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. Supply Voltages Power is delivered by the utility company to the user at supply voltages. Supply voltage is expressed as a nominal voltage because it varies slightly. During normal conditions, supply voltages can vary from about 90 to 105% of nominal voltage. Variations from nominal voltages are caused by a number of reasons, including load variation and changes in conditions at the utility power system. Additionally, transient voltages, caused by phenomena such as lightning strikes, some types of faults, and the switching of some types of user loads, cause variations in voltage avail able to the user. System Voltages The principle voltages available in a building are called the system voltages. Medium and high voltage systems carry volt ages above 600 V may be used in special cases such as the 2400/4160 V three-phase system found in industrial and commercial installations such as for large signage, sports lighting in stadiums, and services for large manufacturing plants and sky scrapers. There are, however, drawbacks to voltages higher than 600 V because significant and costly special precautions such as heavy insulation and conductor shielding are needed. As a result, low voltage systems that carry voltages less than 600 V are typically used in buildings. System voltage is ex pressed as a nominal voltage because it varies slightly for the reasons mentioned earlier. Design of a building's electrical system begins with establishing the desired building system voltage. A higher voltage means that a circuit can carry more current. A 208 V circuit can carry 1.73 times the current of a 120 V circuit (208 V/120 V _ 1.73); a 240 V circuit can carry twice the current of a 120 V circuit; a 277 V circuit can carry 2.31 times the current of a 120 V circuit; and so on. Thus, higher voltage means smaller conductor sizes. The savings for larger conductors (feeders) of moderate length can be quite significant. Higher voltage, however, is more dangerous. There are a numerous system voltage levels and combi nations used in buildings throughout the world. Availability of a particular system voltage is dependent on utility lines and equipment at or near the building site. In the United States and Canada, several system voltage configurations are in use. How ever, a particular system voltage may not be available at a specific building site. UNGROUNDED (HOT) CONDUCTOR GROUNDED (NEUTRAL) CONDUCTOR GROUNDING (GROUND) CONDUCTOR Fig.2 ---Two types of conductors are required to deliver alternating current to the building system: the ungrounded conductor and the grounded conductor. A third conductor, called a grounding conductor, is added to most circuits. The ungrounded conductor is the current-carrying conductor in an alternating current system. A grounded or neutral conductor is required to complete the circuit by connecting the ungrounded conductor to ground. A third conductor, the grounding conductor, provides additional protection. Circuit Wiring Before entering into a detailed discussion on system voltages, it’s necessary to introduce types of wiring found in a circuit. A minimum of two types of conductors is required to deliver alternating current in a building electrical system: the ungrounded conductor and the neutral conductor. A third conductor, called a grounding conductor, is added to most circuits. See Fig.2. Each conductor plays an important role in how electricity is distributed to and circuited in a building. The different function of each conductor is introduced here as a prelude to understanding building system voltages. A more detailed discussion of these conductors is introduced later in this section. Ungrounded Conductor When two associated ungrounded conductors are connected in a single circuit, a higher voltage is delivered. Grounded/Neutral Conductor A grounded (neutral) conductor is required to complete a single phase circuit by connecting the ungrounded (hot) conductor to ground. The neutral conductor is a grounded conductor that serves more than one circuit. It carries the unbalanced load between two ungrounded (hot) conductors. Both conductors complete the circuit(s) by connecting it to ground and, as a result, are treated as current carrying conductors. Grounding Conductor A third conductor known as the grounding conductor provides supplementary but important grounding protection. The grounding conductor is not normally a current-carrying conductor, but is energized only on a temporary, emergency basis when there is a fault between an ungrounded (hot) conductor and any metal associated to the electrical equipment. Confusion often exists between the definitions of the grounded conductor and the grounding conductor because the grounding conductor is commonly referred to as a "ground." It’s more correctly called a "grounding" conductor. In a simple single-phase circuit, the ungrounded conductor provides power to the load and the grounded conductor provides a path from the load back to the power source, which completes the circuit. Voltage in the circuit is equal to the voltage on the un grounded conductor (e.g., 120 V on a 120 V circuit). When two ungrounded (hot) conductors in a single-phase circuit are connected in a circuit, voltage in the circuit is double the voltage available on each ungrounded conductor (e.g., It’s 240 V between two ungrounded conductors that are 120 V each). A three-phase circuit requires three ungrounded conductors that are one-third out of phase of each other. In the case where two ungrounded conductors from a three-phase wye connected transformer are connected, voltage in the circuit is 1.732 times the voltage available on each ungrounded conductor (e.g., 208 V between two ungrounded conductors that are 120 V each but one-third out of phase). The factor 1.732 is the square root of 3. Common Building System Voltages A comparison of the number of conductors and voltages between conductors on common building system voltages is found in Tbl.1. The following is a description of the common building system voltages used in the United States and Canada. 120 Volt, Alternating Current, Single-Phase, Two-Wire System (120 V AC, 1_-2W) The 120 V AC, 1_-2W system is the most basic system voltage used. It was used in the first electrical services to buildings; however, nearly all have since been upgraded. Today, this sys tem is used to serve outbuildings and farm buildings because its use is limited to buildings with loads up to 6000 VA (50 A). The service entrance provided to the service equipment (switch board or panelboard) is by two conductors: one ungrounded (hot) conductor carrying 120 V and one neutral conductor. Volt age measured between the ungrounded (hot) and neutral conductors is 120 V. Before about 1945, only two conductors were used to feed electrical energy beyond the panelboard to the 120 V branch circuits in the building; this configuration relates to the two-slotted convenience outlets found in older buildings. Later, the grounding conductor became a standard part of branch circuits. It relates to the extra slot found in the three-slotted convenience outlets in newer homes. Today, three conductors are used to deliver electrical energy to the branch circuits in the building: one ungrounded (hot) conductor carrying 120 V, one neutral conductor, and one grounding conductor. ====== Tbl. **1 A COMPARISON OF THE NUMBER OF CONDUCTORS AND VOLTAGES BETWEEN CONDUCTORS ON COMMON BUILDING SYSTEM VOLTAGES. Number of Conductors Voltage Between Conductors Voltage Between Voltage Ungrounded Grounded Grounding One Grounded Between Two (Hot) (Neutral) (Ground) Conductor and One Ungrounded Building System Voltage Conductors; Conductors; Conductors Ungrounded Conductor Conductors 120 V, single phase, two wire 1 1 1 120 - 120/240 V, single phase, three wire 2 1 1 120 240 208 Y/120 V, three phase, four wire 3 1 1 120 208 480 Y/277 V, three phase, four wire 3 1 1 277 480 600 Y/346 V, three phase, four wire 3 1 1 346 600 X phase: 120 Y phase: 120 240 /120 V, three phase, four wire 3 1 1 Z phase: 208 240 ====== 120/240 Volt, Alternating Current, Single-Phase, Three-Wire System (120/240 V AC, 1_-3W) The 120/240 V AC, 1_-3W is the most common residential electrical service in use today. It’s also used on a limited basis in light commercial buildings such as small office buildings, churches, and retail shops and stores. The service entrance conductors feeding the service equipment (switchboard or panelboard) are three conductors: two un grounded (live) conductors, each carrying 120 V and one neutral conductor. See Fig.3. The ungrounded (hot) conductors are known as the A and B legs. At the switchboard or panelboard, the grounding conductor is added. The availability of 120 V or 240 V leads to a number of circuit or feeder arrangements that can sup ply the following loads: • 120 V, single-phase, two-wire branch circuit • 240 V single-phase, two-wire branch circuit • 120/240 V single-phase, three-wire feeder or branch circuit On a 120/240 V system, a 120 V branch circuit provides electrical energy to convenience (receptacle) outlets, small appliances, and light fixtures. A 240 V branch circuit serves large appliances and equipment such as electric-resistance baseboard heaters, water heaters, and air conditioning equipment. A 120/240 V branch circuit provides both 120 V and 240 V to an appliance such as a range and clothes dryer; controls, and light fixtures. Typically, small motors run on 120 V and heating elements operate on 240 V. A grounding conductor runs continuously through all branch circuits and serves as a safety circuit in case of a short-circuit. Fig.3 ---Shown is a schematic of a 120/240 V, alternating current, single-phase, three-wire system. The service entrance conductors include ungrounded (live) conductors A and B and one grounded conductor (N). At the panelboard, a grounding conductor that runs continuously through all branch circuits is added. The availability of 120 V or 240 V leads to a number of circuit or feeder arrangements that can supply the following loads: 120 V, single-phase, two-wire branch circuit; 240 V single-phase, two-wire branch circuit; and 120/240 V single-phase, three-wire feeder or branch circuit. Main disconnect is not shown for clarity. Fig.4---Shown above is a schematic of a 208 Y/120 V, alternating current, three-phase, four-wire system. The service entrance conductors include ungrounded (live) conductors A, B, and C, and one grounded conductor (N). At the panelboard, a grounding conductor that runs continuously through all branch circuits is added. Voltage measured across the connection of any single ungrounded conductor (X, Y, or X phase) and the grounded conductor provides 120 V single-phase power. Voltage across any two ungrounded conductor (X, Y, or Z phase) is 208 V single-phase power, because each phase is one-third out of phase. Main disconnect is not shown for clarity. Fig.5 ---Shown is a schematic of a 208 Y/120 V, alternating current, single-phase, three-wire system. The service entrance conductors include ungrounded (live) conductors A and B, and one grounded conductor (N). At the panelboard, a grounding conductor that runs continuously through all branch circuits is added. The availability of 120 V or 208 V leads to a number of circuit or feeder arrangements that can supply the following loads: 120 V, single-phase, two-wire branch circuit; 240 V single-phase, two-wire branch circuit; and 120/208 V single-phase, three-wire feeder or branch circuit. This system is extracted from a 208 Y/120 V, alternating current, three-phase, four-wire system. Main disconnect is not shown for clarity. 208 Y/120 Volt, Alternating Current, Three-Phase, Four-Wire System (208 Y/120 V AC, 3_-4W) The 208 Y/120 V AC, 3_-4W is an older electrical service found in small commercial buildings (e.g., office buildings and schools) and high-rise buildings where three-phase motors (motors above about 1/2 horsepower) and equipment such as large air conditioners are used. It’s not used very often in industry because a 480 V system is more economical for large motor loads. The Y in 208 Y/120 V relates to a wye (Y) winding con figuration of the secondary windings of the transformer producing three-phase current. Service entrance provided to the service equipment (switchboard or panelboard) is by four conductors: three ungrounded conductors, each at 120 V and one third out of phase, and one neutral conductor. See Fig.4. The ungrounded (hot) conductors are known as the X, Y, and Z legs or phases. Voltage measured across the connection of any single ungrounded conductor (X, Y, and Z phase) and the neutral conductor provides 120 V single-phase power. Voltage across any two ungrounded conductor (X, Y, and Z phase) is 208 V single-phase power because each phase is one-third out of phase. Equipment designed to operate on three-phase power, such as a three-phase motor, is connected to the three ungrounded conductors (X, Y, and Z phases). At the panelboard, a grounding conductor is added and it runs continuously through all branch circuits. The availability of 120 V or 208 V in single- or three-phase leads to a number of circuit or feeder arrangements that can supply the following loads: • 120 V, single-phase, two-wire branch circuit • 208 V single-phase, two-wire branch circuit • 208 V three-phase, three-wire branch circuit • 120/208 V three-phase, three-wire feeder or branch circuit As shown in Fig.5, this system is occasionally bro ken down within the building to provide 120/208 V on a single phase, three-wire system. In some cases where 208 Y/120 V is the only system voltage available, it may serve residential customers. The 120/208 V, single-phase, three-wire system works much like a 120/240 V, single-phase system, except 208 V is available instead of 240 V, about 87% of the voltage and thus the power. Distribution is by three conductors and a grounding conductor. In this configuration, voltage measured across the connection of any single ungrounded (hot) conductor and the neutral conductor provides 120 V single-phase power. Voltage across any two ungrounded conductor is 208 V single-phase power. A grounding conductor runs continuously through all branch circuits. 480 Y/277 Volt, Alternating Current, Three-Phase, Four-Wire System (480 Y/277 V AC, 3_-4W) The 480Y/277 V AC, 3_-4W is a common electrical service in most modern medium to large commercial buildings. The 480 V three-phase power is used to power specially designed heavy machinery (e.g., at machine shops and manufacturing plants). High-voltage, 277 V fluorescent lighting and other single-phase devices have also been developed specifically for use with this system. Large retail shopping malls, schools, grocery supermarkets, and office buildings may use this system for its 277 V fluorescent lighting capabilities, where fixtures are not located closer than 3 ft away from windows, platforms, and fire escapes. The Y in 480 Y/277 V relates to a wye-winding configuration of the secondary windings of the transformer producing three-phase current. In this system, service entrance to the service equipment (switchboard or panelboard) is provided by four conductors: three ungrounded conductors, each at 277 V and one-third out of phase, and one neutral conductor. Small trans formers located in electrical rooms or closets in the building step down the voltage from 480 V to 120 V for small equipment and convenience outlets. At the panelboard or switchboard, a grounding conductor is added. The ungrounded (hot) conductors are known as the X, Y, and Z phases. Voltage measured across the connection of any single ungrounded conductor (X, Y, and Z phase) and the neutral conductor provides 277 V single-phase power. Voltage across any two ungrounded conductor (X, Y, and Z phase) is 480 V single-phase power. See Fig.6. Three-phase motors and other equipment that operate on three-phase power are connected to the three ungrounded conductors (X,Y, and Z phases). The availability of 277 V or 480 V in single or three -phase offers the following circuit or feeder configurations: • 277 V, single-phase, two-wire branch circuit • 480 V single-phase, two-wire branch circuit • 480 V three-phase, three-wire branch circuit • 277/480 V three-phase, three-wire feeder or branch circuit When compared with the 208 Y/120 V system, the 480 Y/277 V system has economic advantages from the standpoint of equipment and conductors. Because a given conductor can carry more than twice the VA load at 480 V than at 208 V, the savings in wire size for feeders can be quite significant with the 480 Y/277 V system. Additionally, the smaller current at 480 V for any supply transformer capacity permits the use of protective devices with both smaller frame size and interrupting rating. Both of these factors permit significant savings. Fig.6 Shown is a schematic of a 480 Y/277 V, alternating current, three-phase, four-wire system. In this system, service entrance conductors include three ungrounded conductors, each at 277 V and one-third out of phase, and one grounded conductor. At the panelboard, a grounding conductor is added. The ungrounded (live) conductors are known as the X, Y, and Z phases. Voltage measured across the connection of any single ungrounded conductor (X, Y, and Z phase) and the grounded conductor provides 277 V single-phase power. Voltage across any two ungrounded conductor (X, Y, and Z phase) is 480 V single-phase power. Three-phase motors and other equipment that operate on three-phase power are connected to the three ungrounded conductors (X, Y, and Z phases). Main disconnect is not shown for clarity. 600 Y/346 Volt, Alternating Current, Three-Phase, Four-Wire System (600 Y/346 V AC, 3_-4W) The 600Y/346 V AC, 3_-4W is a less common electrical service in large commercial and industrial buildings that is used to power specially designed heavy machinery. This system is de signed like the 480 Y/277 V AC, 3_-4W described earlier, except that 600 V and 346 V are available in the circuit or feeder configurations. The 600 Y/346 V system has additional economic advantages from the standpoint of equipment and conductor sizing in comparison to the 208 Y/120 V and 480 Y/277 V systems. However, the 550 V or 575 V equipment used on the 600Y/346V system is not as readily available as the 460 V equipment used on the 480 Y/277 V system. As a result, this electrical service is used primarily in industries where the 600Y/346 V systems are a tradition. Fig.7 Shown is a schematic of a 240 /120 V, alternating current, three-phase, four-wire system. In this three-phase, four-wire, delta-connected system, the midpoint of one phase winding is grounded to provide 120 V between Phase X and ground and Phase Z and ground. There are 240 V between the windings of each phase that is, between Phases X and Y, Phases X and Z, or Phases Y and Z. Between Phase Y and ground there is 208 V available. The Phase Y leg is known as the high leg or wild leg because it has a higher voltage to ground than the other legs. Main disconnect is not shown for clarity. === Tbl. **2 THE RELATIONSHIP BETWEEN THE BUILDING SYSTEM VOLTAGE, CURRENT (AMPERAGE), AND REQUIRED CONDUCTOR SIZE FOR A 20 KVA LOAD AND A POWER FACTOR OF 1.0. HIGHER VOLTAGE MEANS LOWER AMPERAGE (AND SMALLER CONDUCTOR SIZE) FOR A SPECIFIC LOAD. ALTHOUGH IT IS LESS SAFE TO TRANSMIT POWER AT HIGH VOLTAGE, IT IS MORE ECONOMICAL BECAUSE OF SMALLER WIRE SIZES REQUIRED AT HIGHER VOLTAGES. Common Building Current Requirement Required Conductor Size System Voltages for 20 kVA Load* (AWG) Copper-75°C (167°F) Volts (V) Amperes (A) Rating 480 41.7 No. 8 277 72.2 No. 4 240 83.3 No. 4 208 96.2 No. 3 120 166.7 No. 4 0 *I _ P/E; For example: 20 000 VA/480 V _ 41.7 A. ==== 240 _/120 Volt, Alternating Current, Three-Phase, Four-Wire System (240 _/120 V AC, 3_-4W) The 240 /120 AC, 3_-4W is another fairly common electrical service found in commercial and industrial buildings where three-phase motors (motors above about 1/2 horsepower) and equipment such as large air conditioners are used. On this three-phase, four-wire, delta-connected ( ) system, the mid-point of one phase winding is neutral to provide 120 V between Phase X and ground and Phase Z and ground only. There are 240 V between the windings of each phase-that is, between Phases X and Y, Phases X and Z, or Phases Y and Z. Between Phase Y and ground there is 208 V available. Phase Y is known as the high leg or wild leg because it has a higher voltage to ground than the other legs. See Fig.7. The availability of 120 V, 208 V, and 240 V in single or three phase offers a number of circuit or feeder options that can supply the following loads: • 120 V, single-phase, two-wire branch circuit (Phase X and ground and Phase Z and ground) • 208 V single-phase, two-wire branch circuit (Phase Y and ground) • 240 V three-phase, three-wire branch circuit • 120/240 V three-phase, three-wire feeder, or branch circuit Tbl.2 shows the relationship between common building system voltage, current (amperage) and conductor size. These are the common building system voltages used in the United States and Canada but other system voltages are used for special applications. Large manufacturing plants use 2400 V or 4160 V distribution systems. A 2400 V distribution system is typically used to power 2300 V motors up to about 2000 hp. Motors at and above 3000 hp are typically 4000 V motors and thus a 4160 V distribution system is the standard. ==== Tbl. **3 A COMPARISON BETWEEN SYSTEM, UTILIZATION, AND OUTDATED VOLTAGES. System Voltages Utilization Voltages Outdated Voltages 120/240 V 115/230 V 110/220 V 208/120 V 200/115 V 208/110 V 480/277 V 460/265 V 440/255 V ==== System, Utilization, and Maximum Voltages A building system voltage may be specified as 120/240 V but is sometimes referred to as 110/220 V, 115/230 V or 125/250 V. The difference in the cited voltages has to do with variations in how the voltage is being defined. There are three ways that a voltage is defined: system voltage, utilization voltage and maximum voltage. A comparison is provided in Tbl.3. System, utilization and outdated voltages are defined as follows. System Voltage System voltage is the target voltage entering the service panel. On a 120/240 V system, the standard for the system voltage is actually 120/240 V; that is, the voltage available at the service equipment is approximately 120/240 V. In practice, this voltage is sometimes a little less and sometimes a little more. System voltage will vary slightly for different buildings because of variations of voltage available at the transformer and voltage drop in the service conductors. It is, however, the target voltage distributed to a building's service equipment. Utilization Voltage A voltage drop occurs as current flows from the service equipment through the branch circuit conductors to the outlet (point of use in the building). Because of these voltage drops, voltage avail able at an outlet in the building is less than the system voltage. The utilization voltage accounts for anticipated voltage drops on branch circuit conductors. On a 120/240 V system, approximately 115/230 V is available at the outlet of the branch circuit and not the 120/240 V available at the service equipment. Appliances and equipment connected to the 120/240 V building system voltages are designed for 115 V, 230 V, or 115/230 V. (The commonly cited 110/220 V standard is obsolete and no longer specified.) Again, there are slight variations in utilization voltage. Measured voltage at an outlet or connection is called the line voltage. Maximum Voltage Wiring devices such as switches, receptacles, relays and conductors, and electrical equipment are manufactured to endure voltages slightly higher than the utilization voltage. The highest voltage to which a wiring device can be exposed is known as the maximum voltage. For example, a 5-15R duplex receptacle that is the wall-mounted receptacle common in most homes and offices is designed to handle a maximum voltage of 125 V, but is intended for use on a 120 V circuit, where the line voltage is likely about 115 V. Prev: Electrical
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