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Sizing Cables
Very high currents can flow in cables connecting inverter to the
batteries and between the batteries. Cables must be adequately sized to
suit the maximum current drawn by the inverter. Low resistance links and
tight connections are essential to maintain even use of a battery bank.
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In order to minimise the voltage drop between
battery and inverter, and so maintain inverter efficiency,
cables should be kept as short as possible and be sufficiently
sized.
Recommended sizes (2m long) are:
|
Battery to Inverter Cable Size (2m long) |
|
Inverter Wattage |
Battery Voltage |
|
12 V |
24 V |
48V |
|
150 W |
10 mm˛ |
6
mm˛ |
- |
|
250 W |
16 mm˛ |
6
mm˛ |
- |
|
500 W |
35
mm˛ |
10 mm˛ |
- |
|
1000 W |
50
mm˛ |
25
mm˛ |
- |
|
1500 W |
50
mm˛ |
35
mm˛ |
- |
|
2000 W |
70 mm˛ |
50
mm˛ |
- |
|
2500 W |
95
mm˛ |
70
mm˛ |
50
mm˛ |
|
3000 W |
- |
95
mm˛ |
50
mm˛ |
|
3500 W |
- |
95
mm˛ |
70
mm˛ |
|
4500 W |
- |
- |
70
mm˛ |
|
|
|
Conversion of American Wire Gauge to Metric |
|
Size AWG |
Amp Rating per single conductor |
Area mm˛ |
|
20 |
7 |
0.52 |
|
18 |
10 |
0.82 |
|
16 |
15 |
1.31 |
|
14 |
20 |
2 |
|
12 |
25 |
3.31 |
|
10 |
40 |
6.68 |
|
8 |
65 |
8.37 |
|
6 |
95 |
13.3 |
|
4 |
125 |
21.15 |
|
2 |
170 |
33.62 |
|
1 |
195 |
42.41 |
|
1/0 |
230 |
53.5 |
|
2/0 |
265 |
67.43 |
|
3/0 |
310 |
85 |
|
4/0 |
360 |
107.2 |
|
250 |
405 |
126.7 |
|
300 |
445 |
152 |
|
500 |
620 |
253.4 |
|
750 |
785 |
300.66 |
|
1000 |
935 |
506.7 |
|
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To calculate
an acceptable size of wire to do a particular job, it is necessary to
know the wattage or amperage of the appliance, it's distance from the
battery, and the system's voltage.
In general
most 12 V lighting and appliances will work on voltages as low as
11-11.5 V. But if possible, wiring should be sized so that losses are
less than 2% ie a 0.25 V drop. (See Table 2.)
The following
equation is useful in choosing the required wire:
R = E / I x L
R = Resistance of wire in
Ohm/m
E = Maximum allowable voltage
drop in the wiring in volts
I = Current drawn by
appliance in amps
= Wattage/Voltage
L = Length of wire of the
complete circuit in metres (count twice - both in & out).
For Example:
If a 60W
(60/12 = 5A) appliance needs to be situated 10m from a 12V battery and
the maximum allowable voltage drop is 2% (0.25 V):
R = 0.25 Volt
/ [(5 Amps)x(20m)] = 0.0025 Ohm/m.
Therefore
resistance of the wire should be less than 0.0025 Ohm/m. Looking at
Table 1 this means that 6 mm˛ wire or thicker must be used. The
thicker the wire the better the efficiency.
Voltage drop
becomes less of a problem with higher voltage systems.
Table 1
|
Cross Sectional Area |
Maximum Current Rating (Amps) |
Resistance |
|
Enclosed Cable In Conduit Or Trunking |
Cable Clipped Direct To Open Surface |
Ohm / metre |
|
mm˛ |
Twin |
3-Core |
Twin |
3-Core |
Per Core |
|
1.0 |
11 |
9 |
12 |
10 |
0.018 |
|
1.5 |
13 |
12 |
15 |
13 |
0.012 |
|
2.5 |
18 |
16 |
21 |
18 |
0.0074 |
|
4.0 |
24 |
22 |
27 |
24 |
0.0046 |
|
6.0 |
30 |
27 |
35
|
30 |
0.0031 |
|
10.0 |
40 |
37 |
48 |
41 |
0.0018 |
|
16.0 |
53 |
47 |
64 |
54 |
0.0012 |
|
25.0 |
60 |
53 |
71 |
62 |
0.00073 |
|
35.0 |
74 |
65 |
87 |
72 |
0.00049 |
Table 2
Maximum length (in m) of cable from power source to load for
less than 2% voltage drop. (For a 12 V system.)
(For 24 V systems cables can be twice as long; 240 V wiring can
be 20 times as long)
|
Cable Size (mm˛ cross-sectional area) |
|
Amps |
1 |
1.5 |
2.5 |
4 |
6 |
10 |
16 |
25 |
35 |
50 |
75 |
100 |
|
1 |
7 |
10.91 |
17.65 |
28.57 |
42.86 |
70.6 |
109.1 |
176.5 |
244.9 |
- |
- |
- |
|
2 |
3.53 |
5.45 |
8.82 |
14.29 |
21.4 |
35.3 |
54.5 |
88.2 |
122.4 |
171.4 |
- |
- |
|
4 |
1.76 |
2.73 |
4.41 |
7.14 |
10.7 |
17.6 |
27.3 |
44.1 |
61.2 |
85.7 |
130.4 |
- |
|
6 |
1.18 |
1.82 |
2.94 |
4.76 |
7.1 |
11.7 |
18.2 |
29.4 |
40.8 |
57.1 |
87 |
117.6 |
|
8 |
0.88 |
1.36 |
2.2 |
3.57 |
5.4 |
8.8 |
13.6 |
22 |
30.6 |
42.9 |
65.25 |
88.2 |
|
10 |
0.71 |
1 |
1.76 |
2.86 |
4.3 |
7.1 |
10.9 |
17.7 |
24.5 |
34.3 |
52.2 |
70.6 |
|
15 |
- |
0.73 |
1.18 |
1.9 |
2.9 |
4.7 |
7.3 |
11.8 |
16.3 |
22.9 |
34.8 |
47.1 |
|
20 |
- |
- |
0.88 |
1.43 |
2.1 |
3.5 |
5.5 |
8.8 |
12.2 |
17.1 |
26.1 |
35.3 |
|
25 |
- |
- |
- |
1.14 |
1.7 |
2.8 |
4.4 |
7.1 |
9.8 |
13.7 |
20.9 |
28.2 |
|
30 |
- |
- |
- |
- |
1.4 |
2.4 |
3.6 |
5.9 |
8.2 |
11.4 |
17.4 |
23.5 |
|
40 |
- |
- |
- |
- |
- |
1.8 |
2.7 |
4.4 |
6.1 |
8.5 |
13 |
17.6 |
|
50 |
- |
- |
- |
- |
- |
- |
2.2 |
3.5 |
4.9 |
6.9 |
10.4 |
14.1 |
|
100 |
- |
- |
- |
- |
- |
- |
- |
1.7 |
2.4 |
3.4 |
5.2 |
7.1 |
|
150 |
- |
- |
- |
- |
- |
- |
- |
- |
- |
2.3 |
3.5 |
4.7 |
|
200 |
- |
- |
- |
- |
- |
- |
- |
- |
- |
- |
2.6 |
3.5 |
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