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Wiring makes up many of the elements in a circuit. Larger sizes of wire have more copper in them, and hence lower resistance. Wire size is specified by a gauge number. The lower the gauge number, the larger the diameter of the copper wire, and thereby the lower its resistance. The actual resistance per 1,000 feet of various copper wire gauges is detailed in Table 1, the Copper Wire Table. We encourage you to use only copper wire in your AE system. Aluminum wire has greater resistance (about twice for the same cross sectional area) and is virtually impossible to interconnect without higher resistance connections. If you don't think so, then try soldering an aluminum wire sometime.

From the Copper Wire Table, we can calculate the resistance of any particular piece of wire. The resistance per foot times the number of feet gives us the total resistance of a length of wire. When estimating the resistance of wiring be sure to include BOTH conductors, i.e. if an appliance is 100 feet from the battery, then the total wiring length is 200 feet (there are two wires actually, each one 100 feet long).

If we know the amount of current being consumed, the resistance per foot of any given wire gauge, and the length of the total wire in the circuit, then how do we determine the actual gauge of wire we should use? The answer is determined by exactly how much loss we find acceptable. In general, consider a 5% loss to be the maximum acceptable (2.5% is better). If we are using 12 VDC, then 5% voltage loss is 0.6 volts (2.5% is 0.3 volts). Consider the following equation to specify exactly which wire gauge to use for any given application.

R = Resistance expressed in Ohms (W) per 1000 feet. E = Maximum allowable voltage loss in the wiring, in Volts. I = Amount of current flowing through the circuit, in Amperes. L = The length of wire in the complete circuit, in feet.

This equation gives us a value in Ohms per 1,000 feet. Simply find the copper wire gauge size that has LESS than this amount of resistance per 1,000 feet, and you've found your wire gauge size.

Consider a PV array that produces 12 amperes. This array is located 100 feet from the batteries. What gauge size of wire should be used to keep the voltage loss in the wiring to less than 0.6 volts? Well, there is 200 feet (two conductors, remember) of wire in the circuit, and a current of 12 amperes flowing. The equation above gives us a maximum resistance of the wire as 0.25W per 1,000 feet. By consulting the Copper Wire Table, we find that 4 gauge wire has a resistance of 0.2485W per 1,000 feet. Since this is less than the 0.25W/1,000 ft. the equation generated, 4 gauge wire is the

TABLE 1- THE COPPER WIRE TABLE

RESISTANCE

DIAMETER

WIRE GAUGE

OHMS PER 1000 FEET

FEET/ OHM

OHMS/ KM.

METERS PER W

MILS

MM.

0000

0.04091

20400

0.1608

6219

460.0

11.68

000

0.06180

16180

0.2028

4932

409.6

10.40

00

0.07793

12830

0.2557

3911

364.8

9.266

0

0.09827

10180

0.3224

3102

324.9

8.252

2

0.1563

6400

0.5127

1951

257.6

6.544

4

0.2485

4025

0.8152

1227

204.3

5.189

6

0.3951

2531

1.296

771.5

163.0

4.115

8

0.6282

1592

2.061

485.2

128.5

3.264

10

0.9989

1001

3.277

305.1

101.9

2.588

12

1.588

629.6

5.211

191.9

80.81

2.053

14

2.525

396.0

8.285

120.7

64.08

1.628

16

4.016

249.0

13.17

75.90

50.82

1.291

18

6.385

156.6

20.95

47.74

40.30

1.024

20

10.15

98.50

33.31

30.02

31.96

0.8118

22

16.14

61.95

52.96

18.88

25.35

0.6438

24

25.67

38.96

84.21

11.87

20.10

0 0

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