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.
Recessed plugs and plates
Can you give me an example of the meaning of the last sentence of 110.12(A) Unused Openings? Why shall metallic plugs or plates be recessed 1/4 inch from the outer surface?
This requirement is applied only where metallic plugs or plates are used with nonmetallic enclosures. The requirement is to prevent inadvertent contact with an energized surface. While the requirement does not specify surface-mounted enclosures only, I believe this was the intent. The definition of enclosure does not include junction or outlet boxes. An example of the intent of this requirement would be a surface-mounted nonmetallic panelboard where the possibility of the metal plugs or plates becoming energized by contact with an energized conductor located in the panelboard is present. The metal plugs or plates would not be grounded (because the panelboard is nonmetallic) and would remain energized, presenting a shock hazard. Recessing the metal plugs or plates would help in preventing inadvertent contact with them.
Electric motor protection
I am an electrician in Alaska. One of my electrician sons wants to see your take on a question about breaker, overload protection and wire sizing for an electric motor. Let’s say you have a polyphase motor with a full amp (A) rating of 20A. According to 430.52, the breaker should be sized to 250 percent of the full-load amperes (FLA) of the motor, which is a 50A breaker. This is supposing the breaker is an inverse-time breaker. The wire sizing should be 10-gauge wire according to 430.22 (A), and 125 percent of FLA equals 24A. Section 430.32 (A)(1) states the overload should be rated at 125 percent of the FLA. The problem is how can 10 AWG wire be used with a 50A breaker?
Protecting the 10 AWG motor branch--circuit conductors with a 50A overcurrent device is permitted because a short circuit or ground fault in the motor branch circuit is of such magnitude that the inverse-time circuit breaker protecting the motor branch circuit will open before any damage would occur to the motor insulation. You and your boys should get a copy of my latest book, Essentials of Motors and Controls,” published by Jones and Bartlett (www.jbpub.com), for complete information on electric motor installations.
Single twin breakers
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?
The applicable NEC requirement would be 110.3(B), where the installation and use of listed or labeled equipment must be in accordance with any instructions included in the listing or labeling. NEC 408.54 requires that a panelboard be provided with physical means to prevent the installation of more overcurrent devices than that number for which the panelboard was designed, rated and listed.
I am installing a photovoltaic (PV) system at an existing residence. The array system is located on the roof of the residence. NEC 690.47 requires the array to be connected to a grounding electrode. If I choose to install an auxiliary grounding electrode for my array system (per 250.54), am I required to bond the auxiliary grounding electrode to the existing electrodes (two ground rods)?
NEC 690.47(A) requires connection to a grounding-electrode system. This means, in your installation, a connection to the existing grounding-electrode system is necessary. You are permitted to install an auxiliary grounding electrode, and it is not required to be bonded to the existing grounding--electrode system; however, it cannot take the place of the grounding-electrode conductor that is required to be connected to the existing grounding-electrode system.
FPE panel replacement
Can we replace an old 20-circuit FPE panel that is currently in an existing bathroom of an old commercial building, located within 6 feet of an existing sink? Common sense tells me that this shouldn’t be allowed, but is there something in the NEC I missed or in the building code that prevents this? Being an old building, is the decision up to the local inspector?
If the panelboard contains the service-disconnecting means, NEC 230.70(A)(2) prohibits installation in a bathroom. There are no requirements in the NEC to prohibit installation of a panelboard in a bathroom of a commercial building or that restricts installation of a panelboard near a sink. Check with the authority having jurisdiction for local requirements.
Drywall and raceway
I have a question regarding NEC 312.5(C) Exception (b). I recently had an issue concerning drywall on wood trusses in a residential garage. The inspector failed the job due to the raceway sleeve penetrating a structural ceiling. There was a structural engineer (PE) present, and he said the drywall on the wood trusses in the garage is not structural because the drywall could be removed with no structural implications.
A structural ceiling in electrical work is a ceiling that cannot be completely accessed by removal of panels designed to permit ready access. Drywall properly fastened to ceiling joists constitutes a structural ceiling. The reason for Exception (b) to 312.5(C) is to provide ready access to the top opening of the nonflexible raceway permitted by the exception.
Where can I find the required clearance between combustible materials and recessed luminaires?
The only NEC requirement relating to clearance from combustible materials for recessed luminaires is 410.116, where the requirements are given for both Non-Type IC (a recess luminaire that is not identified for contact with insulation) and Type IC (a recess luminaire that is identified for contact with insulation).
Is it permitted to run the service-entrance conductor through bored holes in floor joists in an unfinished basement, assuming that the holes would not compromise the joist?
Yes, the service-entrance conductors can be run through bored holes in floor joists in order to achieve a readily accessible location as required by 230.70(A)(1). The length of the service-entrance conductors should be kept at a minimum because they are protected only by the power utilities’ overcurrent devices, which provide limited protection.
Are hospital-grade receptacles with isolated grounds required to be marked with a green dot on their face?
Hospital-grade receptacles are required by 517.18(B) for all receptacles at each patient bed location. These receptacles must be listed “hospital grade” and be so identified. The Underwriters Laboratories’ standard for hospital-grade receptacles requires additional markings, which include the wording “Hospital Grade” or “Hosp. Grade” on the back of the receptacle and a green dot located on the face, which is visible after installation of the cover plate. The green dot requirement doesn’t actually appear in the NEC.
Fiber in conduit
We want to install fiber optic cables to the top floor of a 12-story building. We are going to install these cables in a 1-inch conduit. Can we use the elevator shaft for the installation?
No, this is not allowed by NEC 620.37(A), which permits only such electrical wiring, raceways and cables used directly in connection with the elevator to be installed in the hoistway.
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 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 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 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 cause electrocution.
TROUT answers the Code Question of the Day on the NECA Web site. He can be reached at email@example.com.