ECET4520 Exam II Sample Exam Problems Instructions: This exam is closed book, except for the reference booklet provided by your instructor and one (8.5 x11 ) sheet of handwritten notes that may not contain any numerically-solved problems. To receive full credit for any problem requiring numerical answers, all of the work required to obtain the numerical answers must be shown in a neat and logical manner. No credit will be given for illegible/illogical work or for numerical answers that are not justified by the work shown. You must maintain at least three significant digits of accuracy during all of your calculations. ECET 4520 - Exam II (Fall 2011) Problem #1) Given a three-wire, single-phase, 240V dedicated branch circuit that serves an appliance that is rated at 240V, 6800VA; Assuming that the appliance will operate as a continuous load: a) Determine the smallest, standard-sized circuit breaker that should be used to protect this branch circuit. CB rating = 40 A b) Determine the smallest size, THHN, copper conductor that could be utilized for this branch circuit assuming a 60 C temperature rating, 30 C ambient temperature, and no more than three current-carrying conductors in the conduit. Conductor Size = #8 AWG c) Determine the number of poles for the circuit breaker used to protect this branch circuit. One Pole / Two Poles / Three Poles (Circle the correct answer) d) Based on NEC recommendations, determine the minimum acceptable line-voltage magnitude (rounded to the nearest whole number) that must be present at the terminals of the outlet serving this appliance. V Line(minimum) = 228 V Problem #2) Given a two-story, single-family home (dwelling unit) whose dimensions are (35ft x 60ft); Assuming that the home does not have a basement but that it does have an enclosed garage that takes up an area of (20ft x 20ft) within the first story, determine the general lighting load for the dwelling after any appropriate demand factors are applied. General Lighting Load = 5940 VA
Problem #3) Given a section of 2/0 AWG, aluminum conductor having a temperature of 40 C, determine the DC resistance of the conductor in Ω/1000. R 0.141 Ω/1000 2 / 0, Al.(40 C ) Problem #4) Given a three-phase (3Φ), 150kVA, 13.8kV 208V, Y-Y step-down transformer that provides service to a warehouse; a) Determine the rated phase-voltage magnitude the transformer s primary winding. V Phase ) b) Determine the rated line-current for the transformer s primary winding. ( rated PRIMARY 7,967 V I Line ) c) Determine the rated line-current for the transformer s secondary winding. ( rated PRIMARY 6.28 A I Line ) d) Determine the standard percent impedance of the transformer. ( rated SECONDARY 416.4 A % Z 1.2 e) Assuming that an infinite bus supplies the transformer s primary windings, determine the (line-line-line) short-circuit current available at the secondary terminals of the transformer. I 34,697 A SCA(Sec)
Refer to the following figure for problems #5 and #6. The figure shows a 400-foot long, 3Φ feeder composed of three single 300kcmil, copper conductors enclosed in steel conduit. The feeder is connected to a 480V, 3Φ supply and is providing power to a balanced 3Φ load that draws a line current of magnitude of 220A at a power factor of 0.6 lagging. Note - Assume a 75 C conductor operating temperature (rating) and a 30 C ambient temperature. 480V 3-300 kcmil Copper conductors 400ft. length in steel conduit 220A pf = 0.6 lagging (transient load) Problem #5) Given the 3Φ feeder shown in the above figure, if the line-line-line Short-Circuit Current available at the service end of the feeder is 78,000 amps, determine the line-line-line Short-Circuit Current available at the load end of the feeder. You must use the Pointto-Point method as presented during the course lectures for your solution. I 10,842 A SCA( Load End ) Problem #6) Given the 3Φ feeder shown in the above figure, determine the magnitude of the (line) voltage-drop caused by the AC load current flowing in the feeder. You must use the Effective Impedance method as presented during the course lectures for your solution. V 10.33 V drop(line)
Problem #7) Given the following diagram that shows two rooms within a large, single-family dwelling; 40 ft. 20 ft. 8 ft. 24 ft. 10 ft. 10 ft. 8 ft. 10 ft. 12 ft. 5 ft. a) Based on NEC requirements, determine the minimum total number of receptacle outlets that must be placed within the two rooms and accurately show the location of the outlets on the diagram (using the appropriate symbols). min. # receptacles 17 b) Based on NEC requirements, determine the minimum number of 15A circuits that could be used to supply all of the receptacles in the two rooms is the minimum load rating is assigned to each of the receptacles. # 15A circuits 2
Problem #8) Specify if each statement is True or False based on NEC guidelines and/or standard design practice. Given the following conductors, both operating in a distribution system with rated conductor temperatures of 75 C and ambient temperatures of 30 C: Conductor #1 50 feet long, THHN, 500kcmil, copper conductor Conductor #2 50 feet long, THHN, 500kcmil, aluminum conductor Conductor #1 has a higher ampacity than conductor #2. If the ambient temperature decreases by 5 C to a temperature of 25 C, the ampacity of both conductors will increase by 5%. Conductor #1 weighs more than conductor #2. False Conductor #1 will be solid, but conductor #2 will be stranded. False Both conductors are suitable for use only in dry locations. False The ampacity of conductor #1 is 2x greater than the ampacity of a THHN, copper conductor that has ½ the cross-sectional area of conductor #1 (I.e. 250kcmil). False Decreasing the lengths of the conductors will cause their ampacities to increase. Problem #9) Additional True/False Statements: Branch circuits rated at greater than 50A should only be used to supply non-lighting type outlet loads. False The overcurrent protection device protecting a branch circuit should always be placed at the load-end of the circuit conductors. False The (trip) rating of a circuit breaker that is protecting a circuit must be greater than the ampacity of the conductors used in that circuit. False An overload occurs in a circuit during the existence of a (short-circuit) fault. False The I 2 t (time) curve for a fuse provides the user with a plot showing the number of times that a fuse is able to successfully interrupt a specific amount of current. False A feeder consists of a set of conductors that are used to supply current to utilization equipment (loads) that is connected to a distribution system. False An adjustment factor is applied to the ampacity of the conductors within a raceway only when six or more current-carrying conductors are contained within the raceway. False Switchboards are used within distribution systems to supply only feeders and panelboards are used within distribution systems to supply only branch circuits. False A service lateral is a feeder that interconnects switchboards on the same floor of a multi-floor (high-rise) facility. The interrupting rating of an overcurrent protection device is the maximum fault current that the device can safely interrupt (without harm to personnel or resulting damage to equipment, the premises or the device itself).
ECET 4520 - Exam II (Fall 2010) Problem #1) Specify if each statement is True or False based on NEC guidelines and/or standard design practice. Given a single-phase branch circuit consisting of 2 300kcmil copper conductors operating in a system with rated terminal temperatures of 75 C and an ambient temperature of 30 C: An increase in the allowable terminal temperature for the system to 90 C will cause an effective increase in the ampacity of the conductors. False An increase in the ambient temperature to 40 C will cause an effective increase in the ampacity of the conductors. An increase in the size of the conductors to 400kcmil will cause an increase in the ampacity of the conductors. False Switching from copper to aluminum conductors without changing the size will cause an increase in the ampacity of the conductors. False Running two of these branch circuits together in a single conduit will cause an effective increase in the ampacity of the conductors. Problem #2) Given a dedicated branch circuit that serves an appliance rated at 120V, 22A (continuous); a) The smallest, standard-sized circuit breaker that should be used to protect this load is: b) The calculated (VA) load that this appliance puts on the distribution system is: CB rating = 30 A Load = 2,640 VA c) When calculating the voltage drop on the branch circuit serving this appliance, the value of the current that should be used in the voltage-drop calculation is: Current = 22 A Problem #3) If the resistance of an aluminum conductor is 0.49Ω/1000 at a temperature of 75 C, determine the resistance of the same conductor at a temperature of 105 C. R 105 0.5385 Ω/1000 C
Problem #4) Given the rectangular-shaped bar conductor shown below, determine the cross-sectional area of the conductor in circular mils. 0.15" 0.20" A 38,200 cmils Problem #5) Given the small distribution system shown below that serves three dedicated loads: infinite bus 13.8kV-480V 3 150kVA 1.2%Z CB#1 80A CB#2 90A 3 - #6 AWG THHN Cu. 3 - #3 AWG THHN Cu. Load #1-49kVA pf = 0.95 lagging (continuous load) Load #2-59kVA pf = 0.95 lagging (continuous load) CB#3 150A 3 - #1 AWG THHN Cu. Load #3-99kVA pf = 0.95 lagging (transient load) Determine the things that are wrong or improper in the design of this distribution system, assuming that the smallest allowable conductor sizes and circuit breakers should be utilized and that the system must be able to properly supply the three specified loads: Assume a 75 C terminal temperature rating and a 30 C ambient temperature. Note you do NOT need to calculate the voltage drop on any of the branch circuits. Note - use the blank sheet of paper provided by your instructor for any calculations required while solving this problem. Only write the design errors in the blank space below. The 150kVA transformer rating is too small to serve the 207kVA load. The ampacity of the #6 AWG conductors used in the circuit that supplies load #1 is less than 125% of the 59A rated load current, thus the conductors are too small. Although the 150A rated CB#3 is allowed by the NEC, a 125A CB should be chosen since that would be the smallest allowable circuit breaker rating.
Problem #6) Given an operational 45kVA, 13.8kV 480V, Y-Y, step-down transformer; a) Determine the rated phase voltage of the transformer s primary winding. b) Determine the rated secondary (line) current. c) Determine the rated primary (line) current. V P ) ( phase rated 7,967 V I S ) ( line rated 54.1 A I P ) ( line rated 1.88 A Problem #7) Specify if each statement is True or False based on NEC guidelines and/or standard design practice. Ampacity may be defined as the maximum continuous current that a conductor may carry without causing the conductor s temperature to exceed its temperature rating. False Convenience receptacles should be placed no more than 6 feet apart on all of the walls of any habitable room in a building. An 80V, single phase, 3-wire circuit would have an 80V potential across the two ungrounded conductors and a 40V potential between either of the ungrounded conductors and the (grounded) neutral conductor. False If a service transformer is assumed to be connected to an infinite bus, then the transformer is assumed to be infinitely far away from the power system s generators. False If a branch circuit serves both continuous and transient loads at the same time, then the circuit breaker protecting the circuit should be rated greater than or equal to 125% of the continuous load current. The terminal temperature rating of a branch circuit refers to the temperature rating of the equipment/enclosures connected to the ends of the conductors in the branch circuit. False Increasing the length of the conductors in a branch circuit will typically increase the magnitude of the short circuit current available at the load-end of the circuit. Increasing the length of the conductors in a branch circuit will typically increase the magnitude of the voltage-drop seen at the load-end of the circuit. False An increase in the ambient temperature around a conductor results in an increase in the conductor s ampacity because an increased ambient temperature means that the conductor is allowed to operate at a higher temperature. False A solid conductor having a specific AWG size will always have a larger nominal diameter compared to a stranded conductor having the same (AWG) size. The AC resistance of a conductor will be larger than the conductor s DC resistance.
Refer to the following figure for problems #8 through #11. The figure shows a 3Φ distribution system consisting of a service transformer which feeds two branch circuits. The first branch circuit is fed directly from the secondary of the service transformer. An additional transformer is used to stepdown the service voltage to 208V for the 2 nd branch circuit. Both branch circuits are terminated by a single outlet and the outlets are used to supply the loads specified in the figure. infinite bus Note - Assume a 75 C terminal temperature rating and a 30 C ambient temperature. 13.8kV-480V 3 300kVA 1.2%Z 480V-208V 3 75kVA 1.6%Z 3-250 kcmil Cu. 200ft. 3-1/0 AWG Cu. 200ft. Load #1-200kVA pf = 0.84 lagging (transient load) Load #2-40kVA pf = 0.92 lagging (continuous load) Problem #8) Assuming that the service transformer is supplying rated secondary voltage; determine the percent voltage drop in the line voltage that occurs due the 250kcmil conductors used in the branch circuit that supplies Load #1. V drop % 1.3 % Problem #9) Determine the 3Φ, L-L-L short circuit current available at the secondary terminals of the service transformer using the point-to-point method of calculation. I 30,070 amps SCA(SecT1) Problem #10) Determine the 3Φ, L-L-L short circuit current available at the secondary terminals of the 480-208V transformer using the point-to-point method of calculation. I 10,957 amps SCA(SecT 2) Problem #11) Determine the 3Φ, L-L-L short circuit current available at the outlet serving load #2 using the point-to-point method of calculation. I 3,596 amps SCA(Load 2)