All questions and answers are based on the 2008 NEC.
Q. For a 240V to 208Y/120V transformer, is it better to bond the neutral to the metal frame at the transformer itself, at the panel, or at both locations?
A. A system bonding jumper must be installed at the same location where the grounding electrode conductor terminates to the neutral terminal of the transformer. This connection can be either at the transformer or the system disconnecting means, but not at both locations [250.30(A)(1) and (A)(3)].
Q. What is the required burial depth for a grounding electrode conductor?
A. There is no depth requirement for grounding electrode conductors. Therefore, you can bury them as shallow as you like.
Q. For a duct heater that has a built-in ON/OFF switch, do I still need a disconnect?
A. No. A unit switch with a marked "OFF" position that is an integral part of the equipment can serve as the heater disconnecting means, if it disconnects all ungrounded conductors of the circuit [424.19(C)].
Q. For a wall-mounted emergency light that contains an internal wiring compartment, do I need a box in the wall or can I simply run my cable to the back of the light with a connector and make up the wires inside the light housing?
A. The simple answer is yes. An integral junction box or wiring compartment is allowed in lieu of a box [300.15(B)].
Q. In an apartment building, do the common corridors, offices, and exercise rooms have to be wired in accordance with the dwelling unit rules? For example, do we need a receptacle every 12 ft?
A. No, because these areas do not meet the definition of dwelling unit as outlined in Art. 100: A space that provides independent living facilities, with space for eating, living, and sleeping — as well as permanent facilities for cooking and sanitation.
Q. Where does the Code require all products must be listed by UL or equivalent?
A. The NEC doesn’t require all electrical equipment to be listed, but some Code requirements do specifically require product listing. Organizations like OSHA increasingly require that listed equipment be used when such equipment is available [90.7, 110.2, and 110.3].
Q. Can we install a plenum-rated cable inside of an air duct to wire up the temperature sensor that is inside of the duct?
A. No. Only type MI cable, Type MC cable that has a smooth or corrugated impervious metal sheath without an overall nonmetallic covering, electrical metallic tubing, flexible metallic tubing, intermediate metal conduit, or rigid metal conduit without an overall nonmetallic covering can be installed in ducts or plenums specifically fabricated to transport environmental air. This is only allowed where the wiring is necessary for direct action upon, or sensing of, the contained air [300.22(B) and 725.3(C)].
Q. What are the rules regarding electrical raceways and cables near gas piping?
A. There aren't any rules in the NEC that cover this, so you can put them as close as you would like.
Q. I recently saw a circuit breaker that was locked in the "ON" position. Is this a Code violation? It sure seems like it should be.
A. It may look odd, but there is nothing in the Code that says you can't do this. In fact, if you use the exception to 700.12(F), the Code requires the breaker to be locked on. This is safe because breakers are required to "trip free," meaning that they will trip internally under an overcurrent condition, even if the handle is locked "on."
Q. We have a line voltage fire alarm system in a commercial building. I thought it was illegal to install the supply conductors with the signaling conductors (120V) in the same raceway.
A. Power supply and fire alarm circuit conductors are only allowed in the same cable, enclosure, or raceway where they are connected to the same equipment [760.48(B)].