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¦ The NEC is used to determine the wire size for most installations.
¦ The resistance of a wire is determined by four factors:
a. the type of material from which the conductor is made
b. the length of the conductor
c. the area of the conductor
d. the temperature of the conductor
¦ When conductors are used in high-temperature locations, their current capacity must be reduced.
¦ When there are more than three conductors in a raceway, their current carrying capacity must be reduced.
¦ The amount of current a conductor can carry is affected by the type of insulation around the wire.
¦ In the English system, the mil foot is used as a standard for determining the resistance of different types of wire.
¦ After wires have been installed, they should be checked for shorts or grounds with a MEGGER.
Use of the NEC will be required to answer some of the following questions.
1. What is the maximum temperature rating of Type XHHW insulation when used in a wet location?
2. Name two types of conductor insulation designed to be used underground.
3. A 10 AWG copper conductor with Type THW insulation is to be run in free air. What is the maximum ampacity of this conductor if the ambient air temperature is 40 degree C?
4. Six 1/0 aluminum conductors are to be run in a conduit. Each conductor has Type THWN insulation, and the ambient air temperature is 30°C. What is the ampacity of each conductor?
5. Name five conditions that must be met for running conductors in parallel.
6. What is the largest solid (nonstranded) conductor listed in the wire tables?
7. Can Type TW cable be used in an area that has an ambient temperature of 658C?
8. How is the grounded conductor in a flat multi-conductor cable 4 or larger identified?
9. What three colors are ungrounded conductors not permitted to be?
10. Twenty-five 12 AWG copper conductors are run in conduit. Each conductor has Type THHN insulation. The conduit is located in an area that has an ambient temperature of 958F. What is the ampacity of each conductor?
11. A single-phase load is located 2800 ft from its source. The load draws a current of 86A and operates on 480V. The maximum voltage drop at the load cannot be greater than 3%. What size aluminum conductors should be installed to operate this load?
12. It’s decided to use parallel 0000 conductors to supply the load in . How many 0000 conductors will be needed?
13. A three-phase motor operates on 480 V and is located 1800 ft from the power source. The starting current is 235 A. What size copper conductors will be needed to supply this load if the voltage must not be permitted to drop below 6% of the terminal voltage during starting?
On the Job:
You have been hired by a company to connect an outside lighting system to a panel box located 55 ft from the point of attachment. The conductors are to have Type THWN-2 insulation and the wire is copper. The termination temperature of the connections is not known. The lighting system is rated at 20 kW. The voltage is 240 V. Because the lights are expected to operate more than three hours at a time, they are considered continuous duty. Using the NEC, determine the wire size needed to make this connection.
On the Job:
You are a journeyman electrician in an industrial plant. You have been assigned the task of connecting a 25-kW electric heater to a 240-V, single-phase panel.
The heater is located in an area with an ambient temperature of 90 degree F. The conductors are to be copper with Type TW insulation. The heater is expected to operate for more than three hours at a time. What size conductors would you use to connect this heater?
Problems – in the Real World:
Using Wire Tables and Determining Conductor Sizes
1. A 2 AWG copper conductor is 450 ft long. What is the resistance of this wire? Assume the ambient temperature to be 20 degree C.
2. An 8 AWG conductor is 500 ft long and has a resistance of 1.817 V. The ambient temperature is 20C. Of what material is the wire made?
3. Three 500-kcmil copper conductors with Type RHH insulation are to be used in an area that has an ambient temperature of 588C. What is the maximum current-carrying capacity of these conductors?
4. Eight 10 AWG aluminum conductors with Type THWN insulation are installed in a single conduit. What is the maximum current-carrying capacity of these conductors? Assume an ambient temperature of 30C.
5. A three-phase motor is connected to 480 V and has a starting current of 522 A. The motor is located 300 ft from the power source. The voltage Practical Applications
You are an electrician working in an industrial plant. The company is constructing a maintenance shop located 225 feet from the main plant electrical system. You are to install a 400-ampere three-phase wire system in the new maintenance shop. The service is supplied by the existing plant service. The voltage is 480 volts and the volt age drop is to be kept at 3% or below at full load. The conductors are to be aluminum.
What size conductors should be installed for the new service? Express your answer to the nearest standard wire size. (Note: Determine the wire size by calculating length, resistance, and voltage drop as opposed to using the National Electrical Code.)
In the Real World:
Using the above example, determine the wire size needed for the 400-ampere service in accordance with the National Electrical Code. Assume that the conductors have type THWH insulation. (Note: As a general rule, for each 100 feet of conductor length the wire size is increased one standard size because of voltage drop.) drop to the motor cannot be greater than 5% during starting. What size copper conductors should be connected to the motor?
6. A 50-hp DC motor is connected to 250 volts. The motor does not contain a NEMA code. The motor is expected to operate for more than three hours at a time. The conductors are to be copper with type RHW-2 insulation. The motor is located in an area with an ambient temperature of 112°F. The two conductors are to be run in conduit. What size conductors should be used to connect this motor? (Note: 1 hp = 746 watts.)
7. A bank of electric heaters has a power rating of 25 kW. The heaters are connected to 480 volts. The heaters will operate for more than three hours at a time. The conductors supplying the heaters are to be aluminum with type THWN-2 insulation. The heaters are located in an area with an ambient temperature of 48°C. There will be six conductors in the conduit. What size conductors should be used for this installation?
8. A 15-hp squirrel-cage induction motor is connected to 240 volts. The motor has a NEMA code B. The motor will operate for more than three hours at a time. The motor is in an area with an ambient temperature of 86°F. There are to be three conductors in the raceway. The conductors are copper with THW-2 insulation. What size conductors should be used for this installation? (Note: 1 hp = 746 watts.)
9. Determine the resistance of a 16 AWG copper conductor located in an area with an ambient temperature of 188.6°F (87°C) with a length of 250 feet (76.2 m).
10. Determine the ohms-per-mill foot of an aluminum conductor located in an area with a temperature of 104°F (40°C).
Use the NEC to determine the ampacity of the following conductors.
Size Material Insulation | Ambient Temperature | Conductors in Raceway Amps
#10 AWG Copper RHW 350 kcmil Copper XHH
#2 AWG Aluminum TW 3/0 AWG Aluminum XHHW-2 500 kcmil Copper THWN
#6 AWG Copper THW-2 2/0 AWG Aluminum UF 750 kcmil Aluminum RHW-2
Friday, March 8, 2013 3:31