Q. Can a single-pole AFCI breaker be installed on a multi-wire branch circuit?

A. No, because it won't work. You'll need to use a two-pole AFCI for this purpose, just like you would if the circuit was to be GFCI protected (Figure below).

Q. How many equipment grounding conductors are permitted to terminate under a single screw of a grounding bar located in a panel?

A. The NEC only permits one conductor to terminate on a terminal, unless it's identified for more than one conductor [110.14(A)]. Generally the label located on the cabinet identifies that two or three grounding conductors are permitted on a single grounding terminal. But be careful because 408.21 restricts termination on the grounded neutral bar to a single conductor per terminal.

Q. Can a 15A, 125V duplex receptacle be installed on a 20A, 120V circuit?

A. Yes, as per Table 210.21(B)(3). A 15A duplex receptacle is permitted on a 20A circuit because it's rated 20A internally. However, a single 15A, 125V receptacle isn't permitted on a 20A branch circuit [210.21(B)(2)].

Q. A new school in my area has the classroom light switch installed on the wall across the room from the entrance. Does the NEC permit this?

A. Yes. The NEC doesn't specify the location or placement of wall switches.

Q. Can the optional method load calculation be used to size the feeder conductors for individual dwelling units of a multi-family building? The service is 120/208V, 3-phase, 4-wire from a wye-connected system. The individual feeders are 120/208V, single-phase, 3-wire.

A. Yes. The optional calculation method is permitted for the individual dwelling unit feeders [220.82(A)], but the conductors must be sized in accordance with Table 310.16, not Table 310.15(B)(6) [310.15(B)(6)].

For example, let's say the calculated load according to the optional method is 100A: Table 310.15(B)(6) would permit 4 AWG (for 120/240V 3-wire feeder), but Table 310.16 requires the conductor to be a minimum 3 AWG.

Q. Is it acceptable to install covers or guards over mushroom emergency stop buttons to prevent accidental operation?

A. The NEC doesn't prohibit this, but the general practice in industrial occupancies is to provide a guard surrounding the button in such a way as to allow the button to be depressed by pressing straight in without requiring that a cover be opened. This method will allow rapid operation of the button when necessary, but will prevent inadvertent operation by someone or something leaning on the control station.

Q. Can non-plenum-rated data cable be installed in the under floor area of an information technology (IT) room?

A. Yes, if the cables are associated with the data processing equipment as per 645.5(D)(5)(c).

Q. How many load-side receptacles can be installed on a “feed-through” GFCI receptacle in a dwelling unit?

A. The Code doesn't place a restriction on the number of receptacles on a circuit in a dwelling unit. There's a practical limit based on the expected load, but few circuits in a dwelling unit have an expected load. The 180VA-per-outlet rule contained in 220.14(I) doesn't apply to dwelling occupancies.

Got a Code question for Mike? Send it to mike@mikeholt.com.