Solar DC Cable

 

Solar DC Cable is an essential component of solar power systems, connecting solar panels to inverters, charge controllers, and other electrical devices. To make sure your solar systems work well and safely, it’s important to know the right Solar Cables and Sizing. This easy-to-understand guide will help you learn about the different cables used in solar systems and what you need to think about when choosing and installing them. This way, you can get the most out of your solar power system.

You can check out my article on Everything You Need to Know About Sizing Solar Panels for Your Home

Types of Cables Used in Off-Grid Solar Systems

Off-grid solar systems utilize various types of cables to ensure efficient power transmission and system performance. The cables used in these systems can be broadly categorized into two groups: DC cables and AC cables.

 1. DC Cables

These cables handle the direct current (DC) generated by solar panels and are stored in batteries. They include:

• PV Module Cables: These cables connect the solar panels to the charge controller, which regulates the flow of power to the battery bank. PV module cables are typically 10-12 AWG (American Wire Gauge), double-insulated solar cables designed to handle the DC output from solar panels.

• Battery Cables: Battery cables connect the battery bank to the charge controller and the inverter. They are responsible for carrying the DC power between these components. Battery cables are generally larger in size, ranging from 2-4/0 AWG, depending on the system capacity and the current they need to carry.

• Inverter Cables: These cables connect the inverter to the battery bank, transferring the DC power from the batteries to the inverter. Inverter cables are usually similar in size to battery cables, typically 2-4/0 AWG, to handle the required current between the battery bank and the inverter.

2. AC Cables

These cables handle the alternating current (AC) produced by the inverter and distributed it to the electrical loads. They include:

• Inverter Output Cables: Inverter output cables transmit electricity from the inverter to the main electrical panel or distribution board. The appropriate AC wire size should be chosen in compliance with local electrical codes to ensure safety and efficiency.

• Distribution Cables: Distribution cables are responsible for distributing electricity from the main electrical panel or distribution board to various electrical loads or appliances within the system. They should follow local electrical codes and be appropriately sized based on the expected current and voltage requirements of the connected devices.

 

Factors to Consider When Selecting Solar System Cables

A. Cable size

Cable size is a crucial factor to consider when setting up an off-grid solar system, as it directly affects the system’s efficiency, safety, and overall performance. Selecting the appropriate cable size involves taking into account the following aspects:

• Voltage drop: Voltage drop refers to the reduction in voltage as electricity travels through a cable. To maintain efficient power transmission and minimize energy loss, it’s important to limit the voltage drop. For DC cables in solar systems, aim for a voltage drop of less than 3%, while for AC cables, a drop of less than 5% is acceptable.

• Current carrying capacity: The cable size should be chosen based on its ability to carry the maximum current expected in the system without overheating. A cable’s current carrying capacity is determined by its cross-sectional area, and larger cables can handle higher currents. When selecting a cable, ensure its capacity is greater than the maximum current expected in the system.

• Length of the cable run: The distance between components in the solar system, such as solar panels, charge controllers, batteries, and inverters, influences the cable size selection. Longer cable runs increase the resistance and result in higher voltage drops.

B. Conductor material

Conductor materials are the metallic wires used to conduct electrical energy in cables. The most common conductor materials used in off-grid solar systems are copper and aluminum each with its unique properties and applications.

• Copper Cables

Copper is the most commonly used conductor material in off-grid solar systems due to its excellent electrical conductivity, flexibility, and durability. Copper cables have a lower resistance, which results in lower power losses and higher system efficiency. Additionally, copper cables are more resistant to corrosion, making them suitable for various environments, including humid and coastal areas. However, copper is a more expensive material than aluminum.

• Aluminum Cables

Aluminum cables are a more cost-effective alternative to copper cables. They are lighter in weight and have a larger diameter for the same current-carrying capacity as copper cables, making them suitable for long cable runs. However, aluminum cables have lower electrical conductivity compared to copper, which can result in higher voltage drops and energy losses. They are also more susceptible to corrosion and are not as flexible.

C. Cable Insulation

Cable insulation is a crucial component of electrical cables, providing a protective barrier between the conducting wire and its surroundings. Insulation prevents electrical shocks, short circuits, and other hazards that can result from exposed conductors. It also helps maintain the integrity of the electrical signal by reducing interference and voltage loss. Several insulation materials are used in electrical cables, each with unique properties and applications, such as Polyvinyl Chloride (PVC), Cross-Linked Polyethylene (XLPE), Ethylene Propylene Rubber (EPR), Polyethylene (PE), and Polytetrafluoroethylene (PTFE).

When selecting cable insulation for an off-grid solar system, it is essential to consider factors like temperature range, UV resistance, moisture resistance, and mechanical durability. The choice of insulation material depends on the specific application, environmental conditions, and system requirements.

Understanding Wire Gauge Systems

Wire gauge refers to a system used for measuring the diameter of electrical wire. It’s a standardized system that assigns a numerical value to the thickness of the wire, with lower numbers representing thicker wires. There are several wire gauge systems used around the world, with the most common ones being the American Wire Gauge (AWG), Standard Wire Gauge (SWG), and International Electrotechnical Commission (IEC) system.

1. American Wire Gauge (AWG)

The AWG system is predominantly used in the United States and Canada. In the AWG system, the gauge number is inversely proportional to the wire’s diameter, meaning that as the gauge number increases, the wire diameter decreases, and vice versa.

2. Standard Wire Gauge (SWG)

The SWG system, also known as the British Standard Wire Gauge or Imperial Wire Gauge, is primarily used in the United Kingdom and other countries that were part of the British Empire. The numbering and size increments in the SWG system are different from those in the AWG system.

3. International Electrotechnical Commission (IEC) System

Unlike the AWG and SWG systems, the IEC system measures wire sizes in square millimeters (mm²) of the conductor’s cross-sectional area. The IEC system is more straightforward, as the wire size directly corresponds to its cross-sectional area, eliminating the need for gauge numbers.

Wire Gauge Table

 

 

 

A wire gauge table is an essential reference tool for selecting the appropriate cable size for various electrical applications. It lists wire sizes according to a specific gauge system, typically providing information on wire diameter, cross-sectional area, and resistance per unit length. By consulting a wire gauge table, you can choose the most suitable wire size based on factors such as current-carrying capacity, voltage drop, and power transmission efficiency.

The derated rating is calculated by taking a 25% margin. Derated Ampacity = 1.25 x Max Amperage

Here’s a simplified wire gauge table that includes both copper and aluminum conductors, showing AWG sizes, cross-sectional areas, approximate resistances per unit length, and current capacity:

 

AWG	Cross-sectional Area (mm²)	Resistance (ohm/km)	Maximum Amperage	Derated Ampacity (A)
18	0.823	39.7	7.5	6.0
16	1.31	25.0	10	8.0
14	2.08	15.8	15	12.0
12	3.31	10.0	20	16.0
10	5.26	6.3	30	24.0
8	8.37	4.0	40	32.0
6	13.3	2.5	55	44.0
4	21.2	1.6	70	56.0
2	33.6	1.0	95	76.0
1	42.4	0.794	110	88.0
1/0	53.5	0.628	125	100.0
2/0	67.4	0.498	145	116.0
3/0	85.0	0.395	165	132.0
4/0	107	0.313	195	156.0
0000	135	0.249	230	184.0

 

Please note that this table provides approximate values, and actual values may vary depending on the specific type and manufacturer of the cable. The table doesn’t include factors such as temperature and installation conditions, which can also influence cable performance.

How to Use a Wire Gauge Table:

1. Find a wire size in the AWG table that matches your system’s needs, considering factors like current carrying capacity and voltage drop. The table will show wire sizes, diameters, cross-sectional areas, and resistances per unit length (ohms per 1000 feet or ohms per kilometer).

2.  Compare wire sizes: If choosing between two wire sizes, think about the differences in cost, energy efficiency, and installation ease. Bigger wire sizes usually have less voltage drop and better efficiency but might be more costly and harder to install.

3. Review manufacturer’s recommendations: Check the cable manufacturer’s guidelines to make sure the selected wire size is suitable for your specific project, as actual values can vary based on the cable type and manufacturing process.

Calculating Appropriate Cable Size

 

You can find out the correct size of cable required for your application either by using an Online Calculator or using the following manual method.

Let’s go through an example calculation for an off-grid solar PV system. We will size the cables connecting the solar panels to the charge controller, charge controller to the battery bank, and battery bank to the inverter.

Assumptions:

• 4 solar panels, each with 540W power output, Imp = 12.96A, Vmp = 41.7V, Isc = 13.64A, Voc = 49.5V
• Panels are connected in 2 strings of 2 panels each (series-parallel configuration)
• 48V battery bank with a capacity of 400Ah
• MPPT charge controller with a maximum input current of 40A
• 48V inverter with a maximum input current of 100A
• Cable lengths: 15m (solar panels to charge controller), 2m (charge controller to battery bank), 1m (battery bank to inverter)
• Allowable voltage drop: 3%

 

Step 1: Determine the total current

Total power of the solar array (two strings of two panels each):

• 4 panels * 540W = 2160W

Voltage of one string (two panels in series):

• Vmp = 41.7V * 2 = 83.4V

Current of one string (two panels in parallel):

• Imp = 12.96A * 2 = 25.92A

 

Step 2: Calculate the wire resistance

Wire resistance can be calculated by using Ohm’s Law (R = V/I)

Resistance per kilometer (R/km) = R / Cable length in km

 

Solar panel to charge controller (15m):

• Voltage drop allowed (3%):) = 0.03 * 83.4V = 2.502V
• R = 2.502V / 25.92A = 0.0965 ohms
• Resistance per kilometer = 0.0965 ohms  * 1000 /0. 015km = 6.43 ohms/km

 

Charge Controller to Battery Bank (2m):

Battery bank voltage: 48V and Maximum charge current: 40A (charge controller)

• Voltage drop = (3% of 48V) = 0.03 * 48V = 1.44V
• R = 1.44V / 40A = 0.036 ohms
• Resistance per kilometer = 0.036 ohms / 0.002 km = 18 ohms/km

 

Battery Bank to Inverter (1m):

Inverter input voltage: 48V and Maximum input current: 100A

• Voltage drop = (3% of 48V) = 0.02 * 48V = 1.44V
• R = 1.44V / 100A = 0.0144 ohms
• Resistance per kilometer = 0.0144 ohms / 0.001 km = 14.4 ohms/km

 

Step 3: Determine the wire gauge using the AWG table

Find the AWG value with a resistance closest to or lower than the calculated resistance per kilometer for each segment.

Solar panel to charge controller (6.43 ohms/km):

• From the AWG table, select a copper cable with resistance <= 6.43 ohms/km and derated amperage >= 25.92A. A suitable choice would be AWG 8, with a resistance of 4 ohms/km and adjusted amperage of 32A.

Charge controller to battery bank (18 ohms/km):

• From the  AWG table, select a copper cable with resistance <= 18 ohms/km and adjusted amperage >= 40A. A suitable choice would be AWG 6, with a resistance of 2.5 ohms/km and derated amperage of 44A.

Battery bank to inverter (14.4 ohms/km):

• From the AWG table, select a copper cable with resistance <= 14.4 ohms/km and an adjusted ampacity >= 100A. A suitable choice would be 1/0 AWG, with a resistance of 0.628 ohms/km and a derated ampacity of 100A.

Conclusion

In conclusion, selecting the appropriate cable size and type for your off-grid solar system is crucial for efficient energy transmission and minimizing resistive losses. By considering factors such as the maximum current, safety factor, and voltage drop, you can use the wire gauge table and voltage drop formula to determine the minimum wire gauge required for a specific distance.

Investing in high-quality cables and proper installation techniques can also improve the performance and longevity of your off-grid solar system. High-quality cables can better withstand harsh weather conditions and can reduce the risk of electrical fires and system failures.