Stumped by the Code?

All questions and answers are based on the 2008 NEC.

Q. I am having a hard time understanding the feeder tap rules. Can you please show an example of how the 1/10 rule for 10-ft taps applies?

A. Feeder tap conductors up to 10 ft long are permitted without overcurrent protection at the tap location if all of these conditions are met [240.21(B)(1)]:

The ampacity of the tap conductor must not be less than:

a. The calculated load in accordance with Art. 220, and

b. The rating of the device or overcurrent device supplied by the tap conductors.
The tap conductors must not extend beyond the equipment they supply.
The tap conductors must be installed in a raceway if they leave the enclosure.
The tap conductors must have an ampacity not less than 10% of the ampacity of the overcurrent device that protects the feeder.

Example: A 400A breaker protects a set of 500kcmil feeder conductors. There are three taps fed from the 500kcmil feeders that supply disconnects with 200A, 150A, and 30A overcurrent devices. What are the minimum size conductors for these taps (see Figure )?

200A device: 3/0 AWG is rated 200A at 75°C, and is greater than 10% of the ampacity of 400A breaker (40A).
150A device: 1/0 AWG is rated 150A at 75°C, and is greater than 10% of the ampacity of 400A breaker (40A).
30A device: 8 AWG is rated 50A at 75°C, and is greater than 10% of the ampacity of 400A breaker (40A).

Q. Are AFCIs required in a college dorm room?

A. All 15A or 20A, 120V branch circuits in dwelling units supplying outlets in family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, or similar rooms/areas must be protected by a listed AFCI device of the combination type [210.12(B)]. If the college dorm room meets the definition of a dwelling unit, it must comply with this rule. Remember: A dwelling unit is a space that provides independent living facilities, with space for eating, living, and sleeping — as well as permanent facilities for cooking and sanitation [Art. 100]. It's often the presence of permanent provisions for cooking that determines whether or not a dorm room is a dwelling unit.

Q. We have an apartment complex and are wondering which table in the Code to use to size the service conductors for each 400A apartment (multifamily) building. Can we use Table 310.15(B)(6) instead of Table 310.16?

A. No. Table 310.15(B)(6) cannot be used for this purpose. It's only permitted to be used to size conductor for 120/240Vt, 3-wire, single-phase service-entrance conductors, service-lateral conductors, and feeder conductors that serve as the main power feeder to each dwelling unit.

Q. Are hospital-grade receptacles required in a dentist office?

A. No. Receptacles for inpatient sleeping beds or procedure table beds used in a critical-care area (patient bed location) must be listed as “hospital-grade” [517.18(B)]. Hospital-grade receptacles aren't required in treatment rooms of clinics, medical/dental offices, or outpatient facilities, because these locations don't have a patient bed location as defined in 517.2.

Q. We are remodeling a commercial building that has way too many receptacles on each circuit. The owner says he doesn't need the receptacles, and I really want to have this building Code compliant if my name is on it. Can I simply remove the receptacles and install blank covers on them, or do I need to start removing wires and everything else?

A. The calculations in 220.14(I) apply to receptacles, not outlet boxes; each 15A or 20A, 125V general-use receptacle outlet is considered as a 180VA per mounting strap. That said, removing the receptacles and installing a blank cover on them would result in Code compliance. Assuming that the circuits are 20A, the maximum number of receptacles on a circuit is 13.

Circuit VA = volts × amperes

Circuit VA = 120V × 20A

Circuit VA = 2,400VA

Number of receptacles = 2,400VA ÷ 180VA = 13

Q. Do the metal studs of a building need to be bonded or grounded to anything?

A. No. However, “exposed structural metal that forms a metal building frame that is likely to become energized” must be bonded to the service equipment enclosure, service neutral conductor, grounding electrode conductor of sufficient size, or grounding electrode system [250.104(C)].