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 firstname.lastname@example.org. Answers are based on the 2011 NEC.
Nonmetallic boxes and raceways
Are nonmetallic boxes permitted to be used with metal raceways?
Section 314.3 generally restricts the use of nonmetallic boxes to nonmetallic raceways, but there are two exceptions to this general rule. Exception No. 1 permits metal raceways to be used where internal bonding means are provided between all entries, and Exception No. 2 permits metal raceways to be used where integral bonding means, with a provision for attaching an equipment grounding conductor inside the box, are provided between all threaded entries.
Where are dead-front panels required in the National Electrical Code?
Dead front is defined in Article 100 of the NEC as being “without live parts exposed to a person on the operating side of the equipment.” Section 408.38 requires that panelboards be mounted in cabinets, cutout boxes or enclosures designed for the purpose and shall be dead front. The term “dead front” is used in other places in the NEC, but basically, the NEC requires distribution panels, panelboards (load centers), switchboards (stage and theater) be constructed so that switches, circuit breakers and other electrical components can be operated without the user being exposed to live parts.
Common and neutral conductors
According to Section 310.15(B)(5)(b), “In a 3-wire circuit consisting of two phase wires and the neutral of a 4-wire, three-phase wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors, and shall be counted when applying the provisions of 310.15(B)(3)(a).” Is this common conductor different than the neutral conductor?
No. They are one and the same. However, in this instance where you are using two phase conductors and the “neutral” of a 4-wire, three-phase wye-connected system, we can no longer call this conductor “neutral” because it is carrying the unbalanced current. They are telling you that, when you take a single-phase, 3-wire circuit from a 4-wire, three-phase wye-connected system, you have a common conductor, not a neutral, and it is the conductor that is intentionally grounded in grounded systems.
Ground rods for metal light poles
I have been hearing a lot about whether it is necessary to drive a ground rod at each metal light pole in a parking lot. Is this required?
It is not required to install a ground rod at each light pole. The grounding of the metal pole is required by Section 250.4(A)(2), where normally noncurrent--carrying conductive materials enclosing electrical conductors shall be connected to earth so as to limit the voltage-to-ground on these materials. Sections 250.4(A)(5) and 250.54 have similar wording: “the earth shall not be used as the sole equipment grounding conductor or fault current path.” These sections are the basis for the requirement that an equipment grounding conductor must be run with the circuit conductors. This equipment grounding conductor will establish a fault current path, which will serve to open the overcurrent protective device on the circuit feeding the metal pole. Section 250.54 permits the installation of auxiliary grounding at the light poles. This is a design consideration. All of the safety aspects of grounding relating to personnel have been taken care of by the equipment grounding conductor. If you are in an area where lightning is a probable cause for concern, these poles may attract it, and a strike may feed back into your electrical system. In that case, you may use an auxiliary grounding system.
Truth about well casings
The inspector made me ground the well casing to the grounding conductor. It seems to me that the well casing is pretty well grounded already. Is he right?
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 either or both the building grounding-electrode conductor and the grounding-electrode conductor at the utility transformer location. Sections 250.4(5) and 250.54 both state, “the earth shall not be used as the sole equipment grounding conductor.” The impedance of the earth is too great to permit a sufficient current flow to open the circuit overcurrent device. Please 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.
Using Table 310.15(B)(7)
Is it permitted to use Table 310.15(B)(7) for a 600-ampere (A) service using two parallel-ed 250 kcmil conductors?
Table 310.15(B)(7) can only be used for a single set of 120/240-volt (V), 3-wire, single--phase dwelling services and feeders.
Where is the equipment grounding conductor from an isolated ground receptacle required to be terminated?
The receptacle grounding terminal (which is purposely insulated from the receptacle mounting means) must be grounded by an insulated equipment grounding conductor run with the circuit conductors. NEC 250.146(D) requires the equipment grounding conductor to terminate within the same building directly at the equipment grounding conductor terminal of the derived system or service. This would be the ground bar in the service panel. The insulated grounding conductor is permitted to pass through one or more panelboards without connection to the panelboard equipment grounding terminal (408.40 Exception). The equipment grounding conductor is required to be insulated to prevent contact between the equipment grounding conductor and any grounded surfaces.
Should the metal frame of a toaster be grounded?
Toasters do not come with polarized cord caps. The only specific reference to toasters is in Section 422.4, which permits live parts to be exposed to contact. A severe shock would result under the following conditions: the metal frame of your toaster was grounded and a piece of toast became stuck down in the slot (a condition that is usually remedied by sticking a fork into the slot to retrieve it) and you had a good firm grip on the metal frame of the toaster.
If I tap a feeder supplying a group of motors, how far can I run that tap without overcurrent protection for that tap?
If you are tapping to a single motor and are not reducing the conductor size, overcurrent is not a problem. If you are reducing the conductor size, Section 430.28(2) requires that the tap conductors have protection from physical damage, the conductors are not more than 25 feet in length and that the conductors have an ampacity not less than one-third that of the feeder conductor with a minimum in accordance with Section 430.22. Therefore, the tap conductors must be sized at 125 percent of the motor full-load current and must not be less than one-third the ampacity of the feeder conductor.
Protection for motor control circuits
Are motor control circuits feeding remote-control devices required to have overcurrent protection?
Overcurrent protection for motor control circuits is covered in Section 430.72(B). The requirements for conductors that extend beyond the enclosure (remote) can be found in Table 430.72(B) in Column C. For example, if your motor branch-circuit protective device is rated at 60A and you are using copper control circuit conductors, then you find 60 in the copper column and move to the left to control circuit conductor size where you find 12. This means you need to install control circuit conductors not smaller than 12 AWG copper. A smaller conductor would require supplemental overcurrent protection to protect that conductor.
At an angle
Can nonmetallic sheathed cable be run diagonally and secured to the underside of joists in an unfinished basement in a one-family dwelling unit?
Yes, 334.15(C) permits this where cable is run at angles with joists in unfinished basements or to secure cables not smaller than two 6 AWG or three 8 AWG conductors to the lower edge of the joists. Otherwise, holes must be drilled in the joists for the cable, or it must be secured to a running board.
TROUT answers the Code Question of the Day on the NECA-NEIS website. He can be reached at email@example.com.