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Sizing Cables

Source: Wind and Sun Ltd

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.

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 mm2	6 mm2	-
250 W	16 mm2	6 mm2	-
500 W	35 mm2	10 mm2	-
1000 W	50 mm2	25 mm2	-
1500 W	50 mm2	35 mm2	-
2000 W	70 mm2	50 mm2	-
2500 W	95 mm2	70 mm2	50 mm2
3000 W	-	95 mm2	50 mm2
3500 W	-	95 mm2	70 mm2
4500 W	-	-	70 mm2

Conversion of American Wire Gauge to Metric
Size AWG	Amp Rating per single conductor	Area mm2
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

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 mm² 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
mm2	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 (mm2 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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