. There is no requirement to use oxide inhibitors in Section 110-14. However, this sentence appears at the end of the first paragraph: “Materials such as solder, fluxes, inhibitors, and compounds, where employed shall be suitable for the use and shall be of a type that will not adversely affect the conductors, installation, or equipment.” In general, it is unnecessary to use an oxide-inhibiting compound on all aluminum conductor terminations; however, the connector manufacturers’ instructions must be followed to ensure a good connection, and to comply with Section 110-3(b) of the NEC. Instructions for proper application of the connector may be on the container or on an installation sheet packed with the connector. Some wire connectors are shipped prefilled with conductor termination compound. The connector manufacturer may also specify wire brushing of the conductor. Where aluminum conductors are used in panelboards, switchboards, enclosed safety switches, et cetera, the equipment must be marked (independent of any marking on the terminals or lugs) to indicate the maximum-size aluminum conductors that may be installed. This is important because the bending space in the enclosure may not be adequate for the larger-size aluminum wire. Some conductor termination compounds are listed. These compounds have been tested for use on splice or termination connections of aluminum, copper-clad aluminum, and copper conductors. They are used to retard oxidation at the conductor-connector interface. Where properly used in accordance with the manufacturers’ instructions, these compounds do not have a deleterious effect on the conductor metal, insulation, or equipment. FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at (504) 254-2132.