DC Theory Lesson 7

A person is repairing an emergency light, which normally has two 6-volt lamps in series on a 12-volt battery. One of the lamps is burned out, and no replacement is available.

The lamp is rated at 7 1/2 watts. What size resistor would temporarily be needed while he or she tries to find a replacement lamp? Calculate to one decimal place.

A person is connecting a 250-watt heating element at a jobsite. The heater does not heat correctly so he or she checks the voltage. He or she measures 95 volts across the heater, but the tag says the heater requires 120 volts. How much heat (in watts) is the heater producing? Calculate to two decimal places.

A person has a job installing fixtures in a large room. He or she does not have prints and must figure the number of circuits required. The fixtures are rated 300 watts each, and there are 30 of them to install. The maximum wattage on any one circuit is 1,800 watts.

How many circuits will be needed?

How many 1.5-volt batteries would be required to power a 13.5-volt radio?

A person wants to purchase a generator to power a pump in the basement. The pump

motor is 1 1/2 horsepower. What size generator (in watts) would be required?

What is the maximum wattage lamp that can be connected on a dimmer which is rated

at 8.33 amps at 120 volts? (Round the FINAL answer to the nearest whole number.)

A person wants to individually fuse the speakers on the stereo system. The speakers are each rated 200 watts at 8 ohms. What size fuse should be used to protect each speaker?

A person is connecting heat trace cable on pipe at the jobsite. The cable he or she is

using is rated 2.5 watts/foot. How many feet of this cable can be installed on a 120- volt/15-amp circuit?

The signal from a transducer varies from four to 20 milliamps. The chart recorder a person wishes to control with this signal requires one to five volts to drive the indicator on the recorder. What size resistor would be required to convert the 4-20 milliamp signal

into a 1-5-volt signal?

How much voltage would be required at the source to provide 120 volts to a 500-watt load which is located 1,500 feet from the source and fed with #12 AWG wire? (1.93 ohms per 1,000 feet) (Round the FINAL answer to the nearest whole number.)

A kitchen circuit is rated for 20 amps. The toaster is rated 550 watts, the coffee pot is rated 375 watts and the toaster oven is rated 1,200 watts. If all of these appliances are

on, the current will exceed 20 amps. (All appliances are 120 volts.) Calculate to one decimal place.

Select one:

True

False

Two lamps are installed in parallel on a circuit. With both lamps on, the current to the lamps is 7.5 amps. One lamp is marked 12V/30W. What is the wattage of the other lamp?

Given the circuit drawing, what size resistor would be required to supply the three 30-

volt, 10-watt lamps with their rated voltage if the supply voltage is 45 volts DC as

shown?

R1 = ? Ω

The resistance of 2/0 AWG uncoated copper conductors is 0.0967 ohms per 1,000 feet.

What would be the total resistance for three 2/0 AWG conductors installed in parallel if the total length of each of the three conductors is 243 feet? Calculate to five decimal places.

What would be the voltage drop in the conductors in the previous question if the total current through all three (parallel) conductors was 410 amps? Calculate to two decimal places.

What would be the voltage drop if a fourth conductor was added in parallel with the other three in the previous question? Assume the same current. Calculate to two decimal places.

The current in the original circuit (without R2 ) is 4/15 of an ampere or approximately 267 mA. What value of resistor (R2 ) could be installed in parallel with the lamp to reduce the current through the lamp to half that value?

(Note: The values calculated for this question may be used for additional questions.)

The current in the original circuit (without R2 ) is 4/15 of an ampere or approximately 267

mA. What value of resistor (R2 ) could be installed in series with the lamp to reduce the

current throught the lamp to half that value?

(Note: The values calculated for this question may be used for additional questions.)

Branch-circuit conductors shall have an ampacity of not less than ? of the continuous load.

a. 50%

b. 75%

c. 100%

d. 125%

The NEC recommends limiting voltage drop on branch circuits or feeders to ? and a maximum of ? for feeders and branch circuits combined.

a. 3% / 5%

b. 3% / 7%

c. 5% / 7%

d. 5% / 10%

What would be the recommended maximum allowable voltage drop on a 120 volt branch circuit? Calculate to one decimal place.

If someone had a 120 volt 500 watt heater on a branch circuit with maximum voltage drop recommended by the NEC, what would be the output wattage of the heater?

Calculate to two decimal places.

What percentage would the wattage of the heater in the previous question be reduced due to a 3% reduction in line voltage? Calculate to one decimal place.

Copper has ? per circular-mil foot of resistance at 20°C.

a. 2.5 Ω

b. 3.6 Ω

c. 10.4 Ω

d. 12.5 Ω

Which of the following is not a factor that will affect the resistance of branch-circuit wiring?

a. Circular-mil area or diameter of the wire

b. Device type

c. Length

d. Temperature

e. Type of material

If the resistance of copper is 10.4 ohms per mil foot at 75°F, and #12 copper has a circular-mil area of 6,530 cmil, what would be the resistance of 1,000 feet of #12 copper? (Use the formula R = (K × L) / A.)

Calculate to two decimal places.

The NEC Chapter 9, Table 8, gives 1.93 ohms for the same piece of wire. The difference in resistance is due to a temperature difference. Use the following formula (from the NEC, Table 8-Notes) to determine the resistance of 1,000 feet of #12 wire at 105°C.

Calculate to two decimal places.

R1 = original resistance (1.93 @75°C)

R2 = new resistance

T2 = new temperature

αcu = temperature coefficient (0.00323)

R2 = R1 × [1 + αcu × (T2 − 75)]

Solve the formula [R =(Kcu × L) / A ] to determine the value of Kcu (specific resistance of copper) at 75°C. Use the resistance value and circular-mil area data for uncoated #14 AWG solid wire from Chapter 9, Table 8, of the NEC. Calculate to two decimal places.

Calculate the voltage drop on a 240 volt line, with #10 wire supplying 23 amperes to a resistive load 300 feet from the source. (Use Kcu = 12.6.) Calculate to two decimal places.

(Note: The values calculated for this question may be used for additional questions.)

What is the percentage voltage drop on this line in the previous question? Calculate to two decimal places.

(Note: The values calculated for this question may be used for additional questions.)

Using the previous question(s), determine the circular-mil area of the conductor needed to keep the voltage drop from exceeding 3%.

(Note: The values calculated for this question may be used for additional questions.)

Using the previous question(s), what size wire would be needed to meet the requirements?

(Note: The values calculated for this question may be used for additional questions.)

a. #6

b. #8

c. #10

d. #12

Using the previous question(s), after installing the new conductor, what would be the new voltage drop? Calculate to two decimal places.

(Note: The values calculated for this question may be used for additional questions.)

Using the previous question(s), what is the percentage voltage drop? Calculate to two decimal places.

(Note: The values calculated for this question may be used for additional questions.)

Based upon the previous questions, the circuit meets the recommendations of the NEC.

Select one:

True

False

A 240 volt feeder supplies 150 amperes to a load 200 feet away. What size wire is needed to meet the 3% voltage drop limit? (Use Kcu = 12.6.)

a. #1

b. 1/0

c. 2/0

d. 3/0