If you have a problem related to the National ElectricalCode (NEC), are experiencing difficulty in understanding a Code requirement, or are wondering why or if such a requirement exists, ask Charlie, and he will let the Code decide. Questions can be sent to firstname.lastname@example.org. Answers are based on the 2011 NEC.
I have parallel 4/0 AWG XHHW-2 AL conductors using two conduits with no more than three conductors in each raceway. My position is that the ampacity equals 360 amperes (A) and that I can use a 400A breaker as per 240.6. Is that the case?
You are paralleling two 4/0 AWG AL conductors. Table 310.16 shows 4/0 AWG XHHW AL conductors to have an ampacity of 180A. So yes, you have an ampacity of 360A, and 240.4(B) permits the next higher standard overcurrent device to be used, which, according to 240.6(A), would be a 400A-rated device.
Table 310.15(B)(7) gives you conductor types and sizes for 120/240, 3-wire, single-phase dwelling services and feeders. Are you permitted to parallel conductors for a residential 400A service using this table?
You are permitted to parallel conductors for a residential service rated 400A or any other rating that meets the requirements of 310.10(H). However, you can’t use Table 310.15(B)(7) to size the conductors being paralleled since the table explicitly indicates which conductors must be used for each service or feeder rating. The comment shown in the NEC handbook regarding Table 310.15(B)(7) specifically states that, if a single set of 3-wire, single-phase service-entrance conductors supplies a dwelling unit, the reduced conductor size may be used.
Terminating on the same bar
What’s wrong with terminating the neutrals and the ground wires on the same terminal bar in a panelboard? A bonding wire already connects the neutral bar and the ground bar.
The neutral bar and ground bar are only connected together in panelboards used as service equipment. In panelboards that have this bonding wire installed, it serves as the main bonding jumper required by 250.28. In many areas where metal raceways are used, a grounding terminal bar is not furnished in the panel-board. Where the panelboard is used with a nonmetallic raceway or cable or where separate equipment grounding (bonding) conductors are used, a terminal bar for the equipment grounding (bonding) conductors must be secured inside the cabinet. NEC 408.40 does not permit equipment grounding (bonding) conductors to be connected to a terminal bar provided for grounded (neutral) conductors. NEC 250.142(B) does not permit a connection of the grounded (neutral) conductor and the equipment grounding (bonding) conductors on the load side of the main service-disconnecting means. A connection between the grounded (neutral) conductor and the equipment grounding (bonding) conductors would allow unbalanced (neutral) current to flow on the equipment grounding (bonding) conductors.
Why do we need ground-fault circuit interrupters (GFCIs) when we have grounded systems?
Grounded systems provide a path of low impedance for ground faults. To interrupt the circuit, the ground-fault current must reach the trip level of the overcurrent device. A person who is touching an energized object and is in contact with a grounded surface will be subjected to electric shock. The seriousness of the shock depends on the length of time it takes to open the circuit and the voltage level of the circuit. With a 15A overcurrent-protective device, the current may never reach that trip level, and electrocution may occur. With GFCI protection, the sensing coil will detect a leakage of current not returning to the source through the return conductor (the current that is flowing through your body), and when it reaches a level of 4 to 6 milliamps (mA), the GFCI will open the circuit. The 4 to 6 mA through the body of the person to ground may still result in a severe shock but will not normally cause electrocution.
Are there differences between the appropriate thread pitches for threaded conduit and threaded plumbing pipes? The Code requires 3/4-inch taper per foot for rigid conduit. We are told that threaded plumbing pipe also requires a 3/4-inch taper per foot. That would indicate that the same dies could be used for all same-size piping needs on any given job. Is it correct to thread both with the same set of dies?
NECA 101 2001, Standard for Installing Steel Conduits, Section 4.1.1 states, “Although coupling threads are straight-tapped, conduit threads are tapered. Use a standard 3/4-inch per foot taper National Pipe Thread (NPT) die.” Plumbers use this same die.
Linear versus nonlinear
Please explain linear versus nonlinear loads and how the difference affects counting the neutral conductor as a current-carrying conductor as referenced in NEC 310.15(B)(4)(c)? Could you also give some examples of linear and nonlinear loads?
Harmonics are voltages or currents operating at frequencies that are multiples of the system frequency. If the system frequency is 60 hertz (Hz), the third (triplen) harmonic would be 180 Hz, etc. Harmonics are produced by nonlinear loads that draw current in pulses rather than continuously. Harmonic problems on single-phase lines are generally caused by computer power supplies, copy machines, lighting dimmers or electronic ballasts in fluorescent fixtures. Harmonics cause the alternating current (AC) sine wave to become distorted. Triplen harmonics generally cause overheating of the neutral conductor on three-phase, 4-wire systems. Triplen harmonic currents can cause the neutral current to be higher than the phase current. For this reason, on a 4-wire, three-phase wye circuit, where the major portion of the load is nonlinear, harmonic currents are present in the neutral conductor, and 310.15(B)(5)(c) requires that the neutral conductor be considered a current--carrying conductor. If triplen harmonics are present on the neutral conductor, using a separate neutral for each phase conductor instead of a shared neutral can lessen the effects of harmonics. Or, you could increase the size of the common neutral.
Please explain which table is used for determining the ampere capacity of wires in a metal conduit with three conductors installed in it. In addition, how do you determine temperature rating (60, 75, 90 degree) that must be applied?
Use Table 310.16 for conductors rated 0 through 2,000 volts (V), 60°C through 90°C, not more than three current-carrying conductors in a raceway based on ambient temperature of 30°C. The temperature rating of the conductor that you are permitted to use is based on the rating of the terminations on the equipment you are wiring. For example, if you are wiring an electrical equipment, such as a resistance heater, and the equipment is marked “Suitable for use with 60° conductors only,” you are required to use 60° conductors, or you may use conductors with a higher rating; however, you must use the ampacity shown in the 60° column.
Remote stop button
I have a 10-horsepower (hp), three-phase, 208V motor. I have been asked to add an emergency stop button located 50 feet away from the motor controller. What size conductors should be run from the controller to the stop button?
Table 430.250 shows a 10-hp, three-phase, 208V motor has a full-load current of 30.8A. If you are using dual-element fuses for the motor branch-circuit short-circuit and ground-fault protection, Table 430.52 requires 175 percent of the full-load current, which is 53.9A (30.8 1.75). The next higher standard fuse is rated at 60A. Using 430.72(B)(2), which permits, where the control conductors extend beyond the motor-control equipment enclosure, the rating of the control-conductors protective device shall not exceed the value specified in Column C of Table 430.72. Column C shows, for a 60A protective device, moving to the left, a control circuit conductor size of 12 AWG can be used without providing overcurrent protection for the control circuit.
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