Article 240—Overcurrent Protection
Article 430—Motors, Motor Circuits, and Controllers
Article 440—Air-Conditioning and Refrigeration Equipment
Article 680—Swimming Pools, Fountains, and Similar Installations
Q: Under what conditions am I permitted to omit overcurrent protection for the secondary conductors and the secondary winding for a 5 kVA transformer with 480-volt primary and 120-volt secondary? The transformer will supply a 2-horsepower, 115-volt single-phase motor with a nameplate full-load current of 22.5 amperes.
A: To eliminate transformer secondary winding overcurrent protection, primary overcurrent protection cannot exceed 125 percent of rated primary current. This rule appears in Section 450-3(b) and Table 450-3(b). A note to the table allows an increase in the ampere rating of the overcurrent device to the next larger standard size where 125 percent of the transformer full-load current does not match a standard rating of a fuse or circuit breaker listed in Section 240-6.
The full-load primary current of the transformer is (5,000 divided by 480) 10.4 amperes.
Multiplying this figure by 1.25 increases it to 13. The note to the table allows a 15-ampere overcurrent device to protect the transformer primary and secondary windings. If the primary overcurrent protection is greater than 15 amperes, secondary protection is required.
Since we have satisfied the requirements for overcurrent protection of the transformer, we are finished with Article 450.
Overcurrent protection for the secondary conductors is not required, provided that the requirements in Section 240-3(f) are satisfied. This section allows secondary conductors connected to a two-wire (single-voltage) transformer to be protected by the primary overcurrent device where the secondary conductor ampacity is equal to or greater than the primary overcurrent protection multiplied by the transformation ratio.
This means that the secondary conductors must have an ampacity that is at least (15 x 480 divided by 120) 60 amperes. No. 6 copper conductors with Type THWN insulation may be used if the terminals are suitable for 75 degrees Celsius; otherwise No. 4 copper is the minimum-size conductor permitted without secondary overcurrent protection.
Will the primary overcurrent protection provide short-circuit and ground-fault protection for the 2-horsepower motor? It depends on the type of overcurrent device. If the primary overcurrent protective device is a circuit breaker, Table 430-152 limits its value to 250 percent of motor full-load current. This results in a value of (24 x 2.5) 60 amperes, which when multiplied by the transformation ratio, is 15 amperes.
Therefore, a 15-ampere circuit breaker protecting the primary of the transformer also provides short-circuit and ground-fault protection for the motor. A nontime delay fuse is allowed for short-circuit and ground-fault protection provided that its rating does not exceed 300 percent of motor full-load current. Therefore, a 15-ampere nontime delay fuse is also acceptable.
A 15-ampere dual-element time-delay fuse protecting the primary is not acceptable for short-circuit and ground-fault protection of the motor, because the fuse cannot exceed 175 percent of motor full-load current. The maximum-size time-delay fuse permitted is 45 amperes. This size requires primary overcurrent protection not greater than 10 amperes.
Exceptions to Section 430-52(c) allow for an increase in ampere ratings of overcurrent devices that provide short-circuit and ground-fault protection for motors where the values specified in Table 430-152 are not sufficient to allow starting and running of the motor.
Residential parking garages
Q: Is it permissible to install floor receptacles in a two-car garage that is part of a one-family dwelling unit?
A: The National Electrical Code (NEC) does not restrict the installation of floor receptacles where the authority having jurisdiction judges the garage to be nonhazardous. The electrical inspector should determine that no repair work except exchange of parts, and routine maintenance such as changing oil, spark plugs, fan belt, adding brake fluid, windshield washer fluid, etc., is done. He should also find out if the garage floor will be washed with soap and water to clean up oil leaks and other fluid spills.
I don’t think it is a good idea to install floor receptacles in a residential car garage. The appropriate place for receptacles is on walls or columns 18 inches above the floor.
Q: The engineer who designed the electrical distribution system for a building has specified equipment grounding conductors in some runs of electrical metallic tubing. Circuits that have the copper equipment grounding conductor do not supply isolated-ground receptacles. Is it necessary to terminate this extra equipment grounding conductor in every metal box that is in the circuit?
A: Yes it is. Section 250-148 has this information: “Where more than one equipment grounding conductor enters a box, all such conductors shall be spliced or joined within the box or to the box with devices suitable for the use.”
Another sentence in the Section deserves reproducing: “The arrangement of grounding connections shall be such that the disconnection or the removal of a receptacle, fixture, or other device fed from the box will not interfere with or interrupt the grounding continuity.”
Part (a) of this Section requires that a grounding screw or a listed grounding device be used to ground metal boxes, and part (b) requires that a grounding connection be made to any fitting or device that requires grounding in nonmetallic boxes.
Where the insulated grounding conductor supplies isolated-ground receptacles, this conductor does not have to be connected to every box. This information is in Section 250-146.
Swimming pool wiring
Q: A panelboard is installed to supply central air conditioning equipment, a swimming pool pump motor, and underwater lighting for an in-ground swimming pool at a one-family residence. Am I permitted to use nonmetallic sheathed cable for the feeder that supplies the panel, or must the feeder meet the Article 680 requirements?
A: A list of electrical equipment that must be grounded in and around swimming pools is found in Section 680-24. Items (1) and (2) mention underwater lighting fixtures, Item (4) indicates that the electric pump must be grounded, and Item (7) mentions panelboards. Section 680-25 requires that the feeder to the panelboard include an insulated equipment grounding conductor that is sized according to Table 250-122.
The wiring methods for the feeder are limited to rigid metal conduit, intermediate metal conduit, liquidtight flexible nonmetallic conduit, or rigid nonmetallic conduit. Electrical metallic tubing is acceptable where installed in or on a building. And electrical nonmetallic tubing may be used within the building.
Nonmetallic sheathed cable is not suitable for this application. For a complete understanding of the grounding and wiring requirements for a panelboard that supplies swimming pool electrical equipment, review part (d) of Section 680-25.
Protection for cooling equipment
Q: Will the requirement for Type HACR circuit breakers be removed in the 2002 edition of the NEC?
A: Where the wiring diagram for an air conditioning unit indicates a single branch circuit supplying a herrnetic motor-compressor and other loads or motor(s), short-circuit and ground-fault protection must comply with the requirements in Section 430-53 if the exceptions to Section 440-22(b) do not apply.
Part (c)(3) of Section 430-52 requires that each circuit breaker used for short-circuit and ground-fault protection for two or more motors connected to the same branch circuit be listed for group installation. Circuit breakers that are listed for this application are marked “HACR.” The air conditioning equipment must also be marked to indicate that an HACR-type circuit breaker is acceptable.
If there is no marking on the air conditioning to indicate that an HACR-type circuit breaker may be used, the short-circuit and ground-fault protection must be provided by fuses. If the data plate on the equipment indicates a maximum-size overcurrent device for the short-circuit and ground-fault protection without specifying fuses or an HACR circuit breaker, either fuses or a circuit breaker without the HACR marking are acceptable.
A proposal was submitted and accepted by Code Making Panel No. 11 to delete “listed for group installation” from Item 3 of Section 430-53(c). The revised wording would be, “Each circuit breaker shall be listed and be of the inverse time type.” Part of the substantiation for this proposal points out that all inverse time circuit breakers that meet the requirements of UL 489 will perform safely when supplying group motor installations. However, the Panel rejected the proposal during the Public Comment balloting.
Damp location fixtures
Q: We have a customer who has asked our firm to install recessed incandescent lighting fixtures in the soffet of his building. The fixtures will be outside but protected from rain and snow. Are we permitted by the NEC to install ordinary fixtures at this location or should the fixtures be marked “Suitable for Damp Locations?”
A: According to the definition in the NEC, this is a damp location. Part of the definition reads like this: “Partially protected locations under canopies, marquees, roofed open porches, and like locations subject to moderate degrees of moisture …” “Damp location” fixtures are required or “Wet location” fixtures are also acceptable.
In the 2000 edition of the General Information for Electrical Equipment Directory (White Book), published by Underwriters Laboratories Inc., the following information appears: fixtures or fittings marked “Suitable for Damp Locations” are intended to be installed in damp or dry locations. Fixtures and fittings marked “Suitable for Wet Locations” are intended to be installed in wet, damp, or dry locations.
The installation instructions furnished with the fixture must be followed when selecting fittings, trims, and lamp wattages in order to be in compliance with Section 110-3(b).
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at (504) 254-2132.