The ins and outs of branch circuit installations
The wiring inside a dwelling that you cannot see may be an arc fault waiting to happen, due to improper installation; corrosion; damage caused by rodents, insects, or other pests; penetration by screws or nails after construction; or other problems. The arc-fault circuit interrupter (AFCI) helps prevent an arcing fault from becoming a fire. Unlike the ground-fault circuit interrupter (GFCI), the AFCI doesn’t provide personnel protection from electrical shocks, but it is designed to monitor a circuit for electrical current waveform characteristics that indicate an arcing fault.
AFCIs installed after Jan. 1, 2008, are required to be of the combination type, which includes technology to expand protection against parallel arcing faults as well as series arcing faults. AFCI devices used prior to 2008 were of the branch/feeder type, which offered less protection against parallel arcing faults.
The NEC requires AFCI protection in:
- Dwellings. These are single, two-family, and multi-family residences.
- Lodging units. Guest rooms or suites with permanent provisions for cooking are included in the definition of dwelling units and must have branch circuits installed to meet the rules for dwelling units [210.18].
AFCI protection is required only for 15A or 20A, 120V branch circuits in dwelling units supplying:
- Outlets in “living spaces,” including bedrooms, family rooms, dining rooms, living rooms, dens, closets, hallways, or similar rooms.
- Smoke alarms in 210.12(A) locations. The exemption from AFCI protection for the fire alarm circuit of 760.41(B) and 760.121(B) doesn’t apply to the smoke alarm circuits typically installed in dwelling units. Section 210.12(A), Ex 3 makes it clear that the exception is only for an individual branch circuit to a fire alarm system.
You can omit AFCI protection, where:
- A listed conduit or tubing is encased in at least 2 in. of concrete for the portion of the circuit between the branch circuit overcurrent device and the first outlet. An outlet branch circuit type AFCI at the first outlet can provide protection for the remaining portion of the circuit.
- An individual branch circuit serves a fire alarm system per 760.41(B) and 760.121(B), if the circuit conductors are in RMC, IMC, EMT, or steel sheath Type AC or MC cable that qualifies as an equipment grounding conductor per 250.118, with metal outlet and junction boxes.
You can install an outlet branch circuit type AFCI as the first outlet in a branch circuit to protect downstream wiring and receptacles if:
- Circuit conductors are in RMC, IMC, EMT, or Type MC or steel armored Type AC cable meeting the requirements of 250.118 up to the first outlet, and
- Metal outlet or junction boxes are installed up to and including the first outlet.
Branch circuit ratings
Branch circuit conductors must have an ampacity of at least 125% of the continuous loads, plus 100% of the noncontinuous loads [210.19(A)(1)]. This is based on the ampacities listed in Table 310.15(B)(16), before any ampacity adjustment or correction. When sizing conductors, also remember to select and coordinate conductors so that you do not exceed the lowest temperature rating of any termination or device connected to that circuit [110.14(C)].
All branch conductors must have an ampacity sufficient for the loads served. In addition:
- Branch circuits supplying more than one receptacle must have an ampacity at least that of the rating of the circuit overcurrent device [210.3 and 210.19(A)(2)].
- Branch circuit conductors that supply household ranges, wall-mounted ovens, or counter-mounted cooking units must have an ampacity at least that of the rating of the branch circuit and at least that of the maximum load to be served. For ranges of 8¾kW or more rating, the minimum branch circuit ampere rating is 40A [210.19(A)(3)].
- Conductors tapped from a 50A branch circuit for electric ranges, wall-mounted electric ovens, and counter-mounted electric cooking units must have an ampacity of at least 20A and be sufficient for the load [210.19(A)(3) Ex 2].
Other than tap conductors, all branch circuit conductors must be at least 14 AWG and have sufficient ampacity for the load [210.19(A)(4)]. Tap conductors must have an ampacity of at least 15A for circuits rated less than 40A and at least 20A for circuits rated at 40A or 50A for:
- Luminaires having tap conductors sized per 410.117 [210.19(A)(4)(b)].
- Individual outlets, other than receptacle outlets, with taps up to 18 in. long [210.19(A)(4)(c)].
Branch circuit overcurrent devices must have a rating of at least 125% of the continuous loads, plus 100% of the noncontinuous loads [210.20(A)]. However, if the assembly and the overcurrent devices are both listed for operation at 100% of their rating, the branch circuit overcurrent device could be sized at 100% of the continuous load [210.20(A) Ex]. Overcurrent protection devices are not commonly available with a listing for use at 100% of their rating.
Protect branch circuit conductors against overcurrent in accordance with 240.4, and provide overcurrent protection for equipment based on the individual articles listed in 240.3 and Table 240.3.
Outlet device rating
Lampholders connected to a branch circuit rated more than 20A must be of the heavy-duty type [210.21(A)]. A single receptacle must have an ampere rating of not less than the overcurrent device protecting the branch circuit [210.21(B)(1)].
If connected to a branch circuit that supplies two or more receptacles or outlets:
- The total cord- and plug-connected load must not exceed 80% of the receptacle rating [210.21(B)(2)].
- Receptacles must have an ampere rating per the values listed in Table 210.21(B)(3), as shown in Fig. 1.
Fig. 1. Refer to Table 210.21(B)(3) for guidance on ampere ratings for receptacles.
An individual branch circuit can supply any load for which it’s rated, but the load can’t exceed the branch circuit ampere rating [210.23].
Branch circuits rated 15A or 20A supplying two or more outlets or receptacles must supply loads per 210.23(A) through (D) and 210.24 [210.23].
Cord- and plug-connected equipment not fastened in place, such as a table saw, must not have an ampere rating more than 80% of the branch circuit rating [210.23(A)(1)].
Equipment fastened in place, other than luminaires, must not be rated more than 50% of the branch circuit ampere rating if this circuit also supplies luminaires, receptacle outlets, or both [210.23(A)(2)], as shown in Fig. 2.
Fig. 2. Equipment fastened in place must not be rated more than 50% of the branch circuit rating if thgis circuit supplies luminaires, other receptacles, or both.
Dwelling unit receptacles
In two-family and multi-family dwellings, dwelling unit branch circuits can supply only loads within that dwelling unit or associated with it. Branch circuits installed for public or common areas of a two-family or multifamily dwelling or multi-occupancy building can’t originate from equipment that supplies an individual dwelling unit or tenant space [210.25(A) and (B)].
The two or more 20A, 120V small-appliance branch circuits serving the kitchen, dining room, and similar areas [210.11(C)(1)] must serve all wall, floor and countertop receptacles [210.52(B)(1) and (2)] and the receptacle for refrigeration equipment.
These circuits cannot supply the outlets for luminaires or appliances, but exceptions to this section allow the small appliance circuits to serve clocks and gas-fired ranges, ovens, or counter-mounted cooking units [210.52(B)(2)]. You can supply the refrigeration equipment receptacle from an individual branch circuit rated 15A or greater [210.52(B)(1) Ex 2].
Kitchen countertop receptacles, as required by 210.52(C), must be supplied by at least two 20A, 120V small-appliance branch circuits [210.11(C)(1)]. Either or both can supply receptacle outlets in the same kitchen and in other rooms allowed to be served by the small appliance branch circuits [210.52(B)(1)].
The requirements for the small appliance branch circuits and the receptacle outlets supplied by them must be followed carefully. While drawing out your circuit plan, review it against 210.52(A), (B), and (C) and Figure 210.52 in the NEC.
Bathroom receptacles must be GFCI-protected [210.8(A)(1)]. In bathrooms, install at least one 15A or 20A, 125V receptacle within 3 ft of the outside edge of each basin [210.52(D)]. Locate it on a wall or partition adjacent to the basin counter surface or on the side or face of the basin cabinet not more than 12 in. below the countertop. Receptacle outlet assemblies listed for this specific use may be installed in the countertop [210.52(D)], as shown in Fig. 3.
Fig. 3. Per 210.52(D), receptacle outlet assemblies listed for this specifric use may be installed in the countertop.
For one- and two-family dwellings, install a minimum of two GFCI-protected 15A or 20A, 125V receptacles (one in front, one in back) so they are accessible while standing at grade level and no more than 6½ ft above grade [210.52(E)(1)]. For multifamily dwellings, each unit located at grade level and provided with individual exterior entrances must have at least one receptacle outlet accessible from grade level and not more than 6½ ft above grade.
In all dwelling units, install at least one 15A or 20A, 125V receptacle:
- Within the perimeter and not more than 6½ ft above the surface of any balcony, deck, or porch that is accessible from the inside of the dwelling unit [210.52(E)(3)]. Remember that GFCI protection is required for all 15A and 20A, 125V receptacle outlets installed outdoors at dwellings [210.8(A)(3)].
- In the laundry area [210.52(F)]. Remember that GFCI protection is required for any 15A or 20A 125V receptacle within 6 ft of a sink, in dwellings, except the kitchen which has its own rules [210.8(A)(7)].
- In each basement, garage, or accessory building with electric power (in addition to receptacles for specific equipment) [210.52(G)], as shown in Fig. 4.
- In each hallway that’s at least 10 ft long, measured along the centerline, without passing through a doorway [210.52(H)].
- In foyers greater than 60 sq ft, if not part of a hallway, each unbroken wall space greater than 3 ft must have at least one receptacle installed in it [210.52(I)].
Fig. 4. In a one-family dwelling, at least one 15A or 20A, 125V GFCI-protected receptacle outlet is required in each garage, basement, and accessory building supplied by power.
The requirements of 210.52(A) and (D) also apply to occupancies such as hotel guest rooms and dormitory sleeping rooms. All of the rules of 210.52 apply to guest rooms or guest suites that include cooking provisions [210.60].
For dwelling units, make sure you install wall switch-controlled lighting outlets in these locations [210.70]:
- Habitable rooms
- Attached garages
- Detached garages with electric power
- Entrances/exits to the outdoors with grade-level access
- Storage spaces (switch is allowed to be contained in the lighting outlet)
- Spaces containing equipment requiring servicing (switch is allowed to be contained in the lighting outlet).
For guest rooms or suites, install at least one wall switch-controlled lighting outlet in every habitable room and bathroom [210.70(B).
Install at least one switchable lighting outlet in any attic or underfloor space containing equipment that requires servicing. The switch must be at the usual point of entry; the lighting outlet must be at or near the equipment [210.70(A)(3)].
Although the requirements for branch circuits are more detailed than what we’ve explained in this two-part article, you should have a good understanding of the details, enabling you to more easily apply these rules.
SIDEBAR: What’s a Wall Space?
When it comes to receptacle placement, the most common source of confusion is exactly how do we define a wall space? According to 210.52(A)(2), a wall space is:
- Any space 2 ft or more in width, unbroken along the floor line by doorways, fireplaces, or fixed cabinets.
- The space occupied by fixed panels in exterior walls.
- The space occupied by fixed room dividers, such as freestanding bar-type counters.