Problems

Author: E.E. Kimberly

	Problem	Answer
1	A distribution system has an average power loss of 8 kw at a summer temperature of 86 E. What would be the average power loss at - 20 F with the same current loading?
2	The copper field circuit of a direct-current generator has a resistance of 120 ohms at a room temperature of 22 C. After 4 hours of operation, its resistance is found to be 132 ohms. What is its temperature?
3	A rectangular bus conductor has a cross-sectional area of i sq in. What is its area in circular mils?
4	What is the resistance at 25 C of a coil of commercial copper wire containing 1200 ft of No. 18 conductor?
5	At what temperature would the coil of Problem 4 have no resistance?
6	A load of 50 kw is located 200 ft from a generator. The generator terminal voltage is 248 volts, and the voltage at the load is 230 volts. Calculate the area, in circular mils, of the distribution conductor. Would the National Electric Code permit the use of this size of conductor (rubber-covered) indoors? (See Table B in the Appendix.)
7	What is the power loss in 1500 ft of No. 4 copper conductor carrying 100 amp at 60 C?
8	A coil whose mean diameter is 6 in. contains 4000 turns of No. 22 copper wire. What is its resistance at 50 C?
9	A 6-ft instrument lead of No. 18 copper wire is accidentally connected across a 115-volt a-c supply. Calculate the maximum rate at which heat is generated in the wire.
10	A transformer winding has a resistance of 0.2 ohm at 21 C. What will be its temperature when its resistance measures 0.31 ohm?
11	A tungsten lamp on a 120-volt circuit draws 0.68 amp at a temperature of 2700 C. Calculate the current inrush at the instant of switching when the filament temperature is 25 C. Assume a constant temperature coefficient of 0.0045.	8.02
12	Aluminum has a resistance 1.6 times that of copper and the weight of a unit volume is one-third as great. If the conductor of Problem 7 were of aluminum instead of copper, what would be its weight in pounds to produce the same heat loss at the same current of 100 amp?
13	A 10-ft length of alloy resistance wire has a resistance of 2 ohms. Its diameter is 0.020 in. What is the resistance per circular mil-foot of this alloy?	80 ohms