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. Branch Circuiting A building electrical system consists of several circuits that extend out from the switchboard or panelboard. A branch circuit is that portion of a building wiring system that extends beyond the final overcurrent protection device that is protecting a circuit. It provides power from a circuit breaker or fuse in the panelboard to single or multiple points of use called outlets. An outlet is a point in a wiring system where current is taken to supply an appliance, piece of equipment, or lighting installation. A branch circuit is composed of an overcurrent protection device (fuse or circuit breaker), wiring, and one or more outlets. Types of Branch Circuits Four primary types of branch circuits are recognized for general use. Individual Branch Circuit This type of branch circuit serves only one receptacle or piece of equipment such as for a range, clothes dryer, large copy ma chine, or other piece of machinery. These circuits usually lead directly from the distribution panel to the appliance and don’t serve any other electrical devices. The individual branch circuit is sometimes known in the trade as a dedicated or special purpose circuit. General Purpose Branch Circuit A branch circuit supplies two or more outlets for lighting and appliances. This type of circuit may be referred to as a lighting circuit; this is a carryover from the days when electricity was first used in buildings and its predominant purpose was lighting. There are usually a number of general purpose branch circuits supplying lights and outlets in different rooms around a residence or commercial or industrial building. Appliance Branch Circuit This is the type of branch circuit that supplies energy to one or more outlets to which appliances are to be connected. They supply fixed electric equipment such as refrigerators, washers, and other large appliances and electrical devices. Appliance branch circuits don’t supply lighting fixtures. Appliance branch circuits cannot exceed 20 A. Multiwire Branch Circuit A branch circuit consisting of two or more ungrounded (hot) conductors having a voltage between them and a common neutral (grounded) conductor that is shared between the ungrounded conductors such as in a 120/240 V three-wire circuit. In this circuit, all conductors must originate from the same panelboard. Other branch circuits specific to a particular occupancy may also be required. For example, the following branches are required in health care facilities such as hospitals, nursing homes, and dental facilities. Life Safety Branch Circuit An emergency system of feeders and branch circuits that pro vides adequate power to patients and personnel. It must automatically connect to an alternate power source such as a generator when the normal power source is interrupted. Critical Branch Circuit Split Wiring Receptacles Split wired duplex receptacles are fed with a 120/240 V circuit having two ungrounded (hot) conductors, a grounded (neutral) conductor, and a grounding conductor. One ungrounded (hot) conductor feeds power to the upper outlet and the other un grounded (hot) conductor feeds the lower outlet. The grounded (neutral) conductor is shared between both circuits. Split wiring allows power to be drawn from two separate circuits on one duplex receptacle. Branch Circuit Rating and Loads The branch circuit rating is determined by the rating of the overcurrent protection device (fuse or circuit breaker) used to protect the wiring in the circuit from excessive current flow. The rating of the overcurrent protection device is related to the connected load or loads being fed by the branch circuit. Simply put, the wiring in the circuit must safely deliver current to the connected load and the overcurrent protection device protects this wiring, so the circuit rating matches the rating of the over current protection device. The connected load on a branch circuit is the sum of all loads connected in a circuit. It’s found by totaling the connected volt-amp (VA) load at each outlet connected to the circuit. The volt-amp (VA) is similar in nature to the watt, except that it de scribes the instantaneous current rather than power (wattage). As discussed in Section 17, design of a building electrical installation involves computations with power (P), ex pressed in watts (W); voltage (E), in volts (V); amperage (I), in amps (A); and volt-amps (VA) in volts and amperes. A summary of conversion formulas for direct current and alternating current systems is provided in Tbl.9. These formulas are useful in determining loads in circuiting design. Ex. **12 For a 120 V, 15 A, two-wire branch circuit, determine the maximum load that can be connected to this circuit. From Tbl.9, P _ E · I P _ E _ I _ 120 V _ 20 A _ 2400 VA === Tbl. **9 A SUMMARY OF CONVERSION FORMULAS FOR DIRECT CURRENT AND ALTERNATING CURRENT SYSTEMS WHERE POWER (P) IS EXPRESSED IN WATTS (W); VOLTAGE (E) IN VOLTS (V); AMPERAGE (I) IN AMPS (A); AND VOLT-AMPS (VA) IS IN VOLT-AMPERES.WHEN USED IN THESE FORMULAS, POWER FACTOR (PF) SHOULD BE EXPRESSED AS A DECIMAL. FOR TWO-PHASE, THREE-WIRE ALTERNATING CURRENT SYSTEMS, SUBSTITUTE 1.41 FOR 1.73. FOR TWO-PHASE, FOUR-WIRE ALTERNATING CURRENT SYSTEMS, SUBSTITUTE 2.00 FOR 1.73. Alternating Current (AC) Systems To Find Direct Current (DC) Systems Single Phase Three Phase DC power (watts) P _ E · I - - AC real power (watts) - EI(PF) 1.73 · EI(PF) AC apparent power (VA) - (VA) 1.73 · (VA) Amperes (I) I _ P/E I _ P/E · PF I _ P/1.73 · E · PF -I _ (VA)/E I _ (VA)/1.73 · E Power factor (PF) - PF _ P/E · I PF _ P/1.73 · E · I or or PF _ P/(VA) PF = P/1.73(VA) Note: VA is the rating of the appliance or piece of equipment in volt-amperes (VA). ==== Tbl. **10 VOLT-AMPERAGE (VA) LOADS AND CORRESPONDING AMPERAGE (A) LOADS ON SINGLE-PHASE AND THREE-PHASE CIRCUITS FOR VARIOUS SYSTEM VOLTAGES BASED UPON A POWER FACTOR OF 1.0. Load Amperage (A) Load Volt- Volt Amperage; Amperage (VA) 120 V 240 V 277 V 480 V 240 V 480 V (VA) Single-Phase Circuits Three-Phase Circuits (Balanced Load) ==== Tbl. **11 MAXIMUM CONNECTED LOADS, VOLTAGES, AND REQUIREMENTS FOR COMMON 120 V RESIDENTIAL CIRCUIT RATINGS. Recommended Uses Lighting and individual circuits. Convenience receptacles, small appliance, and individual circuits. Clothes dryer. Small electric range or electric range top. Typical electric range or electric range top. ==== Tbl. **12 STANDARD CONNECTED LOADS, VOLTAGES AND REQUIREMENTS FOR KITCHEN APPLIANCES AND EQUIPMENT. Appliance/Equipment; Range/oven unit (household size); Range/oven unit (apartment size); Cooktop (large, separate); Cooktop (medium size); Oven (built-in, separate) 4 Microwave oven (fixed) Microwave oven (counter top); Dishwasher; Waste disposal (sink) Freezer Exhaust hood (kitchen); Trash compactor; Refrigerator(household size); Refrigerator(apartment size) Coffee maker (10 cup) ==== Tbl. **13 STANDARD CONNECTED LOADS, VOLTAGES, AND REQUIREMENTS FOR ENTERTAINMENT EQUIPMENT AND LIGHTING IN HABI TABLE SPACES. Conductors === Tbl. **14 STANDARD CONNECTED LOADS, VOLTAGES, AND REQUIREMENTS FOR APPLIANCES AND EQUIPMENT IN LAUNDRY SPACES. === Tbl. **15 STANDARD CONNECTED LOADS, VOLTAGES, AND REQUIREMENTS FOR APPLIANCES AND EQUIPMENT IN OFFICE SPACES. === Tbl. **16 STANDARD CONNECTED LOADS, VOLTAGES, AND REQUIREMENTS FOR MISCELLANEOUS EQUIPMENT. ==== Tbl. **17 APPROXIMATE FULL LOAD RATINGS (AMPERES) FOR SINGLE- PHASE AND THREE-PHASE MOTORS BASED ON MOTOR HORSEPOWER. VALUES WILL VARY WITH UNIT. CHECK WITH MANUFACTURER FOR ACTUAL SPECIFICATIONS. === All electric-resistant appliance or pieces of equipment such as an oven, water heater, and space heater (no blower) and lights generally have power (wattage) ratings equal to their connected load (VA rating). For example, a 4500 W water heater has a 4500 VA rating; ten 150 W lamps have a connected load of 1500 VA; and an oven rated at 12 kW has a connected load of 12 000 VA. In contrast, all electromechanical (having a motor) appliances or pieces of equipment generally will have a VA rating larger than the power (W) rating. At startup, an electric motor consumes substantially more electrical power than when it’s operating at its rated speed and load. For example, a piece of equipment containing a 230 V, 1 hp motor that normally draws about 1000 W (4.3 A) under full load can draw 6000 VA (26.1 A) at startup. Because voltage remains constant, this motor's starting amperage is much greater than amperage required when it’s at full operation. Thus, this device may be rated at 6000 VA even though it only consumes 1000 W when operating at full speed. This initial power surge is evident by the temporary dimming of lights at the instant a heavy-duty, motor-driven appliance such as a vacuum cleaner or table saw is switched on. Design of a branch circuit involves sizing conductors and overcurrent protection (fuse or circuit breaker) that match the circuit rating. Volt-amperage (VA) loads and corresponding amperage (A) loads on single-phase and three-phase circuits for various system voltages are provided in Tables **10 and **11. Maximum connected loads, voltages, and requirements for common circuit ratings are provided in Tables **12 through **16. Approximate full load ratings (amperes) for single-phase and three-phase motors based on motor horsepower are provided in Tbl.17. Check with manufacturer for actual specifications. Manufacturers' data should always be used in computations. Requirements for common branch circuits are introduced in the following. Fig.10 Shown is a cut-away pictorial view of a room with separate circuits for lighting, convenience receptacles, and window air conditioner. Wiring is shown as a dashed line because it’s hidden from view (within the walls). The related one-line schematic drawing of electrical symbols is also shown. The arrows indicate that each circuit originates at a panelboard. General Purpose Circuits General purpose circuits feed more than one outlet for lighting or other purpose. According to requirements, the rating of general purpose branch circuits must be 15 A, 20 A, 30 A, 40 A, or 50 A. Maximum connected loads, voltages, and other requirements for general purpose circuit ratings are shown in Tbl.11. General purpose circuits typically provide power to convenience receptacles and lighting outlets with ratings of 20 A and 15 A because of their ease of running and pulling the slender conductors required (No. 12 AWG and No. 14 AWG cop per, respectively). Some electricians use 15 A circuits for lighting installations (No. 14 AWG copper conductors) and 20 A circuits for receptacles (No. 12 AWG copper conductors). General purpose circuits are typically limited according to what will be connected to them: 1. When a general purpose circuit feeds fixed appliances and luminaires or portable appliances, the total of the fixed appliances should be no more than 50% of the branch circuit rating. Assuming a 15 A, 120 V branch circuit, it would have a maximum rating of 15 A · 120 V _ 1800 VA (refer to Section 18 for formula explanation). In this case, the fixed appliances would be limited to 900 VA, leaving the other 900 VA available to supply luminaires or portable appliances also served by the branch circuit. Refer to Tbl.11. 2. A 20 A, 120 V branch circuit would have a theoretical maximum of 2400 VA (20 A ·120 V _ 2400 VA), but it’s common practice to limit the connected load to 80% of the circuit rating (e.g., 20 A · 120 V · 80% _ 1920 VA). Refer to Tbl.11. 3. When the load on the circuit will be a continuous operating load (e.g., for store lights), the total load should not exceed 80% of the circuit rating. The lighting load must include any ballasts, transformers, or autotransformers, which are part of the lighting system. Because a 15 A branch has a full rating of 1800 VA, the limit would be 80%, or 12 A and 1440 VA (e.g., 15 A · 120 V · 80% _ 1440 VA). A 20 A, 2400 VA branch would be limited to 16 A and 1920 VA of connected load. Again, refer to Tbl.11. 4. When portable appliances will be used on a general purpose circuit, the limit for any one portable appliance is 80% of the branch circuit rating. 5. In commercial applications, convenience receptacles are computed at a load of 1.5 A (180 VA) per receptacle and are limited to 80% of the rating. This limits a branch circuit serving only receptacles to its rating divided by 1 1/2 A. For example, a 15 A circuit is limited to a maxi mum of 8 outlets and a 20 A circuit to 10 outlets. Fig.11 Shown are the switching configuration and related one-line schematic drawing for a lighting circuit serving two rooms. Wiring from each room is connected at a junction (J) box were the circuit returns to a panelboard. Individual Circuits: These circuits provide power to a single outlet such as a receptacle serving a range, clothes dryer, or copy machine. Although there are no size limitations for an individual circuit rating, appliances and equipment rated at above 25 A must be placed on a separate individual circuit because of the 50% maximum single load limitation in general purpose circuiting. Usually connected loads above 20 A are placed on an individual circuit. It’s good practice to provide individual circuits for loads above 1500 W. Motors above 1/8 hp should also be placed on an individual circuit. Generally, individual circuits are required for the following appliances and equipment: • Kitchen range (both stand-alone and counter-mounted units) •Oven • Microwave (built-in) • Waste disposal • Dishwasher • Clothes washer • Clothes dryer • Electric water heater • Furnace • Boiler circulating pump motor (large commercial and industrial) • HVAC air-handling unit • Large machinery (e.g., table saw, lathe, milling ma chine, machining center, elevator) • Large equipment (e.g., large copy machines, compressors, HVAC blowers) Appliance Circuits: These circuits serve two or more outlets to which only appliances are connected. In dwelling units, two or more 20 A small appliance circuits for convenience receptacle outlets in the kitchen, dining room, pantry, and breakfast room are required. These are in addition to the other outlets required (e.g., range, waste disposal, and so on). Small appliance circuits can supply any refrigerators and freezers, but not an electric range or oven. A light fixture is not permitted to be permanently connected to a small appliance circuit, unless the light fixture is part of an appliance (e.g., an oven or clothes dryer light). A minimum of one 20 A circuit is required to serve the laundry room convenience receptacle. Any other special requirements, such as an electric clothes dryer requiring 120/240 V, must also be provided. Tables **12 through **17 provide recommendations for circuiting common appliances and pieces of equipment. Manufacturers' data should always be used in computations. Continuous Loads A continuous load is a connected load that operates for 3 hr or more at any time. Many electrical loads fit within this category such as circuits serving office and classroom lighting installations. When determining a circuit rating, most loads deemed continuous must have a circuit rating calculated at 125% of the circuit's connected load. The intent behind the 125% factor is from the inability of the overcurrent protection device to handle a continuous load without overheating; that is, most circuit breakers trip if they carry their rated load for any significant time period. In many installations, it’s good practice for all connected loads not to exceed 80% of the individual circuit rating. Ex. **14 A lighting installation containing 16 to 100 W incandescent luminaires on one circuit will be operated approximately 10 hr per workday. Determine the circuit rating. The 1600 W connected load is a continuous load be cause it operates for 3 hr or more. The continuous load multiplier (125%) must be applied: The circuit rating is found by algebraically manipulating the power equation, P _ IE: A 20 A circuit breaker is the closest circuit breaker available for overcurrent protection (see Section 18 for sizes available). The rating for this circuit is 20 A. Another way of looking at this is that the connected load on a continuous-load circuit should not exceed 80% of the individual circuit rating. Thus, the maximum continuous load on a 20 A circuit is 16 A (80% of 20 is 16). I _ P>E _ 2000 VA>120 _ 16.7 A 1600 VA _ 1.25 _ 2000 VA There are several exceptions to application of the continuous-load multiplier. On circuits involving electric space heating, the connected load is taken at 100% of the load even though it may operate for more than 3 hr at a time. Any branch circuit serving a single motor or a device containing a motor should have an ampacity (amperage rating) of not less than 125% of the motor's full load current rating (this is the same as saying that no motor can exceed 80% of the branch circuit rating). Ex. **15 A fluorescent lighting fixture has a ballast with a nameplate rating of 185 W. Determine the number of such fixtures that can be connected to a two-wire circuit protected by a 20 A circuit breaker if the fixtures are used in an office building where they will be on for 8 hr continuously each day. a. For a 277 V, two-wire lighting system: From Tbl.9, P _ E _ I. The continuous-load multiplier (80%) must be applied. b. For a 120 V, two-wire lighting system: From Tbl.9, P _ E · I · PF. The continuous-load multiplier (80%) must be applied. Branch Circuit Conductor Size On a branch circuit, conductor size is tied to circuit rating. Generally, ungrounded (hot) and grounded (neutral) conductors in the circuit must be sized so that conductor ampacity is at least the branch circuit rating. The ampacity of a conductor can be larger than the circuit rating but not smaller. For example, the No. 8 AWG copper conductor with THW insulation that will be used in a 120 V, two-wire circuit in an environment with an average ambient air temperature of no greater than 86°F (30°C) has an ampacity of 50 A. It can safely carry up to 50 A without overheating. A 50 A overcurrent protection device (fuse or circuit breaker) would adequately protect a circuit wired with this conductor size, because the overcurrent protection device would disconnect if current drawn exceeded 50 A. A 40 A overcurrent protection device would also protect a circuit wired with the No. 8 AWG copper conductors because the 50 A ampacity of the conductor would not be exceeded. How ever, a 60 A overcurrent protection device would be too large and unsafe. It would not adequately protect the circuit, because it will allow current to flow that exceeds the 50 A ampacity of the conductor. Ex. **16 A clothes dryer rated at 5 kW is connected to a 120/240 V, single-phase three-wire, individual branch circuit. Determine the following: a. The connected full load, in amps. From Tbl.9, P _ E _ I. Therefore, I _ P>E. b. The minimum size circuit rating (of the overcurrent device) and the minimum copper wire size based on THWN-2 insulation at a temperature rating 167°F/75°C (see Section 18). Neglect any correction factors for temperature and conductor bundling. From Tbl. 13, the minimum size overcurrent device that can be used on this circuit is 25 A. From Tbl. 11, an AWG No. 10 copper conductor rated at 30 A is the minimum required. The designer would likely select a 30 A circuit rating, and thus a fuse or circuit breaker rated at 30 A. c. The rating of the receptacle and plug used to connect the clothes dryer. From Tbl. 16 based on a 30 A circuit rating, a three pole, four-wire 14-30R receptacle and 14-30P plug rated at 30 A, 125/250 V. Residential Branch Circuit Wiring Type NMB cable (e.g., Romex) is the most widely used wiring method in residential dwellings. NM cable must have 194°F (90°C) conductor insulation rating, which is designated by a "B" on the cable sheath. Typically, AWG No. 12, and AWG No. 14 are used for receptacle and lighting circuits; AWG No. 10/2 is commonly used for electric water heaters; AWG No. 10/3 with ground for electrical dryers and cooktops; and AWG No. 8/3 with ground or AWG No. 6/3 with ground for ranges and wall mounted ovens. Type SER or other four-wire cable is used for electrical ranges, cooktops, wall ovens, and clothes dryers. Prev: Cable, Raceway,
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