Yash
The right wire size is very important for the system to work effectively.
It is the wire which joins all the components and transfers the output of one to the next in line, making the current reach the electrical appliances.
You cannot connect the components with any wire size.
An under-sized wire may get heated up soon and will eventually burn out.
While an oversized 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.
Understand 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.
It is measured in ohms.
While experimenting, our engineers found that:
The opposition to the flow of current is more in long wires than the shorter ones.
The wires with a wider cross-sectional area allow current to flow more easily than ones with a smaller cross-sectional area.
The wires of different materials, having the same length and equal cross-sectional area, offer different resistance. In simple words, a copper wire will offer different resistance to the flow of current than a silver wire of the same dimensions.
Finally, the engineers encapsulated their findings in the form of an equation which is shown below:
Where “p” (rho) is the resistivity of the wire whose value depends on the type of material.
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 of treating copper at the 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 of treating copper at the right temperature to make it soft and flexible is called annealing).
Ohm’s law: An important concept for sizing the wire
It states that for certain materials at a particular range of temperature, the voltage drop across the wire is proportional to the amount of current flowing in it.
We can substitute this value of resistance in the very first formula.
My final formula becomes:
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 basic roof-top solar power system is made of solar panels, an inverter, a solar charge controller, batteries, and obviously wires.
I haven’t shown the 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.
Know energy consumption before sizing the wire
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:
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:
Find the wire size (diameter)
When you look at the wire closely, you can that it is cylindrical in shape with both ends forming the circle.
Find the wire gauge
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.
The nearest gauge comes out to be 13. Therefore, I need to purchase a wire of gauge 13 to connect my components of the solar power system.
