Article 210—Branch Circuits; Article 410—Lighting Fixtures, Lampholders, Lamps, Receptacles; Article 430—Motors, Motor Circuits, Controllers
Motor branch circuit protection
Q: Your March column included a question about No. 12 Type THHW copper conductors’ ampacity. I understood the discussion pertained to derating, because there were more than three current-carrying conductors in a raceway or cable assembly. Derating, because of the ambient temperature, is above 60 degrees C or 86 degrees F.
However, I do not understand how a No. 12 copper conductor can be adequately protected when the overcurrent device is rated 50 amperes for a motor branch circuit that supplies a 1.5-horsepower, 120-volt single-phase motor. How can a No. 12 copper conductor that is rated to safely carry 25 amperes be connected to a 50-ampere breaker? What is protecting the conductor from carrying more than 25 amperes? I have seen this done in several installations; however, I don’t understand it.
A: Generally, circuit conductors must be protected from overcurrent in accordance with their ampacities. This requirement is found in Section 240-3 of the NEC, which reads like this: “Conductors, other than flexible cords and fixtures wires, shall be protected against overcurrent in accordance with their ampacities as specified in Section 310-15, unless otherwise permitted or required in (a) through (g).” Part (g) allows overcurrent protection for motor branch circuit conductors to be selected from parts C, D, E, F, and H of “Article 430—Motors, Motor Circuits, and Controllers.”
To determine the minimum branch-circuit-conductor ampacity, use Sections 430-6 and 430-22. Section 430-6 requires the use of Table 430-148 to obtain the full load current of a 1.5 horsepower, 120-volt single-phase motor. Although there is no 120-volt column, the commentary that is part of the Table allows the 115-volt column to be used for motor nameplate voltages of 110 to 120 volts. Therefore, the full load current of the 1.5 horsepower motor is 20 amperes.
Section 430-22(a) dictates the minimum-branch-circuit conductor ampacity. This section requires a minimum ampacity of 125 percent of table full load current. Therefore, the branch circuit conductor ampacity cannot be less than 25 (20 x 1.25). According to Table 310-16, the ampacity of 60 degrees C or 75 degrees C No. 12 copper wire is 25, which satisfies Section 430-22 (a).
The next step is to size the overload relays in the motor starter. Part C of Article 430 has the title: “Motor and Branch-Circuit Overload Protection,” and part of Section 430-31 has this sentence: “Part C specifies overload devices intended to protect motors, motor-control apparatus, and motor branch-circuit conductors against excessive heating due to motor overloads and failure to start.” Notice that the overload relays are intended to protect the branch circuit conductors as well as the motor and controller.
Section 430-32 provides the information necessary to select the overload relays based on motor nameplate full-load current, service factor, and motor temperature rise. These values are 125 percent of motor nameplate full-load overcurrent. Motors with a 1.0 service factor or a marked temperature rise of over 40 degrees C should be protected at 115 percent of motor nameplate full-load current.
If the nameplate full load current is 19.4 amperes for the 1.5 horsepower motor and the motor has a 1.15 service factor, the overload relay should have an ampere rating of 24.25. If this overload relay trips before the motor reaches rated speed, the ampere rating of the overload relay may be increased, but it cannot exceed 140 percent of motor nameplate current for motors, with a 1.15 service factor or temperature rise of not over 40 degrees C. However, a class 20 or 30 overload relay with the same ampere rating (24.25) should be tried before increasing the ampere rating of the overload relay does. Class 20 and 30 overload relays have more time delay before tripping than a class 10 relay.
The final step is sizing the motor branch circuit short-circuit and ground-fault protective device. Because motors have high inrush currents when starting, the short-circuit and ground-fault protection must have an ampere rating that varies between 150 and 1,100 percent of motor full load current, depending on the type of overcurrent device, to allow the motor to start and accelerate its load. Section 430-52 and Table 430-152 are used to size the branch circuit fuses or circuit breaker. If a circuit breaker is used for the motor branch short-circuit and ground-fault protection, Section 430-52 allows the values in Table 430-152 to be used. This table allows the circuit breaker to be rated at (2.5 x 20) 50 amperes. The exceptions to Section 430-52 allow an increase in the ampere rating of the circuit breaker to a maximum of 80 amperes, if the 50-ampere device does not allow the motor to start.
As you can see from this rather long discussion, the overload relay protects the branch circuit conductors. Since there is no load between the branch circuit short-circuit and ground-fault protection and the motor controller, the only failures that the circuit breaker has to protect from are short-circuits and ground-faults. The branch circuit conductors are protected from excessive current caused from overloads by the overload relay in the motor controller.
Testing ground-fault circuit-interrupter (GFCI) receptacles
Q: I am an electrical inspector. For GFCI-type receptacles, I use a hand-held tester that is listed and designed for testing the operation of GFCI receptacles. Sometimes when I plug the tester into a GFCI receptacle, it does not trip. However, when I press the test button on the receptacle, it does trip. Should I be concerned because the tester does not trip the GFCI?
A: Most, if not all, GFCI receptacle manufacturers recommend using the test button on the device to ensure proper operation. If a small load, such as a table lamp or floor lamp, is plugged into the receptacle and turned on, then the test button on the GFCI is pressed, and the lamp goes out, the GFCI receptacle is working, and is properly wired. If the unit trips, but the lamp remains energized, the GFCI receptacle is miswired. To correct this situation, the wires connected to the receptacle’s “line and load” terminals must be reversed.
A hand-held GFCI tester causes current to flow from the ungrounded conductor to the equipment-grounding conductor when it is plugged into the receptacle. This method allows an unbalanced current flow through the GFCI. If the tester does not cause the GFCI to trip, the tester could be defective, the receptacle could be miswired, or there could be an open or no-equipment grounding conductor in the circuit.
An equipment-grounding conductor is not required for GFCI receptacles that are being installed as replacements for existing receptacles during remodeling of a dwelling unit. Section 210-7(d)(2) requires GFCI receptacles as replacements where other Sections of the NEC specify locations that must have this protection. And Section 210-7(d)(3)(b) requires that these receptacles be marked “No Equipment Ground.”
If the hand-held GFCI tester does not trip the GFCI and you know the tester is functioning properly, the receptacle should be removed from the outlet box to check the presence/absence of an equipment grounding conductor. If the conductor is present, it is not continuous to the outlet. If it is not present, the receptacle should be marked “No Equipment Ground.”
Wiring fluorescent fixtures
Q: The electrical inspector has questioned the way I am wiring fluorescent fixtures in a supermarket. I am wiring the 277-volt fixtures by running armored cable from one fixture to the next. The distance between fixtures is 12 feet, and the cable enters and leaves the ends of the fixtures. The inspector said the cable should originate at an outlet box to conform with Section 410-67 (c). Am I permitted to wire fluorescent fixtures in this manner?
A: Yes you are, if you meet the requirements in Section 410-31 and the Exceptions. I am assuming that the fixtures do not have the marking, “Suitable for Use as a Raceway.” Without this marking the maximum number of branch circuits allowed to enter and exit each fixture is two.
According to Exception No. 2 to Section 410-31, either a two-wire or multiwire branch circuit is allowed to supply the fixtures. Exception No. 3 permits an additional two-wire circuit to be carried through the fixtures. This extra circuit is generally used to supply security or night lighting; however, it may be used to energize work lights, or other fixtures the occupant selects.
Since you are using branch circuit conductors to go from fixture-to-fixture, Section 410-67(c) does not apply. This part deals with tap conductors, but you are not using taps to supply the fixtures. Also, the cable must be secured to comply with Section 333-7.
I believe that all armored cable manufacturers with listings are using only 90 degrees C insulated conductors in their production of armored cable. Therefore, it is not necessary to change conductor insulation to one of the types specified in the last paragraph of Section 410-31 where the branch circuit conductors pass within 3 inches of the ballast. The requirement for 90 degrees C insulation where the branch circuit conductors are within 3 inches of the ballast within the ballast compartment appears as the last paragraph of Section 410-31.
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at (504) 254-2132.