Alkalinity: The acid neutralizing capacity of a water. It is usually expressed as "M" alkalinity (the methyl orange endpoint at a pH (4.3) and "P" alkalinity (the phenolphthalein endpoint at a pH ≅ 8.3).
Several ions contribute to alkalinity, but it is generally due to bicarbonate (HCO3 –1), carbonate (CO3 –2), and hydroxyl (OH–1) ions.
Alkalinity and pH relationships: The alkalinity of naturally occurring waters is usually due to calcium and magnesium bicarbonate and occasionally to some sodium bicarbonate. Carbonate or hydroxide alkalinity is rarely encountered in raw waters.
"M" alkalinity = all bicarbonate + all carbonate + all hydroxide
"P" alkalinity = 1/2 carbonate + all hydroxide
If 2P < M, hydroxide = 0
If 2P = M, bicarbonate and hydroxide = 0
If 2P > M, bicarbonate = 0If P = 0, all alkalinity is bicarbonate
If P = M, all alkalinity is hydroxyl
M-2P = bicarbonate alkalinity
2P-M = hydroxide alkalinity
Ampacity: The continuous current-carrying capacity of electric conductors expressed in amperes under specific operating temperatures.
Biochemical Oxygen Demand (BOD): A measure of the oxygen consumed in the oxidation of organic and oxidizable inorganic materials in wastewater (expressed in ppm).
Blowdown: The removal of a portion of the water in a boiler or cooling water system to reduce concentrations of dissolved and/or suspended solids.
Bonding: The permanent joining of metallic parts to form an electrically conductive path which will assure electrical continuity and the capacity to conduct safely any current likely to be imposed. Branch Circuit: The circiut conductors between the final overcurrent device protecting the circuit and the outlet(s).
Bus: A conductor, or group of conductors, that serve as a common connection for two or more circuits.
Cathodic Protection: A means of reducing or preventing corrosion of a metal by making it the cathode in a conducting medium by means of an impressed or galvanic direct electric current.
Caustic Embrittlement: A peculiar type of boiler corrosion characterized by cracking of the metal along the grain boundaries which may occur when highly stressed metal is exposed to concentrated boiler water. It is usually associated with high concentrations of sodium hydroxide.
Circuit Breaker: A device designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined overcurrent without injury to itself when properly applied within its rating. The automatic opening means can be integral, direct acting with the circuit breaker, or remote from the circuit breaker. Adjustable: A qualifying term indicating that the circuit breaker can be set to trip at various values of current and/or time within a predetermined range. Instantaneous Trip: A qualifying term indicating that no delay is purposely introduced in the tripping action of the circuit breaker.
Inverse Time: A qualifying term indicating there is purposely introduced a delay in the tripping action of the circuit breaker, which delay decreases as the magnitude of the current increases.Nonadjustable: A qualifying term indicating that the circuit breaker does not have any adjustment to alter the value of current at which it will trip or the time required for its operation. Setting: (of Circuit Breaker) The value of current and/or time at which an adjustable circuit breaker is set to trip.
Conductivity: The ability of water to conduct an electrical current. As such, it is the opposite of resistivity and is related to the concentration of dissolved (ionizable) solids in the water. The measure of conductivity is conductance (the reciprocal of resistance) whose units are normally microhms equal to 1,000,000/ohms. The relationship between conductance and dissolved solids concentration will vary depending on the compounds present in the water. Temperature is also a factor and most conductivity meters provide for temperature compensation.
Conductor: A wire or combination of wires not insulated from one another, suitable for carrying a single electric current.
Continuous Load: A load where the maximum current is expected to continue for three hours or more.
Controller: A device or group of devices that serves to govern, in some predetermined manner, the electric power delivered to the apparatus to which it is connected.
Demand Factor: The ratio of the maximum demand of a system, or part of a system, to the total connected load of a system or the part of the system under consideration.
Duty: Continuous Duty: Operation at a substantially constant load for an indefinitely long time. Intermittent Duty: Operation for alternate intervals of (1) load and no load; or (2) load and rest; or (3) load, no load, and rest. Non-plugging, Non-jogging Duty: The rate of operation shall not exceed five openings or closings per minute and shall be not more than 10 in a 10-minute period. Periodic Duty: Intermittent operation in which the loads conditions are regularly recurrent. Plug-Stop, Plus-Reverse, and Jogging Duty: Require repeated interruption of stalled motor current or repeated closing of high transient currents encountered in rapid motor reversal. Short-Time Duty: Operation at a substantially constant load for a short and definitely specified time. Varying Duty: Operation at loads, and for intervals of time, both of which may be subject to wide variation. Enclosure: The case or housing of apparatus, or the fence or walls surrounding an installation to prevent personnel from accidentally contacting energized parts, or to protect the equipment from physical damage.
Explosion-proof Apparatus: Apparatus enclosed in a case that is capable of withstanding an explosion of a specified gas or vapor which may occur within it and of preventing the ignition of a specified gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within, and which operates at such an external temperature that a surrounding flammable atmosphere will not be ignited thereby.
Feeder: All circuit conductors between the service equipment, or the generator switchboard of an isolated plant, and the final branch-circuit overcurrent device.
Fitting: An accessory such as a locknut, bushing, or other part of a wiring system that is intended primarily to perform a mechanical rather than an electrical function.
Flammable and Combustible Liquids: A flammable liquid is one that has a closed cup flash point below 100°F (38°C) and a vapor pressure not exceeding 40 psia (276 kPa) at 100°F (38°C). Combustible liquids have a closed cup flash point at or above 100°F (38°C).
Ground: A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth.
Ground-Fault Circuit Interrupter: A device intended for the protection of personnel that functions to de-energize a circuit or portion thereof within an established period of time when a current to ground exceeds some predetermined value that is less than that required to operate the overcurrent protective device of the supply circuit.Ground Water: Water from wells and springs is classified as ground water. Ground waters are usually relatively free of suspended solids. The composition of a ground water usually remains fairly constant with time.
Hertz: The unit of frequency, one cycle per second.
Hardness: A property of water which is usually caused by the presence of calcium and magnesium ions. Hardness causes excessive soap consumption domestically and is the source of most scale formation in boiler and cooling water systems. Waters with up to 50 ppmw hardness (as CaCO3) are considered soft; waters with more than 200 ppmw hardness (as CaCO3) are considered hard. Carbonate hardness is usually due to calcium and magnesium bicarbonate; noncarbonate hardness is due to calcium and magnesium salts of sulfate, chloride, and nitrate.
Impedance: The equivalence of resistance in an alternating current circuit, consisting of the sum of orthogonal components of resistance and reactants, measured in ohms.
InSight From (Within Sight From, Within Sight): Where the NEC specifies that one equipment shall be "in sight from," "within sight from," or "within sight," etc., of another equipment, one of the equipments specified shall be visible and not more than 50 feet (15.24 m) distant from the other.
Interrupting Rating: The highest current at rated voltage that an overcurrent protective device is intended to interrupt under specified test conditions. Equipment intended to break current at other than fault levels may have its interrupting rating implied in other ratings, such as horsepower or locked rotor current.
Intrinsically Safe: Equipment and wiring that are incapable of releasing sufficient electrical energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture in its most easily ignited concentration.
Ions, Anions, Cations: Electrically charged atoms or groups of atoms. In electrolysis, the negatively charged ions (anions, containing an excess of electrons denoted by minus signs such as Cl–1, SO4 –2) move toward the anode while the positively charged ions (cations, deficient in electrons denoted by plus signs such as Na+1, Ca+2) move toward the cathode.
Langelier Scaling Index (LSI), Ryznar Scaling Index (RSI): Empirical indices often used to predict the tendency of cooling water to form scale.
LSI = pH – pHS
RSI = 2pHS – pH
pH = a measurement of the acidity or alkalinity of a system. The reference temperature for pH is 25°C (77°F) and the pH scale runs from 0 (highly acidic) to 14 (highly basic) with pH = 7.0 being neutral.
Motor-Circuit Switch: A switch, rated in horsepower, capable of interrupting the maximum operating overload current of a motor of the same horsepower rating as the switch at the rated voltage.
Motor Control Center: Motor Control Centers consist of a mechanical grouping of combination motor control, feeders, or other units arranged in a convenient assembly. They may or may not include interwiring or interlocking between units or to remotely mounted devices.
Outlet: A point of the wiring system at which current is taken to supply utilization equipment.
Overcurrent: Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload (see definition), starting, short circuit, or ground fault. A current in excess of rating may be accommodated by certain equpment and conductors for a given set of conditions. Hence, the rules for overcurrent protection are specific for particular situations.
Overload: Operation of equipment in excess of normal, fullload rating, or of a conductor in excess of rated ampacity which, when it persists for a sufficient length of time, would cause damage or dangerous overheating. A fault, wuch as a short circuit or ground fault, is not an overload. (See "Overcurrent").
Panelboard: A single panel or group of panel units designed for assembly in the form of a single panel; including buses, automatic overcurrent devices, and with or without switches for the control of light, heat, or power circuits; designed to be placed in a cabinet or cutout box placed in or against a wall or partition and accessible only from the front. (See "Switchboard").
Power Factor: Ratio of total watts to the total root-meansquare volt-amperes.
Raceway: An enclosed channel designed expressly for holding wires, cables, or busbars, with additional functions as permitted in the NEC. Raceways may be of metal or insulating material, and the term includes rigid metal conduit, rigid nonmetallic conduit, intermediate metal conduit, liquid- tight flexible metal conduit, electrical metallic tubing, underfloor raceways, cellular concrete floor raceways, cellular metal floor raceways, surface raceways, wireways, and busways.
Receptacle: A receptacle is a contact device installed at the outlet for the connection of a single attachment plug. A single receptacle is a single device on the same yoke. A multiple receptacle is a single contact device containing two or more receptacles.
Remote-Control Circuit: Any electric circuit that controls any other circuit through a relay or an equivalent device.
Service Drop: The overhead service conductors from the last pole or other aerial support to and including the splices, if any, connecting to the service-entrance conductors at the building or other structure.
Service Equipment: The necessary equipment, usually consisting of a circuit breaker or switch and fuses, and their accessories, located near the point of entrance of supply conductors to a building or other structure, or an otherwise defined area, and intended to constitute the main control and means of cutoff of the supply.Short Circuit: A connection of comparatively low resistance accidentally or intentionally made between points on a circuit between which the resistance is normally much greater.
Solids (suspended, dissolved, total): Suspended solids are those which can be removed by filtration. Dissolved solids are in true solution, while total solids are the sum of the suspended and dissolved solids. Usually expressed in ppmw or mg/liter. Surface Water: Water from lakes and streams is considered to be surface water. The composition of surface waters tends to vary widely with time.
Synchronism: The state where connected alternating-current systems, machines, or a combination operating at the same frequency and where the phase-angle displacements between voltages in them are constant, or vary about a steady and stable average value.
Synchronous Speed: Speed of rotation of the magnetic flux, produced by or linking the primary winding.
Switchboard: A large single panel, frame, or assembly of panels on which are mounted, on the face or back or both, switches, over current and other protective devices, buses, and usually instruments. Switchboards are generally accessible from the rear as well as from the front and are not intended to be installed in cabinets. (See "Panel board").
Switchgear: A form of switchboard in which all the equipment required to control an individual circuit, including bus, circuit breaker, disconnecting devices, current and potential transformers, controls, instruments, and relays, is assembled in one metal cubicle and the circuit breaker is provided with means for ready removal from the cubicle. Circuit breakers can be of the oil or air type, although the trend is strongly to the use of air circuit breakers.
Thermal Cutout: An over current protective device that contains a heater element in addition to and affecting a renewable fusible member which opens the circuit. It is not designed to interrupt short-circuit currents.
Thermal Protector: (As applied to motors) A protective device for assembly as an integral part of a motor or motor- compressor and which, when properly applied, protects the motor against dangerous overheating due to overload and failure to start.
Voltage (of a Circuit): The greatest root-mean-square (effective) difference of potential between any two conductors of the current concerned. Some systems, such as 3-phase 4-wire, single-phase 3-wire, and 3-wire direct-current may have various circuits of various voltages.
Voltage, Norminal: A nominal value, assigned to a circuit or system for the purpose of conveniently designating its voltage class (as 120/240, 480Y/277,600, etc.). The actual voltage at which a circuit operates can vary from the nominal within a range that permits satisfactory operation of equipment.
Voltage to Ground: For grounded circuits, the voltage between the given conductor and that point or conductor of the circuit that is grounded; for ungrounded circuits, the greatest voltage between the given conductor and any other conductor of the circuit.
Several ions contribute to alkalinity, but it is generally due to bicarbonate (HCO3 –1), carbonate (CO3 –2), and hydroxyl (OH–1) ions.
Alkalinity and pH relationships: The alkalinity of naturally occurring waters is usually due to calcium and magnesium bicarbonate and occasionally to some sodium bicarbonate. Carbonate or hydroxide alkalinity is rarely encountered in raw waters.
"M" alkalinity = all bicarbonate + all carbonate + all hydroxide
"P" alkalinity = 1/2 carbonate + all hydroxide
If 2P < M, hydroxide = 0
If 2P = M, bicarbonate and hydroxide = 0
If 2P > M, bicarbonate = 0If P = 0, all alkalinity is bicarbonate
If P = M, all alkalinity is hydroxyl
M-2P = bicarbonate alkalinity
2P-M = hydroxide alkalinity
Ampacity: The continuous current-carrying capacity of electric conductors expressed in amperes under specific operating temperatures.
Biochemical Oxygen Demand (BOD): A measure of the oxygen consumed in the oxidation of organic and oxidizable inorganic materials in wastewater (expressed in ppm).
Blowdown: The removal of a portion of the water in a boiler or cooling water system to reduce concentrations of dissolved and/or suspended solids.
Bonding: The permanent joining of metallic parts to form an electrically conductive path which will assure electrical continuity and the capacity to conduct safely any current likely to be imposed. Branch Circuit: The circiut conductors between the final overcurrent device protecting the circuit and the outlet(s).
Bus: A conductor, or group of conductors, that serve as a common connection for two or more circuits.
Cathodic Protection: A means of reducing or preventing corrosion of a metal by making it the cathode in a conducting medium by means of an impressed or galvanic direct electric current.
Caustic Embrittlement: A peculiar type of boiler corrosion characterized by cracking of the metal along the grain boundaries which may occur when highly stressed metal is exposed to concentrated boiler water. It is usually associated with high concentrations of sodium hydroxide.
Circuit Breaker: A device designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined overcurrent without injury to itself when properly applied within its rating. The automatic opening means can be integral, direct acting with the circuit breaker, or remote from the circuit breaker. Adjustable: A qualifying term indicating that the circuit breaker can be set to trip at various values of current and/or time within a predetermined range. Instantaneous Trip: A qualifying term indicating that no delay is purposely introduced in the tripping action of the circuit breaker.
Inverse Time: A qualifying term indicating there is purposely introduced a delay in the tripping action of the circuit breaker, which delay decreases as the magnitude of the current increases.Nonadjustable: A qualifying term indicating that the circuit breaker does not have any adjustment to alter the value of current at which it will trip or the time required for its operation. Setting: (of Circuit Breaker) The value of current and/or time at which an adjustable circuit breaker is set to trip.
Conductivity: The ability of water to conduct an electrical current. As such, it is the opposite of resistivity and is related to the concentration of dissolved (ionizable) solids in the water. The measure of conductivity is conductance (the reciprocal of resistance) whose units are normally microhms equal to 1,000,000/ohms. The relationship between conductance and dissolved solids concentration will vary depending on the compounds present in the water. Temperature is also a factor and most conductivity meters provide for temperature compensation.
Conductor: A wire or combination of wires not insulated from one another, suitable for carrying a single electric current.
Continuous Load: A load where the maximum current is expected to continue for three hours or more.
Controller: A device or group of devices that serves to govern, in some predetermined manner, the electric power delivered to the apparatus to which it is connected.
Demand Factor: The ratio of the maximum demand of a system, or part of a system, to the total connected load of a system or the part of the system under consideration.
Duty: Continuous Duty: Operation at a substantially constant load for an indefinitely long time. Intermittent Duty: Operation for alternate intervals of (1) load and no load; or (2) load and rest; or (3) load, no load, and rest. Non-plugging, Non-jogging Duty: The rate of operation shall not exceed five openings or closings per minute and shall be not more than 10 in a 10-minute period. Periodic Duty: Intermittent operation in which the loads conditions are regularly recurrent. Plug-Stop, Plus-Reverse, and Jogging Duty: Require repeated interruption of stalled motor current or repeated closing of high transient currents encountered in rapid motor reversal. Short-Time Duty: Operation at a substantially constant load for a short and definitely specified time. Varying Duty: Operation at loads, and for intervals of time, both of which may be subject to wide variation. Enclosure: The case or housing of apparatus, or the fence or walls surrounding an installation to prevent personnel from accidentally contacting energized parts, or to protect the equipment from physical damage.
Explosion-proof Apparatus: Apparatus enclosed in a case that is capable of withstanding an explosion of a specified gas or vapor which may occur within it and of preventing the ignition of a specified gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within, and which operates at such an external temperature that a surrounding flammable atmosphere will not be ignited thereby.
Feeder: All circuit conductors between the service equipment, or the generator switchboard of an isolated plant, and the final branch-circuit overcurrent device.
Fitting: An accessory such as a locknut, bushing, or other part of a wiring system that is intended primarily to perform a mechanical rather than an electrical function.
Flammable and Combustible Liquids: A flammable liquid is one that has a closed cup flash point below 100°F (38°C) and a vapor pressure not exceeding 40 psia (276 kPa) at 100°F (38°C). Combustible liquids have a closed cup flash point at or above 100°F (38°C).
Ground: A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth.
Ground-Fault Circuit Interrupter: A device intended for the protection of personnel that functions to de-energize a circuit or portion thereof within an established period of time when a current to ground exceeds some predetermined value that is less than that required to operate the overcurrent protective device of the supply circuit.Ground Water: Water from wells and springs is classified as ground water. Ground waters are usually relatively free of suspended solids. The composition of a ground water usually remains fairly constant with time.
Hertz: The unit of frequency, one cycle per second.
Hardness: A property of water which is usually caused by the presence of calcium and magnesium ions. Hardness causes excessive soap consumption domestically and is the source of most scale formation in boiler and cooling water systems. Waters with up to 50 ppmw hardness (as CaCO3) are considered soft; waters with more than 200 ppmw hardness (as CaCO3) are considered hard. Carbonate hardness is usually due to calcium and magnesium bicarbonate; noncarbonate hardness is due to calcium and magnesium salts of sulfate, chloride, and nitrate.
Impedance: The equivalence of resistance in an alternating current circuit, consisting of the sum of orthogonal components of resistance and reactants, measured in ohms.
InSight From (Within Sight From, Within Sight): Where the NEC specifies that one equipment shall be "in sight from," "within sight from," or "within sight," etc., of another equipment, one of the equipments specified shall be visible and not more than 50 feet (15.24 m) distant from the other.
Interrupting Rating: The highest current at rated voltage that an overcurrent protective device is intended to interrupt under specified test conditions. Equipment intended to break current at other than fault levels may have its interrupting rating implied in other ratings, such as horsepower or locked rotor current.
Intrinsically Safe: Equipment and wiring that are incapable of releasing sufficient electrical energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture in its most easily ignited concentration.
Ions, Anions, Cations: Electrically charged atoms or groups of atoms. In electrolysis, the negatively charged ions (anions, containing an excess of electrons denoted by minus signs such as Cl–1, SO4 –2) move toward the anode while the positively charged ions (cations, deficient in electrons denoted by plus signs such as Na+1, Ca+2) move toward the cathode.
Langelier Scaling Index (LSI), Ryznar Scaling Index (RSI): Empirical indices often used to predict the tendency of cooling water to form scale.
LSI = pH – pHS
RSI = 2pHS – pH
pH = a measurement of the acidity or alkalinity of a system. The reference temperature for pH is 25°C (77°F) and the pH scale runs from 0 (highly acidic) to 14 (highly basic) with pH = 7.0 being neutral.
Motor-Circuit Switch: A switch, rated in horsepower, capable of interrupting the maximum operating overload current of a motor of the same horsepower rating as the switch at the rated voltage.
Motor Control Center: Motor Control Centers consist of a mechanical grouping of combination motor control, feeders, or other units arranged in a convenient assembly. They may or may not include interwiring or interlocking between units or to remotely mounted devices.
Outlet: A point of the wiring system at which current is taken to supply utilization equipment.
Overcurrent: Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload (see definition), starting, short circuit, or ground fault. A current in excess of rating may be accommodated by certain equpment and conductors for a given set of conditions. Hence, the rules for overcurrent protection are specific for particular situations.
Overload: Operation of equipment in excess of normal, fullload rating, or of a conductor in excess of rated ampacity which, when it persists for a sufficient length of time, would cause damage or dangerous overheating. A fault, wuch as a short circuit or ground fault, is not an overload. (See "Overcurrent").
Panelboard: A single panel or group of panel units designed for assembly in the form of a single panel; including buses, automatic overcurrent devices, and with or without switches for the control of light, heat, or power circuits; designed to be placed in a cabinet or cutout box placed in or against a wall or partition and accessible only from the front. (See "Switchboard").
Power Factor: Ratio of total watts to the total root-meansquare volt-amperes.
Raceway: An enclosed channel designed expressly for holding wires, cables, or busbars, with additional functions as permitted in the NEC. Raceways may be of metal or insulating material, and the term includes rigid metal conduit, rigid nonmetallic conduit, intermediate metal conduit, liquid- tight flexible metal conduit, electrical metallic tubing, underfloor raceways, cellular concrete floor raceways, cellular metal floor raceways, surface raceways, wireways, and busways.
Receptacle: A receptacle is a contact device installed at the outlet for the connection of a single attachment plug. A single receptacle is a single device on the same yoke. A multiple receptacle is a single contact device containing two or more receptacles.
Remote-Control Circuit: Any electric circuit that controls any other circuit through a relay or an equivalent device.
Service Drop: The overhead service conductors from the last pole or other aerial support to and including the splices, if any, connecting to the service-entrance conductors at the building or other structure.
Service Equipment: The necessary equipment, usually consisting of a circuit breaker or switch and fuses, and their accessories, located near the point of entrance of supply conductors to a building or other structure, or an otherwise defined area, and intended to constitute the main control and means of cutoff of the supply.Short Circuit: A connection of comparatively low resistance accidentally or intentionally made between points on a circuit between which the resistance is normally much greater.
Solids (suspended, dissolved, total): Suspended solids are those which can be removed by filtration. Dissolved solids are in true solution, while total solids are the sum of the suspended and dissolved solids. Usually expressed in ppmw or mg/liter. Surface Water: Water from lakes and streams is considered to be surface water. The composition of surface waters tends to vary widely with time.
Synchronism: The state where connected alternating-current systems, machines, or a combination operating at the same frequency and where the phase-angle displacements between voltages in them are constant, or vary about a steady and stable average value.
Synchronous Speed: Speed of rotation of the magnetic flux, produced by or linking the primary winding.
Switchboard: A large single panel, frame, or assembly of panels on which are mounted, on the face or back or both, switches, over current and other protective devices, buses, and usually instruments. Switchboards are generally accessible from the rear as well as from the front and are not intended to be installed in cabinets. (See "Panel board").
Switchgear: A form of switchboard in which all the equipment required to control an individual circuit, including bus, circuit breaker, disconnecting devices, current and potential transformers, controls, instruments, and relays, is assembled in one metal cubicle and the circuit breaker is provided with means for ready removal from the cubicle. Circuit breakers can be of the oil or air type, although the trend is strongly to the use of air circuit breakers.
Thermal Cutout: An over current protective device that contains a heater element in addition to and affecting a renewable fusible member which opens the circuit. It is not designed to interrupt short-circuit currents.
Thermal Protector: (As applied to motors) A protective device for assembly as an integral part of a motor or motor- compressor and which, when properly applied, protects the motor against dangerous overheating due to overload and failure to start.
Voltage (of a Circuit): The greatest root-mean-square (effective) difference of potential between any two conductors of the current concerned. Some systems, such as 3-phase 4-wire, single-phase 3-wire, and 3-wire direct-current may have various circuits of various voltages.
Voltage, Norminal: A nominal value, assigned to a circuit or system for the purpose of conveniently designating its voltage class (as 120/240, 480Y/277,600, etc.). The actual voltage at which a circuit operates can vary from the nominal within a range that permits satisfactory operation of equipment.
Voltage to Ground: For grounded circuits, the voltage between the given conductor and that point or conductor of the circuit that is grounded; for ungrounded circuits, the greatest voltage between the given conductor and any other conductor of the circuit.