If you have a problem related to the National Electrical Code (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 codefaqs@earthlink.net. Answers are based on the 2011 NEC.


Parallel arc-fault and series arc-fault?
What does the National Electrical Code (NEC) mean by a parallel arc-fault and a series arc-fault, and upstream and downstream?
A parallel arc-fault is an arc caused by a short circuit between a phase conductor to ground or to a grounded or neutral conductor or to another phase conductor of opposite polarity. Series arc-faults are caused by a current-carrying conductor that is broken and making intermittent contact at the break or by a loose connection at a conductor termination point or a loose connection at a splice point. References to upstream or downstream relate to whether the arc-fault protective device will protect the circuit in front of the device, after the device, or both.

Marking the high-phase conductor
I’m running a circuit to a three-phase motor from a 120/240-volt (V), three-phase, delta service. Is it required to mark the high-phase conductor with an orange color? Article 430 doesn’t state anything about this.
You are required to mark the high-phase conductor only when a connection is made where the grounded conductor is present. This information is in sections 110.15, 230.56 and 408.3(F)(1).

Neutral and ground wires together
What’s wrong with terminating the neutral and ground wires on the same terminal bar in a panelboard? The neutral bar and ground bar are connected together by a bonding wire already.
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 panelboard. Where the panelboard is used with a nonmetallic raceway or cable or where separate equipment grounding conductors are used, a terminal bar for the equipment grounding conductors must be secured inside the cabinet. NEC 408.40 does not permit equipment grounding conductors to be connected to a terminal bar provided for grounded conductors. NEC 250.142(B) does not permit a connection of the grounded (neutral) conductor and the equipment grounding conductors on the load side of the main service disconnecting means. A connection between the grounded conductor and the equipment grounding conductors would allow unbalanced (neutral) current to flow on the equipment grounding conductors.

Grounding to a well casing
Am I required to run a grounding conductor to a submersible pump well casing? The inspector made me ground the well casing to the grounding conductor. It seems to me that the casing is pretty well grounded already?
Section 250.112(M) requires that, “where a submersible pump is used in a metal well casing, the well casing shall be bonded to the pump circuit equipment grounding conductor.” If the well casing were not bonded to the equipment grounding conductor and the ungrounded pump circuit conductor were to accidentally energize the well casing, the only path for the ground-fault current would be through the earth to the building grounding electrode conductor, the grounding electrode conductor at the utility transformer location, or both. Sections 250.4(5) and 250.54 state, “the earth shall not be used as the sole equipment grounding conductor.” The impedance of the earth is too great to permit sufficient current flow to open the circuit overcurrent device. Note that, while there is not sufficient current flow to open the circuit overcurrent device, there is sufficient current flow to cause severe electric shock or electrocution to a person who may become part of the load by touching the energized well casing and the earth simultaneously.

Sharing neutral conductors
I’m working on a project, and the contractor is telling me that I do not need a separate neutral with each circuit. They are in single-pole breakers. Is this right?
Two or three circuit conductors of opposite phases can share neutral conductors.

The 10-foot tap rule
I want to install two feeders rated at 200 amperes (A) each from a set of 400A fuses with the appropriate lugs to two panelboards with 200A mains. The distance is less than 10 feet in EMT. Isn’t this covered by the 10-foot tap rule?
Yes, taps not longer than 10 feet are permitted where the conditions shown in 240.21(B)(1) are followed.

Lamp difference
What is the difference between a 75-watt (W), R30 lamp and a 75W, R40 lamp?
An R30 lamp is 33/8 inches in diameter. An R40 lamp is 5 inches in diameter.

Primary and secondary sides
Does a transformer installation require overcurrent protection on both the primary and secondary sides if the current is more than 9A?
Section 450.3(B) requires, for transformers rated 600 volts (V) or less, that overcurrent protection be provided in accordance with Table 450.3(B). Table 450.3(B) does not require overcurrent protection for the secondary of a transformer if the primary current is 9A or more and the primary overcurrent protection is limited to 125 percent of the transformer-rated current. Table 450.3(B) permits primary overcurrent protection to be 250 percent of the transformer-rated current if the secondary current is 9A or more and secondary overcurrent protection limited to 125 percent is provided. The requirement for overcurrent protection for the secondary winding of a transformer depends on the protection provided for the primary winding. The overcurrent protection of the feeder conductors for the transformer, if not larger than 250 percent of the transformer primary current, may serve as the overcurrent protection for the transformer primary winding. Protection of the transformer secondary conductors must be provided. Section 240.21(C)(1) through (6) provides the requirements for this protection. The requirements for overcurrent protection for a panelboard supplied by the secondary of a transformer are shown in 408.16(D).

Plastic conduit, wet location?
Is it acceptable to use Type NM wiring outdoors/underground if installed inside plastic conduit? Specifically, it will be used to supply power to a pool filter from a service panel located on the side of the home.
NEC 334.12(B)(4) does not permit the use of Type NM cable in wet or damp locations. NEC 300.5(B) considers the interior of raceways installed underground to be a wet location.

Securing Type NM cable
NEC Section 312.5(C) requires each NM cable to be secured to the panel enclosure. The exception permits running Type NM cables through a conduit or raceway not less than 18 inches or more than 10 feet in length. Part (g) limits the cable fill to what Table 1 and all applicable notes permit. Table 1 shows an allowable fill of 40 percent for more than two conductors. Note 4 to Table 1 permits a fill of 60 percent with no derating factors if the raceway does not exceed 24 inches. Is this raceway considered a nipple? And if I don’t exceed 24 inches, can I use Note 4 to Table 1?
Section 312.5(C), Exception Part (g) permits the allowable fill to be that permitted for a complete conduit system by Table 1 and all applicable notes thereto. In my opinion, this qualifies the raceway to be treated as a nipple, regardless of the fact that it is not joining two pieces of equipment together. If you don’t exceed a length of 24 inches, you can use Note 4 and a 60 percent fill with no derating factors. If you use a raceway longer than 24 inches, you are limited to 40 percent fill and are required to use the derating factors shown in Table 310.15(B)(2)(a). To use this exception, and not secure each NM cable to the enclosure, this raceway must be installed in a surface-mounted enclosure, and all of the parts of the exception to 312.5(C) must be followed. Note 9 to Table 1 requires a multiconductor cable of two or more conductors to be treated as a single conductor for calculating percent conduit fill area. For cables with elliptical cross-sections, the calculation shall be based on using the major diameter of the ellipse as the circle diameter.