DC CABLES IN SOLAR PROJECTS
When planning electrical projects, one of the first questions to ask if whether the application calls for use AC or DC power which might mean the use of different type of cables. When it comes to electrical applications, there is a great difference
between “cables” and “wires”. And therefore, it is essential to ensure
you are using the appropriate “wiring” for a project. People tend to use these two terms interchangeably, however, there is a
difference between the two. Not to get too technical, but wires are
single conductors of electricity, while cables contain multiple
conductors.
There are two basic types of currents, alternating current (AC) and Direct current (DC). The characteristics of the current intended to flow through those cables
impacts the choice of an AC or DC cable for the project. With AC current, as supply flows through the cables, the current’s
polarity (negative versus positive) changes, alternating as it flows in
one direction while DC current does not change polarity.
Most common typical applications that involve DC power involve low voltages especially in the Solar energy sector. DC cables are a very crucial component in solar power as they have a significant role and impact on the power output and with poor selection or design leading to performance issues.
DC cable wires are made from copper due to the metal's high flexibility, superior current carrying capacity and better thermal performance. Cable sizing is one of the most crucial engineering problems in PV system design. Correct cable selection ensures reduction on the overall cost, increase in performance, guaranteed safety in design and compliance with international standards.
The following requirements are looked into when selecting an appropriate cable
1. Current carrying capacity- This is the maximum current that a conductor can carry under its installation conditions.
2. Voltage drop - This is the decrease of voltage along the cable run due to the cable's internal resistance. Voltage drops are undesired because generate energy is lost as heat. Acceptable maximum voltage drop for DC loads is 5% of nominal voltage.
3. Short Circuit Temperature limit - This is the temperature that cables can adequately handle when exposed to their maximum short circuit current.
Voltage drop = (2 x L x I x R) / A where
L- length in meters I - Current measured in Amps R- Resistance of the cable for copper =0.017, aluminum= 0.028 A - Cross sectional area of cable in mm^2.
Below are charts to help in selecting the best cable size. However this should be checked against the individual manufacturer recommendations