Underlined text indicates a change in the rule for the 2017 NEC.

Q1. What is the Code rule regarding using portable cords for permanent wiring?

A1.Unless specifically permitted in 400.10, flexible cords sets (extension cords) and power‑supply cords aren’t permitted for the following [400.12]:

(1) Flexible cord sets (extension cords) and power‑supply cords aren’t permitted to be a substitute for the fixed wiring of a structure.

(2) Flexible cord sets (extension cords) and power‑supply cords aren’t permitted to be run through holes in walls, structural ceilings, suspended or dropped ceilings, or floors.

(3) Flexible cord sets (extension cords) and power‑supply cords aren’t permitted to be run through doorways, windows, or similar openings.

(4) Flexible cord sets (extension cords) and power‑supply cords aren’t permitted to be attached to building surfaces.

(5) Flexible cord sets (extension cords) and power‑supply cords aren’t permitted to be concealed by walls, floors, or ceilings, or located above suspended or dropped ceilings.

Ex: Flexible cords can be located above suspended or dropped ceilings if installed in a metal enclosure in accordance with 300.22(C)(3).

(6) Flexible cord sets (extension cords) and power‑supply cords aren’t permitted to be installed in raceways, except as permitted by 400.17 for industrial establishments where the conditions of maintenance and supervision ensure that only qualified persons will service the installation.

(7) Flexible cord sets (extension cords) and power‑supply cords aren’t permitted where they’re subject to physical damage.

Author’s Comment:
• Even cords listed as “extra‑hard usage” must not be used where subject to physical damage.

Q2. What is the Code rule for sizing service conductors for dwellings?

A2. (7) Single‑Phase Dwelling Services and Feeders [310.15(B)(7)].

For dwelling units, service and feeder conductors supplied by a single‑ phase, 120/240V and 120/208V system can be sized in accordance with the requirements contained in 310.15(B)(7)(1) through (3).

(1) Service Conductors. Ungrounded service conductors supplying the entire load of a one‑family dwelling or an individual dwelling unit in a two‑family or multifamily dwelling can have the conductor ampacity sized to 83 percent of the service overcurrent protection device rating.

Example: What size service conductors are required if the calculated load for a dwelling unit equals 195A, and the service disconnect is rated 200A?

Solution: 200A rated circuit breaker multiplied by 83% = 166A

Answer: 2/0 AWG service conductors rated 175A at 75°C [Table 310.15(B)(16)] are required.

Author’s Comment:
• Section 310.15(B)(7) can’t be used for service conductors for two‑family or multifamily dwelling buildings.

(2) Feeder Conductors. Ungrounded feeder conductors not over 400A supplying the entire load of a one‑family dwelling, or an individual dwelling unit in a two‑family or multifamily dwelling, can have the ungrounded feeder conductors sized to 83 percent of the feeder overcurrent protection device rating.

Author’s Comment:
• Section 310.15(B)(7)(2) can’t be used to size feeder conductors where a feeder doesn’t carry the entire load of the dwelling unit, except as permitted in 310.15(B)(7)(3).

Example: What size feeder conductors are required if the calculated load for a dwelling unit equals 195A, the service disconnect is rated 200A, and the feeder conductors carry the entire load of the dwelling unit?

Solution: 200A rated circuit breaker multiplied by 83% = 166A

Answer: 2/0 AWG feeder conductors rated 175A at 75°C [Table 310.15(B)(16)] are required.

(3) Feeder Conductors Not Greater Than. Feeders conductors for an individual dwelling unit aren’t required to be larger than the service conductors.

(4) Neutral Conductors. For one‑family dwellings and individual dwelling units of two‑family and multifamily dwellings, service and feeder neutral conductors supplied by a single‑phase, 120/240V system can be smaller than the ungrounded conductors, provided the requirements of 220.61 and 230.42 for services and 220.61 and 215.2 for feeders are met.

CAUTION: Because the service neutral conductor is required to serve as the effective ground‑fault current path, it must be sized so it can safely carry the maximum fault current likely to be imposed on it [110.10 and 250.4(A)(5)]. This is accomplished by sizing the neutral conductor in accordance with Table 250.102(C), based on the area of the largest ungrounded service conductor [250.24(C)(1)].

Example: What size neutral conductor is required if the calculated load for a dwelling unit equals 195A, the maximum unbalanced load is 100A, and the service disconnect is rated 200A with 2/0 AWG conductors?

Answer: A 3 AWG neutral conductor is required because it’s rated 100A at 75°C in accordance with Table 310.15(B)(16), and 310.15(B)(7)(3) doesn’t allow for the 83 percent deduction for neutral conductors. In addition, 250.24(C) requires the neutral conductor to be sized no smaller than 4 AWG based on 2/0 AWG service conductors in accordance with Table 250.102(C).

Where conductor ampacity correction or adjustment factors are required by 310.15(B)(2) or (3), we can use the ampacity associated with the temperature rating of the conductor in accordance with Table 310.15(B)(16) as the basis for adjustment or correction.

Note 1: The service or feeder ratings addressed by this section are based on the standard ampacity ratings from 240.6(A).

Q3. Where is Arc Fault Circuit Interrupter protection required by the NEC?

A3. Arc‑fault circuit‑interrupter protection must be provided in accordance with 210.12(A), (B), and (C). AFCI devices must be installed in readily accessible locations.

(A) Required Locations. A listed combination AFCI breaker is required for all 15A or 20A, 120V branch circuits in dwelling units supplying outlets or devices in kitchens, family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, laundry areas, or similar rooms or areas.

(B) Dormitory Units. A listed combination AFCI breaker is required for all 15A or 20A, 120V branch circuits for outlets and devices in dormitory unit bedrooms, living rooms, hallways, closets, bathrooms, and similar rooms in accordance with 210.12(A).

(C) Guest Rooms and Guest Suites. A listed combination AFCI breaker is required for all 15A or 20A, 120V branch circuits supplying outlets and devices in guest rooms and guest suites of hotels and motels.

(D) Branch‑Circuit Extensions or Modifications in Dwelling Units and Dormitory Units. Where 15A or 20A, 120V branch‑circuit wiring is modified, replaced, or extended in a dwelling unit or dormitory where AFCI protection is required [210.12(A)], the modified, replaced, or extended branch‑circuit wiring must be AFCI protected by one of the following methods:

(1) A listed combination AFCI circuit breaker

(2) A listed AFCI receptacle located at the first receptacle outlet of the branch circuit

Ex: AFCI protection isn’t required for extension wiring that’s less than 6 ft in length if no outlets or devices are added.