DC Cable Sizing for Solar Projects
DC cable sizing is a crucial aspect of solar projects as it directly affects the performance and efficiency of the entire solar PV system. The main purpose of the DC cables in a solar project is to connect the solar panels to the inverters and then to the grid. Proper sizing of DC cables ensures that there are no power losses and voltage drops in the solar system.
In this article, we will discuss the DC cable sizing for a solar project and the factors that need to be considered while selecting the right size of cables.
Connection Type and Cable Sizing
There are three main connections in a solar project where DC cables are used:
- Array to Combiner Box
- Combiner Box to DC Cabinet
- DC Cabinet to Inverter
The table below shows the power loss and voltage drop percentages for different types of cables at 75°C temperature.
| Connection Type | Power Loss (%) | Voltage Drop (%) |
|---|---|---|
| Array to Combiner Box – 6 sqmm Cu cable | 0.40 (Maximum), 0.55 (Average) | 0.40 |
| Combiner Box to DC Cabinet – 1C x 240 sqmm AL cable | 0.53 (Maximum), 1.27 (Average) | 0.52 |
| DC Cabinet to Inverter – 1C x 300 sqmm Cu cable | 0.036 (Maximum), 0.035 (Average) | 0.04 |
As we can see from the table, the power loss and voltage drop percentages vary for different types of cables. Therefore, it is crucial to select the appropriate size of cable for each connection based on the distance and the current that needs to be carried.
DC Cable Sizing Calculation
To calculate the appropriate size of DC cable, we need to consider the following factors:
- Current: The amount of current that needs to be carried by the cable depends on the power of the solar panels and the distance between the panels and the inverter. We can use the following formula to calculate the current:Current (A) = Power (W) / Voltage (V)For example, if the power of the solar panel is 300 W and the voltage is 24 V, then the current will be:Current (A) = 300 / 24 = 12.5 A
- Distance: The distance between the solar panels and the inverter is also an important factor to consider while sizing the DC cables. Longer distances require larger cables to reduce power losses and voltage drops. The voltage drop can be calculated using the following formula:Voltage Drop (V) = (Current (A) x Cable Length (m) x Resistance (ohm/km at 75°C)) / 1000
- Cable Size: The appropriate size of the cable can be calculated using the following formula:Cable Size (mm²) = (Current (A) x 1.5) / Cable Current Carrying Capacity (A)The cable current carrying capacity depends on the type and size of the cable. For example, a 6 sqmm Cu cable can carry a current of up to 52 A.
DC Cable Sizing Example
Let’s consider an example to understand the DC cable sizing calculation. We have a solar project where the solar panels are connected to the inverter through the following connections:
- Array to Combiner Box – 6 sqmm Cu cable
- Combiner Box to DC Cabinet – 1C x 240 sqmm AL cable
- DC Cabinet to Inverter – 1C x 300 sqmm Cu cable