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We may calculate voltage drops in conductors by taking the equation R = 2pL/A and multiplying by I:
Ed = IR = -^r-d A
A certain motor draws 22 amperes at 230 volts and the feeder circuit is 150 feet in length. If No. 10 copper conductors are selected, what would the voltage drop be? Would No. 10 conductors be permissible for the 3 percent maximum voltage drop allowed by the NEC?
2 X 12 X 150 X 22 79,200 ,
Ed =---------10380----------= 10:380 = 7 63 v°',s
However, 230 X 0.03 = 6.90 volts, which is the voltage drop permitted by the NEC for feeders, so No. 10 conductors wouldn’t be large enough. The next larger size would be the proper selection.
In the Appendix, you will find a table to determine volt loss in copper conductors in iron and nonmagnetic conduits; you will also find a similar table for aluminum conductors.
In measuring the diameter of conductors, there are two very popular instruments used. One is the wire gauge, illustrated in Figure 7-5. The other is the micrometer (Figure 7-6), which measures one-thousandths of inches and fractions thereof.
Many conductors are composed of stranded conductors. The total of the circular mils of the strands equals the total circular mils of the conductor. Conductors are stranded mostly for flexibility and ease of pulling into raceways. This is especially true of the larger sizes of conductors, such as No. 8 and larger.
Figure 7-5 Standard wire gauge.
Figure 7-6 Micrometer.
1. What losses occur in conductors when electrical currents flow through them?
2. What is an ohm?
3. What is a mho?
4. How many ohms in a megohm?
102 Chapter 7
5. The resistance of a conductor is directly proportional to its
6. The resistance of a conductor is inversely proportional to its
7. Does temperature affect the resistance of copper conductors?
8. What is a circular mil?
9. Give a formula for changing circular mils to square mils.
10. Give a formula for changing square mils to circular mils.
11. What is designated by rho?
12. Give the symbol for rho.
13. What is skin effect?
14. Give the conductivity of the following metals, as compared to silver: copper, gold, Nichrome, aluminum, and iron.
15. Explain superconductivity.
16. Give the voltage-drop formula for DC circuits.
Resistance in Series and Parallel
A series circuit is one in which all the electrical devices and components are connected end to end so that the same current flows throughout the circuit. Such an electrical circuit is illustrated in Figure 8-1. In Figure 8-1, there are five resistors connected in series with a source of electrical energy, which in this case will be shown as the battery B.
Figure 8-1 Series resistive circuit.
Resistances in Series
There are rules governing resistances in series, which are as follows:
1. The total resistance in a series circuit is equal to the arithmetic sum of the resistances of the individual resistors in the series circuit; e.g., in Figure 8-1, we have five resistors in series, namely, R1, R2, R3, R4, and R5. The resistance of the individual resistors are Rt = 10 ohms, R2 = 15 ohms, R3 = 5 ohms, R4 = 25 ohms, and R5 = 20 ohms. Thus, to arrive at the total resistance of this series circuit, we add all five individual resistances:
10 + 15 + 5 + 25 + 20 = 75 ohms, that is, Rt = 75 ohms.
2. The voltage drop in a series circuit equals the sum of the voltage drops of the resistors in series, and this in turn equals the voltage of the supply source. Also, the sum of the voltage drops across the individual resistors will be the same as the voltage drop across all resistors in the series circuit.
3. The current in a series circuit is the same throughout the entire circuit.
104 Chapter 8
Now for rules 2 and 3, make use of Ohm’s law: E = IR. It was found in (1) that Rt = 75 ohms and the source voltage is given as 150 volts, so I = E/R, or I = 150/75 = 2 amperes, and this 2 amperes flows through all five resistors. Further, the voltage drop in each resistor is as follows:
E1 = I X R1 = 2 X 10 = 20 volts
E2 = I X R2 = 2 X 15 = 30 volts
E3 = I X R3 = 2 X 5 = 10 volts
E4 = I X R4 = 2 X 25 = 50 volts
E5 = I X R5 = 2 X 20 = 40 volts
total = 150 volts
This proves that the total voltage drop is equal to the sum of the voltage drops across the resistors and this total must equal the voltage of the electrical source.
Resistances in Parallel
A parallel circuit is also known as a multiple circuit or a shunt circuit. Figure 8-2 illustrates such a circuit.
150V RjSlOO f?2>15fi R3>50 R4S250 R5<20fi
Figure 8-2 Parallel resistive circuit.
Again, as for series circuits, we have rules governing parallel circuits:
1. The total resistance of the combined resistances in a parallel circuit is always less than the resistance of the lowest-value resistor in the parallel circuit. This will be calculated mathematically as proof.
2. The combined total resistance of a number of unequal resistances in parallel is equal to the reciprocal of the sum of the reciprocals of the individual resistances.