Article 210 covers branch circuits except those that supply only motor loads. Provisions for motor branch circuits are covered in Article 430. Where a circuit contains combination loads (motor and nonmotor), the installation must comply with Articles 210 and 430. Branch circuits supplying certain equipment must comply with Article 210 and also with the applicable provisions within that equipment’s own article.
For example, branch circuits feeding air-conditioning equipment must not only comply with Article 210, but must also comply with Article 440. See Section 210-2 for a complete listing of equipment, including applicable articles or sections.
Branch circuits, as defined in Article 100, are the circuit conductors between the final overcurrent device protecting the circuit (such as the last breaker or fuse) and the outlet(s). Four categories of branch circuits are defined in Article 100, including appliance, general purpose, individual, and multi-wire.
An appliance branch circuit supplies energy to one or more outlets to connect appliance(s). These circuits have no permanently connected lighting fixtures unless they are part of the appliance being connected. The two or more small-appliance branch circuits, required by Section 210-11(c)(1) for dwelling units, are examples of appliance branch circuits.
A general-purpose branch circuit supplies a number of outlets for lighting and appliances. This type of circuit supplies lights, receptacles, appliances, or any combination thereof. Of the four branch circuit types, this is the most common.
An individual branch circuit supplies only one piece of utilization equipment. Utilization equipment uses electric energy for electronic, electromechanical, chemical, heating, lighting, or similar purposes. A circuit feeding a single receptacle, supplying one piece of utilization equipment, is an individual branch circuit. [Article 100]
A multi-wire branch circuit contains three or more conductors (not counting equipment-grounding conductors). The branch circuit must consist of two or more ungrounded (hot) conductors, and there must be a potential difference between those conductors. This simply means that the hot conductors must be connected to different phases . . . not the same phase. For example, a 120/240-volt, single-phase panel board is energized and the leads of a voltmeter are touching the first and third breaker on the right side.
Although both breakers are energized and both have a potential difference of 120 volts to ground, there is no potential difference between them. With both leads of a voltmeter touching Phase A (or line 1), the meter’s display will show zero volts. A multi-branch circuit must also have one grounded (neutral) conductor, and there must be an equal potential difference between the grounded conductor and each ungrounded conductor of the circuit. This grounded conductor must be connected to the systems neutral or grounded conductor.
Multi-wire branch circuits supplied from a single-phase voltage source must contain two ungrounded conductors (one from each phase) and one grounded conductor. Multi-wire branch circuits supplied from a three-phase voltage source can contain either two or three ungrounded conductors and one grounded conductor.
When two (or three) circuits share a common neutral, it is essential to make sure the ungrounded conductors terminate on different phases. Improperly installed multi-wire branch circuits can create a potential hazard to persons and/or property. Moving or relocating ungrounded conductors in an existing panel-board could also create a hazard.
For example, two 120-volt heaters are supplied from a multi-wire branch circuit. While one heater draws 10 amperes, the other draws 15. The panel board is fed from a 120/240-volt, single-phase source. If properly installed and both heaters are energized, the unbalanced neutral current will be 5 amperes. The current on the neutral will be the difference between the two loads.
To find the maximum current on the neutral, turn off the circuit drawing the least amount of current. In this example, the maximum neutral current is 15 amperes. If one of the ungrounded conductors is relocated and ends up on the same phase as the other ungrounded conductor, the neutral conductor will become overloaded. With both ungrounded conductors on the same phase, the neutral will no longer carry the unbalanced current, but will carry the total current of each ungrounded conductor.
Before, the neutral current was only 5 amperes when both heaters were energized. Now, since the neutral will carry the total load of both heaters, the neutral current will be 25 amperes. (See Figure 8.) Note that the ungrounded conductors draw the same amount of current. Therefore, since there is not an overload on either ungrounded conductor, the breakers will not trip.
Branch circuits recognized by Article 210 must be rated in accordance with the maximum permitted ampere rating or setting of the overcurrent device. The rating for other than individual branch circuits shall be 15, 20, 30, 40, and 50 amperes. [210-3] Supplementary overcurrent protection, as covered in Section 240-10, is not considered a branch-circuit overcurrent protective device.
The exception to 210-3 states that multioutlet branch circuits greater than 50 amperes are permitted if they supply nonlighting outlet loads on industrial premises where maintenance and supervision indicate that qualified persons will service the equipment.
Next month’s In Focus, beginning with Section 210-4, will continue discussing the general branch-circuit provisions covered in Article 210.
MILLER, owner of Lighthouse Educational Services, teaches custom-tailored classed and conducts seminars covering various aspects of the electrical industry. He is the author of Illustrated Guide to the National Electrical Code. For more information, visit his Web site at www.charlesRmiller.com. He can be reached by phone at (615) 333-3336, or via e-mail at charles@charlesRmiller.com.