Start with the overcurrent protection device to correctly rate branch-circuit conductors
Article 210 contains the requirements for conductor sizing, conductor identification, overcurrent protection, and GFCI and AFCI protection of branch circuits. It also contains requirements for receptacle outlets and lighting outlets. The sidebar, “Branching Out” below, lists other applicable Code sections related to branch circuits.
Branch circuits are the most commonly installed of all circuits, so a good working knowledge of Art. 210 is essential. That knowledge starts with understanding what a branch circuit is. It consists of the conductors between the final overcurrent protection device (OCPD) and the receptacle outlets, lighting outlets, or other outlets [Art. 100] (Fig. 1).
A key point to remember about branch circuits is that the overcurrent protection device (OCPD) defines the circuit. Thus, the rating of the OCPD determines the rating of the branch circuit; the conductor size does not [210.3].
Multiwire branch circuits. Multiwire branch circuits are circuits that have more than one ungrounded conductor sharing a common grounded (neutral) conductor. These circuits are very beneficial in that they use less material, result in a lower circuit voltage drop, and ultimately result in cost savings. They do, however, have some specific Code rules that can't be ignored.
To prevent inductive heating and reduce conductor impedance for fault currents, all multiwire branch-circuit conductors must originate from the same panelboard or distribution equipment [210.4(A)].
Multiwire branch circuits must supply only line-to-neutral loads [210.4(C)].
Exception 1: A multiwire branch circuit can supply line-to-line utilization equipment, such as a range or dryer.
Exception 2: A multiwire branch circuit can supply both line-to-line and line-to-neutral loads if the circuit is protected by a device (multipole circuit breaker) that opens all ungrounded conductors of the multiwire branch circuit simultaneously (common internal trip) under a fault condition.
If the continuity of the grounded (neutral) conductor of a multiwire circuit is interrupted (open), the resultant over- or undervoltage could cause a fire and/or destruction of electrical equipment. See 300.13(B) for the requirements relating to the continuity of the grounded (neutral) conductor on multiwire circuits.
Disconnecting means. Where two or more branch circuits supply devices (or equipment) on the same yoke, you must provide a means to disconnect simultaneously all ungrounded conductors that supply those devices or equipment. A “yoke” is the metal mounting structure for a device (e.g., switch, receptacle, pilot light). It's also called a strap. Locate it at the point where the branch circuit originates [210.7(B)] (Fig. 2 on page 60). Individual single-pole circuit breakers with handle ties identified for the purpose can be used for this application [240.20(B)(1)]. So can a breaker with a common internal trip. This rule prevents people from working on energized circuits they thought were disconnected.
Multiwire branch circuits that supply devices (or equipment) on the same yoke must also be provided with a means to disconnect simultaneously all ungrounded conductors that supply those devices (or equipment) at the point where the branch circuit originates [210.4(B)] (Fig. 3).
Dwelling unit branch circuits can supply only loads within (or associated with) the dwelling unit. Common area branch circuits for house lighting, central alarm, signal, fire alarm, communications, or other public safety needs must not originate from any single dwelling unit [210.25]. This rule prohibits common area branch circuits in two-family or multifamily dwellings from being supplied from an individual dwelling unit. In addition, this prevents common area circuits from being turned off by individual tenants or by the utility because of nonpayment of electric bills.
Identification. The grounded (neutral) conductor of a branch circuit must be identified per 200.6 [210.5]. Equipment grounding (bonding) conductors can be bare, covered, or insulated. Insulated equipment grounding (bonding) conductors sized 6 AWG and smaller must have a continuous outer finish that is either green or green with one or more yellow stripes [250.119]. Equipment grounding (bonding) conductors larger than 6 AWG, if insulated, can be permanently reidentified with green marking at the time of installation. This marking has to be present at every point where the conductor is accessible [250.119(A)].
Where premises wiring systems contain branch circuits supplied from more than one voltage system, each ungrounded conductor (where accessible), must be identified by system. Identification can be by color-coding, marking tape, tagging, or other means approved by the authority having jurisdiction (AHJ). Such identification must be permanently posted at each branch-circuit panelboard or distribution equipment [210.5(C)]. The NEC does not require any particular color scheme (such as black, red, and blue for 120/208V, or brown, orange, and yellow for 277/480V).
Conductors with insulation that is green or green with one or more yellow stripes cannot be used for an ungrounded or grounded (neutral) conductor [250.119]. See the sidebar, “Phase Identification,” below, for the normal wire marking color scheme.
Voltage limitations. In dwelling units (and in guest rooms or guest suites of hotels, motels, and similar occupancies), the branch-circuit voltage can't exceed 120V (nominal), if those circuits are [210.6(A)]: luminaires or cord-and-plug connected loads rated not more than 1,440 volt-amperes (VA), or less than ¼ horsepower.
In nondwelling units, you can use 277V (phase-to-ground) circuits to supply any of the following [210.6(C)]:
Listed electric-discharge luminaires.
Luminaires with mogul base screw shells.
Lampholders other than the screw-shell type.
Equipment rated at 277V.
If an Edison-base lampholder is rated for 120V, don't put it on a 277V circuit.
Conductor sizing. You must size conductors no less than 125% of the continuous loads, plus 100% of the noncontinuous loads [210.19]. Base this on the terminal temperature rating ampacities as listed in Table 310.16 before any ampacity adjustment [110.14(C)].
In addition, conductors must have sufficient ampacity, after applying adjustment factors, to carry the load — and they must be protected against overcurrent per their ampacity [210.19(A)(1), 210.20(A) and 240.4].
Overcurrent protection. Branch-circuit OCPDs must have an ampacity not less than 125% of the continuous loads, plus 100% of the noncontinuous loads [210.20(A)].
Outlet ratings. A single receptacle on an individual branch circuit must have an ampacity not less than the rating of the OCPD [210.21(B)(1)]. A single receptacle has only one contact device on its yoke [Article 100]. This means you treat a duplex receptacle as two receptacles.
If a branch circuit supplies two or more receptacles:
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 that complies with the values listed in Table 210.21(B)(3) (Fig. 4).
Permissible loads. An individual branch circuit can supply any load for which it's rated [210.23]. A multioutlet branch circuit can supply only lighting and/or equipment loads as summarized in Table 210.24 and as specified in 210.19, 210.20, and 210.21. If you're installing a multioutlet branch circuit, read those requirements carefully.
Cord-and-plug connected equipment not fastened in place, such as a drill press or table saw, for example, must not have an ampere rating more than 80% of the branch-circuit rating [210.23(A)(1)]. UL and other testing laboratories list portable equipment (such as hair dryers) up to 100% of the circuit rating. The NEC is an installation standard, not a product standard, so it cannot prohibit this practice. There really is no way to limit the load to 80% of the branch-circuit rating if testing laboratories permit equipment to be listed for 100% of the circuit rating.
Equipment fastened in place (not a luminaire) must not be rated more than 50% of the branch-circuit ampere rating if this circuit supplies luminaries, other receptacles, or both [210.23(A)(2).
To keep your mind clear when sizing conductors for branch circuits, remember that the OCPD defines the circuit. If a 20A circuit contains 8 AWG conductors because of voltage drop, it is still a 20A circuit — the size of the OCPD determines the rating of the branch circuit. That's because the conductors must be able to handle the current supplied by the OCPD. Going forward, you should now be able to handle branch-circuit requirements.
Sidebar: Branching Out
Other sections of the Code relating to branch circuits include:
Air-conditioning and refrigeration equipment [440.6, 440.31, and 440.32]
Data-processing (information technology) equipment [645.5]
Electric space heating equipment [424.3]
Sidebar: Phase Identification
Electricians often use the following color system for power and lighting conductor identification:
120/240V, single-phase — black, red, and white
120/208V, 3-phase — black, red, blue, and white
120/240V, 3-phase — black, orange, blue, and white
277/480V, 3-phase — brown, orange, yellow, and gray; or, brown, purple, yellow, and gray