Determine the size of wire of the solar power system

2/10/2019

Tell me one thing which connects all the components of any solar power system?

It is the wire which joins all the components and transfers the output of one to the next in line, making the current to reach to the electrical appliances.

You cannot connect the components with any wire size. As under-sized wire may get heat up soon and will eventually burn out while an over-sized wire will cost you more, reducing the financial feasibility of the system.

Therefore, the right sizing is very important for the system to work efficiently.

In this post, we will learn to size the wire in accordance with the current provided by the panels.

The concept of resistance

Everything which has mass or exists in this universe, offers some kind of resistance.

Resistance is the measure of the opposition offered by the wire to the flow of the current and it is measured in ohms.

While experimenting, our engineers found that:

The opposition to the flow of current are more in long wires than the shorter ones.

The wires with wider cross-sectional area allow current to flow more easily than ones with smaller cross-sectional area.

The wires of different materials, having the same length and the equal cross-sectional area, offer different resistance. In simple words, a copper wire will offer different resistance to the flow of current than the silver wire of the same dimensions.

Finally, the engineers encapsulated their findings in the form of an equation which is shown below:

The resistance formula

Where, "p" (rho) is the resistivity of the wire whose value depends on the type of material like whether it is silver wire, copper wire or any other wire.

The copper wires are used for making connections as they are cheap and offer less resistance than other materials.

Resistivity is an important term used in sizing the wire.

(*The process treating copper at right temperature to make it soft and flexible is called annealing)

Resistivity is the resistance offered by the wire of 1 meter length

(*The process treating copper at right temperature to make it soft and flexible is called annealing)

Ohm's law: An important concept

It states that for certain materials at particular range of temperature, the voltage drop across the wire is proportional to the amount of current flowing in it.

The value of resistance using Ohm's law

We can substitute this value of resistance in the very first formula.

My final formula becomes:

Cross-section area of wire

Before moving forward, it is important to know the above terms more clearly.

Let me make you understand with the help of the following picture:

A roof-top solar power system

A basic roof-top solar power system is made of solar panels, inverter, solar charge controller, batteries and obviously wires.

I haven't shown charge controller and batteries in the picture as the things can be explained without them.

You are going to mount the panels on the roof and keep the remaining components in your home.

You will be needing wires to connect the panels to the inverter. The length of the wires is decided by the distance between the panels and the inverter.

If the distance between them is 3 meters then you need 2 set of 3 meter wires, a total of 6 meters, to connect the positive of panel with the positive of inverter and another 3 meters to connect the negative of panel with the negative terminal of the inverter.
The voltage drop would be the difference between the voltage at A and B. For example if the voltage at A is 12 volts and at B is 11.4 volts the voltage drop is 0.6 volts

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Know your energy consumption

The wire sizing depends on your energy consumption. The more you consume, the thicker is the wire used to connect the components.

Let us understand all this with one simple example:

The energy consumption table is shown below:

The energy consumption chart

My solar feasibility spreadsheet recommends 400 watts of solar panels for fulfilling this energy demand.

Configuring the solar panels

One can use two 200 watts panels to make it 400 watts each having a nominal voltage of 12 volts and the current at maximum power is 7.41 amperes.
When we connect two panels in parallel, the nominal voltage will remain 12 volts while the current will add up that is 7.41 + 7.41 = 14.82 amperes.
We take the voltage drop as 5% of the nominal voltage which is 5% of 12 volts = 0.6 volts

Now, we have all the values to know the cross-section area of the wire.

Voltage drop = 0.6 volts
Length = 6 meters
Current = 14.82 amperes
Resistivity = 1.72 x 10-8 ohm.meter

Calculating the cross-section area of wire

Putting all the values in the formula:

Calculating cross-section area of wire

Know the diameter of the wire

The shape of the wire

When you look the wire closely, you can that it is cylindrical in shape with both ends forming the circle.

Know the gauge of the wire

In the market the wires are differentiated by the gauge, therefore we have to find the relation between the wire diameter and the wire gauge.
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American wire gauge(AWG)	Diameter (mm)	Area(mm2)
0000	11.68	107.1
000	10.4	84.9
00	9.27	67.5
0	8.25	53.4
1	7.35	42.4
2	6.54	33.6
3	5.83	26.7
4	5.19	21.1
5	4.62	16.8
6	4.11	13.3
7	3.67	10.6
8	3.26	8.3
9	2.91	6.6
10	2.59	5.3
11	2.3	4.2
12	2.05	3.3
13	1.83	2.6
14	1.63	2.1
15	1.45	1.7
16	1.29	1.3
17	1.15	1
18	1.02	0.8
19	0.91	0.7
20	0.81	0.5
21	0.72	0.4
22	0.65	0.3
23	0.57	0.3
24	0.51	0.2
25	0.45	0.2
26	0.4	0.1