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 email@example.com.
Twin breaker limit
Is there a Code requirement that specifically states how many single twin breakers are allowed in a panel, or is it a percentage of the breakers in the panel?
Article 408.54 requires a panelboard to be provided with physical means to prevent the installation of more overcurrent devices than the number for which the panelboard was designed, rated and listed.
I am working with San Diego Gas and Electric to supply the necessary power at 460 volts (V) to connect a concrete batch plant, which the owner plans to install inside a large enclosed metal building. His full-sized concrete trucks will be driving into the building to be loaded with the concrete mixture of sand, gravel and cement the plant makes. Looking at Article 500 in my 2008 NEC book, I do not see the dust from the cement classified as a hazardous material subject to any of the classifications that require special fittings. I thought it would be prudent to ask for an opinion regarding anything I might miss.
The scope of Hazardous (Classified) Locations relates to locations where fire or explosion may exist due to flammable gases or vapors, combustible dusts or ignitable fibers/flyings. I don’t believe dust from cement fits into that category and would not be covered by Article 500. You should check with the authority having jurisdiction (AHJ) to be sure of its interpretation and for any additional local ordinances.
We are having a discussion in our office about the working space requirements at the rear of switchgear. The switchgear sections are composed of three compartments: breakers in the front, busing in the center and outgoing feeders in the rear. The rear sections of the switchgear have bus runbacks from the front-mounted circuit breakers. I believe the working space requirements of Table 110-26(A)(1) apply to the rear of the switchgear, since there is no interlock to prohibit removal of the rear panel and accessing energized components. A co-worker believes only 30 inches of working space is required, since a tool (wrench) is required to expose the energized wiring terminations. What is your opinion of the working space requirements for this type of installation?
Your installation falls under 110.26(A)(1)(a) where all parts are accessible from locations other than the back. A minimum working space of 30 inches is required at the rear of the switchgear.
Primary vs. branch-feeder protection
I am an estimator and project manager for an industrial contractor who doesn’t believe in installing a disconnect switch for transformers. The Code requires a primary overcurrent protection device (OCPD) for a 480V to 120/208, three-phase, 4-watt (W) transformer. His argument is the OCPD is the circuit breaker feeding the transformer, even if it is several rooms away. My argument is the Code addresses primary protection, not branch feeder protection feeding the transformer. My other argument is the Code in 450.3(C), which states voltage transformers installed indoors or in an enclosure require fuses to protect the primary, not a circuit breaker. Finally, my argument is the transformer is not only a piece of electrical equipment but also utilization equipment by definition. An appliance also is, by definition, a piece of utilization equipment, and Section 422.31(B) relating to appliances rated over 300 volt-amperes requires a disconnecting means. What is your take?
The requirements for the installation of transformers are found in Article 450. There is no specific requirement for a disconnecting means for a transformer within sight of the transformer. The primary OCPD can be and usually is the circuit breaker feeding the transformer (even if it is not within sight). Voltage transformers are another subject and have no bearing on the question. I believe that calling a transformer a utilization device is a stretch. Transformers feed utilization devices. The Code-making panel feels there is no need for a transformer disconnecting means to satisfy the purpose of the NEC (90.1) and that the designer can incorporate a disconnecting means in the design if desired.
Patient care areas
Do the examining rooms of an eye-care facility or an optometrist’s office fall under the scope and requirements of Article 517 Health Care Facilities? Would the receptacles served by the branch circuits in these areas be required to be installed per 517.13(A) and (B), per the 2008 NEC? The definition of a patient care area is any portion of a healthcare facility where patients are intended to be examined or treated. As an inspector, I believe these areas fall within the scope of Article 517.
Examining rooms of an eye-care facility are considered patient care areas and must comply with the requirements of Article 517, which includes the receptacle requirements of 517.13(A) and (B).
Metal vs. PVC conduit
We are being told to use PVC conduit and not metal conduit to enclose the grounding-electrode conductor. The reasoning is the metal conduit will cause a “choke” effect under fault conditions. Is this a valid request, and should PVC conduit be used?
Yes, it is a valid request. When metal conduit is used, any currents imposed on the grounding-electrode conductor (either fault currents or normal grounded-neutral current) will cause magnetic fields that will induce currents on the conduit. Induced currents are in direct opposition to the normal current flow. The conduit must be bonded at both ends in accordance with 250.64(E) to avoid a “choke coil effect.”
When it’s OK to pigtail and when it’s not
I have a 120V, 20A general-purpose branch circuit. I believe I am allowed to use a 15A-rated receptacle device; however, am I allowed to pig-tail using 14 AWG conductors to the outlets?
Yes, you are permitted by 210.21(B)(3) to use a 15A-rated receptacle on a 20A-rated circuit. You are not permitted to pigtail 14-AWG conductors to the 20A circuit conductors. NEC 240.21 requires overcurrent protection to be located at the point where the conductor receives its supply. A short-circuit, ground-fault or an overload could damage the insulation on the unprotected conductor and introduce a fire or shock hazard.
When roughing in a multigang switch box containing a switched leg and a ground-fault circuit interrupter-protected outlet from two different circuits, is it a Code violation to not combine the grounds? I ask this because when it came time for the final, the ground-fault circuit interrupter (GFCI) would not function properly because of the combined grounds downstream. Only when the bonding was separated would the GFCI outlet test properly. The switched leg controlled a fluorescent fixture.
Apparently you are using the term “grounds” instead of grounded conductors or neutral conductors. It also appears that you are using a multiwire circuit and sharing the grounded (neutral) conductor. This causes an imbalance between the current flowing in the grounded (neutral) conductor and the GFCI-protected ungrounded circuit conductor. The imbalance is detected by the GFCI device and the protected circuit is opened. Get your NEC Handbook (you should have one) and check out Exhibit 210.6, which shows the circuitry and components of a typical GFCI.
Most architects and engineers prefer to install receptacle outlets for scheduling boards and projectors above a suspended ceiling for aesthetic reasons. Can factory-installed power cords actually be plugged into an above-the-ceiling receptacle?
No, 400.8(5) does not permit flexible cords to be used where located above suspended ceilings unless specifically permitted in 400.7. NEC 400.7 permits a variety of uses for flexible cords but does not permit any of the uses shown to be specifically located above a suspended ceiling.
When reading through the Code, 517.18 requires two duplex receptacles, one normal and one emergency with the exception No. 2, which states sleeping rooms of nursing homes shall not apply. My question is what is the definition of a sleeping room.
The NEC does not define sleeping rooms of nursing homes. The intent of this part of the requirement in 517.18 is to eliminate the necessity for these receptacle outlets in rooms where patient care procedures are not a part of the normal use of these rooms.
TROUT answers the Code Question of the Day on the NECA Web site. He can be reached at firstname.lastname@example.org.