All questions and answers are based on the 2008 NEC.

Q. When a single generator is used to provide standby power for emergency and nonemergency loads, can one transfer switch be used?

A. No, because transfer equipment for emergency power systems must only supply emergency loads. So, two transfer switches would be required (see Figure) — one for emergency loads and the other for other loads [700.6(D)].

Q. How many 12-2 NM cables can be installed in an 18-cubic-inch plastic box with a duplex receptacle?

A. Two 12-2 NM cables. Each device yoke counts as two conductor volumes based on the largest conductor that terminates on the device in accordance with Table 314.16(B) [314.16(B)(4)], and each conductor that terminates in a box is counted as a single conductor volume [314.16(B)(1)]. Equipment-grounding conductors in a box count as a single conductor volume [314.16(B)(5)]. The conductor volume for 12 AWG conductors is 2.25 cubic inches each [Table 314.16(B)]. Follow these steps to get your answer.

Step 1: Determine the number and size of conductor volumes for the device and the equipment-grounding conductors.

One receptacle: 2 - 12 AWG conductor volumes

One equipment-grounding conductor: 1 - 12 AWG conductor volumes

Total: 3 - 12 AWG conductor volumes

Step 2: Determine the volume of those conductor volumes [Table 314.16(B)].

12 AWG conductor = 2.25 cubic inches × 3 wires = 6.75 cubic inches

Step 3: Determine the space remaining for conductors.

Total space = 18 cubic inches Remaining space = 18 cubic inches - 6.75 cubic inches = 11.25 cubic inches

Step 4: Determine the number of 12 AWG conductors permitted in the spare space.

Conductors added = Remaining space/added conductors' volume

Conductors added = 11.25 cubic inches ÷ 2.25 cubic inches = 5 conductors

An 18-cubic-inch box will hold two 12-2 NM cables with a duplex receptacle.

Q. When a cable tray enters through the outside wall of a metal building, is the opening required to be fireproofed or just weatherproofed?

A. The locally adopted building code would give you the answer for this. If the exterior wall is required to be of fire resistance-rated construction, it would have to be firestopped.

Q. Is a receptacle required within 25 feet of a rooftop exhaust fan?

A. No. A 15A or 20A, 125V receptacle outlet must be installed at an accessible location for the servicing of heating, air-conditioning, and refrigeration equipment [210.63]. An exhaust fan is not heating, air-conditioning, or refrigeration, so a receptacle is not required.

Q. I have two duplex receptacles in the same two-gang box, each on a separate circuit. Are these circuits required to be fed from a double-pole circuit breaker?

A. No, but if the circuits share a neutral (multiwire branch circuit), they require a means to simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates [210.4(B)], such as a 2-pole breaker or single-pole breakers with an identified handle tie [240.15(B)(1)].

Q. Is the lighting in a bathroom allowed to be fed from the load side of the bathroom GFCI receptacle, which means that the lights will go out when the GFCI trips?

A. Yes, as long as the circuit to the bathroom receptacles only supplies that bathroom [210.11(C)(3) Exception], and no single load fastened in place is rated more than 10A [210.23(A)].

Q. Section 680.51 of the NEC requires GFCI protection for fountain equipment. Is there a maximum fault current-to-ground requirement with this section that should be used when providing ground fault protection for fountain equipment?

A. When the Code requires GFCI protection, it is a “Class A” type. A “Class A” ground-fault circuit interrupter opens the circuit when the current-to-ground has a value of 6 milliamps or higher and doesn't trip when the current-to-ground is less than 4 milliamps [Art. 100 Ground-Fault Circuit Interrupter FPN].