By Mike Holt for EC&M Magazine

Here's the follow up to yesterday's newsletter. This includes all of the answers to the questions sent, so you can see how you did.

Note: These questions are based on the 2014 NEC. Any underlined ext indicates a change to the Code rule for the 2014 NEC.

Q1. What does the Code define as a branch circuit?

A1. Article 100 defines a “branch circuit” as the conductors between the final overcurrent device and the receptacle outlets, lighting outlets, or other outlets.

Q2. What designates the rating of a branch circuit in the NEC?

A2. The rating of a branch circuit is determined by the rating of the branch-circuit overcurrent device, not the conductor size [210.3].

Author’s Comment: For example, the branch-circuit ampere rating of a 10 THHN conductor on a 20A circuit breaker is 20A.

Q3. What does the Code define as a multiwire branch circuit?

A3. A multiwire branch circuit consists of two or more ungrounded circuit conductors with a common neutral conductor. There must be a difference of potential (voltage) between the ungrounded conductors and an equal difference of potential (voltage) from each ungrounded conductor to the common neutral conductor [210.4].

(A) General. A multiwire branch circuit can be considered a single circuit or a multiple circuit.

To prevent inductive heating and to reduce conductor impedance for fault currents, all conductors of a multiwire branch circuit must originate from the same panelboard.

Author’s Comment: For more information on the inductive heating of metal parts, see 300.3(B), 300.5(I), and 300.20.

Informational Note 1: Unwanted and potentially hazardous harmonic neutral currents can cause additional heating of the neutral conductor of a 4-wire, three-phase, 120/208V or 277/480V wye-connected system, which supplies nonlinear loads. To prevent fire or equipment damage from excessive harmonic neutral currents, the designer should consider: (1) increasing the size of the neutral conductor, or (2) installing a separate neutral for each phase.

Author’s Comment: See the definition of “Nonlinear Load” in Article 100.

Informational Note 2: See 300.13(B) for the requirements relating to the continuity of the neutral conductor on multiwire branch circuits.

CAUTION: If the continuity of the neutral conductor of a multiwire circuit is interrupted (opened), the resultant over- or undervoltage can cause a fire and/or destruction of electrical equipment.

(B) Disconnecting Means. Each multiwire branch circuit must have a means to simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.

Informational Note: Individual single-pole circuit breakers with handle ties identified for the purpose can be used for this application [240.15(B)(1)].

CAUTION: This rule is intended to prevent people from working on energized circuits they thought were disconnected.

(C) Line-to-Neutral Loads. Multiwire branch circuits must supply only line-to-neutral loads.

Ex 1: A multiwire branch circuit is permitted to supply an individual piece of line-to-line utilization equipment, such as a range or dryer.

Ex 2: A multiwire branch circuit is permitted to supply both line-to-line and line-to-neutral loads if the circuit is protected by a device such as a multipole circuit breaker with a common internal trip that opens all ungrounded conductors of the multiwire branch circuit simultaneously under a fault condition.

(D) Grouping. The ungrounded and neutral conductors of a multiwire branch circuit must be grouped together by cable ties or similar means at the point of origination.

Ex: Grouping isn’t required where the circuit conductors are contained in a single raceway or cable unique to that circuit that makes the grouping obvious, or if the conductors have circuit number tags on them.

Author’s Comment: Grouping all associated conductors of a multiwire branch circuit together by cable ties or other means within the point of origination makes it easier to visually identify the conductors of the multiwire branch circuit. The grouping will assist in making sure that the correct neutral is used at junction points and in connecting multiwire branch-circuit conductors to circuit breakers correctly, particularly where twin breakers are used. If proper diligence isn’t exercised when making these connections, two circuit conductors can be accidentally connected to the same phase or line.

CAUTION: If the ungrounded conductors of a multiwire circuit aren’t terminated to different phases or lines, the currents on the neutral conductor won’t cancel, which can cause an overload on the neutral conductor.

Q4. What are the Code rules for the identification of branch circuit conductors?

A4. The neutral conductor of a branch circuit must be identified in accordance with 200.6 [210.5(A)].

(B) Equipment Grounding Conductor. Equipment grounding conductors can be bare, covered, or insulated. Insulated equipment grounding conductors size 6 AWG and smaller must have a continuous outer finish either green or green with one or more yellow stripes [250.119].

On equipment grounding conductors 4 AWG and larger, insulation can be permanently reidentified with green marking at the time of installation at every point where the conductor is accessible [250.119(A)].

(C) Ungrounded conductors must be identified as follows [210.5(C)]:

(1) More Than One Voltage System. If the premises wiring system contains branch circuits supplied from more than one voltage system, each ungrounded conductor must be identified by phase and system at all termination, connection, and splice points in accordance with 210.5(C)(1)(a) and (b).

(a) Means of Identification. Identification can be by color coding, marking tape, tagging, or other means approved by the authority having jurisdiction.

(b) Posting. The method of identification must be documented in a manner that’s readily available or permanently posted at each branch-circuit panelboard.

Author’s Comment: When a premises has more than one voltage system supplying branch circuits, the ungrounded conductors must be identified by phase and system. This can be done by permanently posting an identification legend that describes the method used, such as color-coded marking tape or color- coded insulation.

Conductors with insulation that’s green or green with one or more yellow stripes can’t be used for an ungrounded or neutral conductor [250.119].

Although the NEC doesn’t require a specific color code for ungrounded conductors, electricians often use the following color system for power and lighting conductor identification:

• 120/240V, single-phase—black, red, and white
• 120/208V, three-phase—black, red, blue, and white
• 120/240V, three-phase—black, orange, blue, and white
• 277/480V, three-phase—brown, orange, yellow, and gray; or, brown, purple, yellow, and gray

Q5. What is the Code voltage limit for branch circuits?

A5. In dwelling units, the voltage between conductors must not exceed 120V, nominal, when they supply the terminals of [210.6(A)]:

(1) Luminaires.

(2) Cord-and-plug-connected loads of 1,440 VA or less or less than ¼ hp.

Q6. What is the Code rule that applies if two or more branch circuits supply devices or equipment on the same yoke?

A6. If two or more branch circuits supply devices or equipment on the same yoke, a means to disconnect simultaneously all ungrounded conductors that supply those devices or equipment is required at the point where the branch circuit originates [210.7].

Author’s Comment: A yoke, also called a strap, is the metal mounting structure for a receptacle, switch, switch with pilot light, switch-receptacle, and so forth.

Individual single-pole circuit breakers with handle ties identified for the purpose, or a circuit breaker with a common internal trip, can be used for this application [240.15(B)(1)].