| Home | Fire
Safety | Skyscrapers Home Emergencies | Glossary |
|
. The grounded and ungrounded (neutral) conductors are necessary to complete an electrical circuit. They provide a continuous path from the load to ground. Both conductors are connected to the earth (to ground), but there are differences in the location and function of each of these conductors. These differences are addressed in the following sections. Grounded Conductor In single-phase branch circuits (beyond the service equipment and any feeders and originating at the panelboard), a grounded conductor serves as the grounded leg of the circuit. It completes the circuit by connecting the ungrounded (hot) conductor to ground. Thus, in circuit design, the grounded conductor is considered to be a current carrying conductor because it serves as a return path back to the circuit's power source. On a two-wire branch circuit (e.g., a 120 V circuit with one ungrounded conductor and one grounded conductor), the grounded conductor carries current equal to the load. For ex ample, if the ungrounded (hot) conductor is feeding a load of 12 A, then the grounded conductor carries 12 A to ground. As a result, the grounded conductor must be sized at the same ampacity as the ungrounded (hot) conductor. Neutral Conductor A neutral conductor performs the function of a grounded conductor for at least two ungrounded (hot) conductors that have sources from different voltage phases, such as on a multiwire branch circuit, multiwire feeder, and the electrical service. The conductors served by a neutral must measure voltage between the ungrounded conductors and be protected by a double or triple-pole breaker or set of fuses. A neutral conductor can not by definition serve a single 120 V circuit because it has only one ungrounded conductor; a neutral conductor is a grounded conductor that is shared between two or more ungrounded conductors. Thus, a neutral conductor is frequently called a shared neutral or common neutral. In a 120/240 V three-wire service entrance, feeder, or branch circuit, there is a neutral conductor and two ungrounded (hot) conductors, each with 120 V to ground. A neutral conductor is also present in 208 Y/120 V and 480 Y/277 V services, feeders, and circuits. Grounded and neutral conductors should always be insulated because they can carry current. They should be continuous because they provide a path to complete the circuit or circuits. The neutral conductor is grounded at the transformer and at the service equipment (switchboard or panelboard). In these cases it’s connected to ground much like the grounding conductor. At subpanelboards served by feeders, the equipment grounding conductor and the neutral conductor remain separate. This way, each circuit extending from the subpanelboard has a separate equipment grounding conductor and the neutral conductor. Grounded and neutral conductors should be wired so a circuit breaker, fuse, or switch does not interrupt them. A switch or overcurrent device located in the grounded or neutral conductor allows the remainder (and much of) of the circuit wiring to remain electrically hot if it opens the circuit. Consider the following example: A branch circuit serves several light fixtures with a switch connected to the grounded conductor in stead of the ungrounded (hot) conductor. When the switch is closed (on), the circuit is complete and the lamps light; when the switch is open (off), the lamps are off. It appears the circuit is controlled and wired properly, but there are concerns. With the switch open (off), the circuit wiring and light fixtures are still hot; that is, power runs through the ungrounded wiring to the lamps and through the grounded wiring to the open switch where it’s interrupted. The lamps in the light fixtures don’t light but the circuit wiring is hot back to the open switch. Similarly, a circuit breaker or fuse located on the grounded conductor may open the circuit because of a short-circuit or current overload, but the circuit wiring will remain hot. A hot circuit is potentially hazardous condition because it unnecessarily exposes occupants to hot wiring. Interruption of the grounded or neutral conductor with a circuit breaker, fuse, or switch is considered poor practice in most instances. (There are unique cases when the grounded and ungrounded conductors are controlled with a switch.) When switching control is desired in a circuit, the ungrounded (hot) wire should be the conductor that is switched. Also, the circuit should be protected with a circuit breaker or fuse in the un grounded (hot) wire so, if a short circuit or current overload occurs, the entire circuit is interrupted. In both cases, power is interrupted near the source (e.g., at the panelboard) so that circuit wiring does not remain hot. Load Balancing In multiwire branch circuits, feeders, and services (e.g., a 120/240 V or 480 Y/277 V circuits with two or more un grounded conductors and one neutral conductor), the common neutral conductor carries current only when dissimilar loads exist on the ungrounded (hot) conductors in the circuit. For ex ample, in a 120/240 V single-phase, three-wire circuit, if one ungrounded (hot) conductor feeds a 16 A load on one circuit and the other feeds a 4 A load on the other circuit, the common neutral conductor carries 12 A to ground. This is the difference between the two loads. A common neutral conductor carries only the unbalanced load. In multiwire circuits, feeders, and services the neutral load can be quite high if loads on different circuits are uneven from occupant use of electrical equipment or poor design. High neutral loads have been known to overheat neutral conductors and distribution transformers without tripping overcurrent protection. For example, in office buildings, computer use is very sporadic (unlike lighting, which is fairly constant), which causes disproportionate loading on individual circuits and a heavy load on the neutral conductor. Under extreme conditions of use, this unbalanced load can be excessive and unsafe. Designing circuits to avoid or to accommodate the high neutral currents can avert the conditions created by unbalanced loading on a common neutral. Typically, the steps are balancing the circuit loads, treating the neutral as a current carrying conductor, and properly sizing the common neutral conductor in multiwire branch circuits, feeders, and services. Load balancing is the practice of dividing loads as evenly as possible between the ungrounded conductors on a multiwire circuit, feeder, or service. A well-designed neutral is balanced so that under load little or no current flows through the neutral conductor. Similarly, the designer should attempt to balance the loads in feeders and building services. This is accomplished by dividing loads between the A and B legs or X, Y, and Z legs; that is, on a three-wire, single-phase system the circuit loads should be evenly shared (as possible) between the A and B hot conductors, and on a four-wire, three-phase system the circuit loads should be evenly divided between the X, Y, and Z hot conductors. On a multiwire circuit, each ungrounded conductor should carry loads equally. Load balancing of 240 V circuits on a 120/240 V three wire, single-phase system is easily accomplished because the load is the same on each phase conductor serving the circuit. Two-pole circuit breakers connect to the panelboard so that one breaker connects to the Phase A bus and the other breaker to the Phase B bus. Balancing 120 V circuits on a 120/240 V three-wire, single-phase system is more difficult. Loads must be evenly divided between hot conductors. These breakers or fuses serving ungrounded conductors must be connected to the hot buses in the panelboard so that loads on each bus tend to equate with loads on the other bus. On a three-phase system, the neutral conductor is common to all phases. Three-phase motors and other equipment operating on three-phase systems are connected to the three ungrounded (hot) conductors and a common neutral. Balancing is easily accomplished because the common neutral conductor is shared and carries no load. Three-pole circuit breakers connect to the panelboard such that one breaker connects to the Phase X bus, a second connects to the Phase Y bus, and the third breaker connects to the Phase Z bus. A two-wire circuit on a three-phase system (e.g., 277 V circuit) is more difficult be cause it requires that loads be evenly divided between phase conductors. The basic way to attempt to balance the loads at a panel board is to connect an equal number of branch circuits to each ungrounded bus, but this only accomplishes good balancing when all circuits are equally loaded. On lighting panelboard, this is accomplished by connecting equal loads to each circuit. When circuit or feeder loads are not equal, circuits must be divided so that the total load on each phase conductor is equal. This requires the designer to identify the load on each circuit. In most cases the load on a circuit is not known exactly so the designer predicts an anticipated load and uses it to balance the neutral load. Because a neutral conductor will typically not carry cur rent equivalent to the ungrounded conductors in the multiwire circuit, feeder, or service it serves, it’s generally not required to be sized at the same ampacity rating as the ungrounded (hot) conductor. Code prescribes methods to de-rate (reduce) the size to an acceptable size. Discretion must be exercised by the de signer in this instance because it’s possible to undersize the neutral, particularly if loading of a panelboard is changed or expanded as a result of future renovation. As a current-carrying conductor, an undersized neutral will overheat, producing a hazardous condition, and of course, this condition can be avoided by using a larger neutral conductor. The neutral conductor is sized to carry the maximum un balanced current in the circuit (for instance, the largest load between the neutral and any one ungrounded phase conductor). On large circuits, feeders, and the service, the first 200 A of neutral current is computed at 100%. After the first 200 A, additional resistive loads on the neutral can be reduced by a demand factor of 70%, but all inductive neutral current must be computed at 100% (no demand factor). Additionally, for cooking equipment or clothes dryers, the feeder neutral load should be computed at 70% of the demand load. Ex. **1 The neutral load on a 120/240 V, three-wire feeder circuit carries an unbalanced load on the ungrounded conductors of 68 A on Phase A and 58 A on Phase B. Determine the grounded neutral conductor load. Unbalanced load _ 68 A _ 58 A _ 10 A The grounded neutral conductor load is 10 A for these conditions. Prev: Building System
Voltages |